Offset reduction and three-dimensional field sensing wigh magnetotransistors

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**X i l «*.«'» '**

N

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1

OFFSET REDUCTION

AND THREE-DIMENSIONAL FIELD

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OFFSET REDUCTION

AND THREE-DIMENSIONAL FIELD

SENSING WITH MAGNETOTRANSISTORS

Offsetreductie en driedimensionale veldmetin

met magnetotransistoren

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de

Rector Magnificus, prof.dr. J.M. Dirken, in het openbaar te verdedigen ten overstaan van een commissie aangewezen door het College van Dekanen, op donderdag 10 december 1987, te 14.00 uur

door

Srdjan Kordic

elektrotechnisch ingenieur

geboren te Belgrado, Joegoslavië T R CÜSS

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promotor prof.dr.ir. S. Middelhoek

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Bismilahir-rahmanir-rahim!

I call as a witness, the inkwell and the quill, and everything they have written;

I call as a witness the wavering twilight of the evening, the night and all that it awakens;

I call as a witness the moon when it mellows and the dawn when it emerges;

I call as a witness Judgement Day, and the soul chastising itself;

I call as a witness Time, the beginning and the end of everything;

— that man is always at a loss.

The Koran

Adapted by M. Selimovic in "The Dervish and Death"

To Nedja and Mara, for they willed this book, and to Julija, who lived through it.

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When I was seventeen my teachers were so stupid that I could hardly bear them, but by the time I was twenty-four I was amazed to see how much they had learned in seven years.

Mark Twain - adaptation

PREFACE

This book represents the Ph.D. thesis resulting from the research which I have been carrying out during the past four years at the Electrical Engineering Department of the Delft University of Technology. It can roughly be divided in two parts: the first part deals with the causes of offset and its reduction in silicon magnetic-field sensors, while in the second part a family of magnetic-field sensors which are sensitive to all three components of the field vector (3-D sensors) is examined. The work on offset and its reduction has been performed in cooperation with Philips - The Netherlands. 3-D sensors have been developed for Océ - The Netherlands.

In Chapter 1 I have tried to justify the use of silicon as a material for sensors. A review of silicon-based magnetic-field sensors is given in Chapter 2 along with an extensive reference list which, I hope, will spare the reader working on silicon magnetic sensors many tedious visits to the library. The first part of Chapter 3 deals with the causes of offset in magnetotransistors, Hall plates and other magnetic sensors. A definition of offset is given and ways of reducing it are discussed. The second part of Chapter 3 deals with the sensitivity-variation offset-reduction method developed at Delft University, which forms an addition to already existing techniques of offset reduction. In Chapter 4 the influence of high electric fields on the Hall angle is discussed, which is important for the understanding of the magnetotransistor sensitivity characteristics presented

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in Chapter 3. Finally, in Chapter 5 - the last chapter - a new class of multicollector devices developed at Delft University, which are sensitive to the full magnetic-field vector, is presented.

I have already published many of the aspects of silicon magnetic-field sensors discussed in this thesis, while a few manuscripts are in preparation. These papers are listed at the end of the thesis.

The devices in this thesis were manufactured in the University's IC-Workshop, and I am deeply indebted to its staff members: J. Groeneweg, E. J. G. Goudena, F. J. de Jong, E. J. Linthorst, W. de Koning, Ir. P. K. Nauta, E.Smit, Ir. J. M. G. Teven, W. Verveer and L. Wubben. Without their dedicated work most of this thesis would have been reduced to pure speculation without any experimental proof. I am further indebted to my students: Ir. Y.Xing, Ir. D. W. de Bruin, P. J. A.Munter, Ir. J. M. van den Boom and H. Mol, whose enthusiastic work tied up many loose ends. I am also grateful to Ir. J. H. H. Janssen of Philips and Th. Siebers of Océ for their support of my work and many fruitful discussions. The technical support of F. Schneider and P. C. M. van der Jagt is greatly appreciated. The many excellent drawings have been furnished by W. J. P. van Nimwegen and J. W.Muilman of the Electrical Engineering's Drafting Department. Photographs have been made by W. G. M. M. Straver, J. C. Schipper and J. C. van der Krogt. I am very much indebted to Mrs. S. Massotty, who corrected the manuscript under intense time pressure and never laughed at my sometimes ridiculous spelling, and to Miss. V. Kordié, who reviewed the final version.

The work on this thesis was supported by the Foundation for Technical Sciences (Stichting voor Technische Wetenschappen - STW) under contract DEL 46.0580. The project has also been receiving significant financial support from Philips, Delft University Fund (DHF) and Océ. Royal Dutch Shell has made a contribution to the costs of a trip to Tokyo, where the 4th International Conference on Sol id-State Sensors and Actuators was held.

Finally, I wish to express my gratitude to my thesis advisor, Prof. S. Middelhoek, because his door was always open.

S. Kordic Delft

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1 INTRODUCTION

The past 30 years have been a period of a continuous microelectronic revolution. Since 1959 the integrated-circuit complexity has been doubling every year. At the same time the performance/price ratio has shown a dramatic increase: a factor of 1018 for digital signal processing

and 1012 for analog circuits. In comparison, if the aircraft industry had

made the same progress today's Boeing 767 would be able to fly around the world in 20 minutes while consuming only 20 liters of fuel; at the same time the aircraft would cost only $ 500 [1.1].

This tremendous improvement in the performance/price ratio has made the proliferation of microelectronics into non-electronic products and industries possible. Today, as a result of the unprecedented, rapid development of microelectronics, electronic watches, games, sewing machines, personal computers, etc. are commonplace. The advent of microelectronics into traditionally non-electronic industries is, however, seriously impeded by the lack of appropriate input transducers (or sensors) having a performance/price ratio comparable to the microelectronic circuits [1.2]. With the exception of military and professional applications where the cost aspect does not seem to be of primary importance, in the vast majority of the consumer goods in which information is processed

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electronically the input side of the system is usually formed by very simple sensors such as push buttons. The fact that a large number of products such as mechanical scales, clinical thermometers, rotating vane flow meters, etc. still do not use any form of electronic information processing indicates that there is still a lot of room for innovation and new products. However, the lack of low-cost, dependable and mass-produced sensors, which are at the same time immune to hostile environments and do not exhibit significant drift of the characteristics, is a problem which should be solved before further penetration of electronics into more traditional markets can take place.

RAD — MECH — THERM MAG — CHEM — INPUT TRANSDUCER 1 SENSOR) EL MODIFIER EL OUTPUT TRANSDUCER /ACTUAT0R\ \ DISPLAY J r— RAD — MECH — MAG — CHEM

Fig. 1.1 Functional block diagram of a measurement or a control system.

The field of consumer goods is not the only one in need of appropriate sensors. The performance capabilities of robots, robot systems and control systems in general are to a large extent dependent on the sensors with which they are equipped [1.3].

In the above applications the interface formed by sensors between the information-processing circuitry and the outside world is a bottleneck which justifies the current interest and efforts in the field of sensors and silicon integrated sensors, in particular.

1.1 INFORMATION-PROCESSING SYSTEMS

In Fig. 1.1 the three components which invariably form a measurement or a control system are depicted [1.4]. The input transducer, usually called a sensor, transforms the input signal of interest into a signal suitable for the information-processing unit. This signal need not be an electrical signal, as there are still quite a few measurement and control systems which internally do not use electronic information processing. An example is the ordinary mercury thermometer in which the thermal input signal is transformed into a mechanical expansion of mercury, which is subsequently processed mechanically in the glass capillary tube. However, the distinct advantages of electronic signal processing such as flexibility, low power consumption, speed, reliability, low cost, low weight, etc. are

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Silicon Integrated Sensors 3

the reasons that this form of information processing has become the most attractive one. As a consequence, sensors which transform the signal of the measurand into an electrical signal suitable for electronic information processing are becoming increasingly important, and we will concentrate on these only.

There are five possible signal types which the input transducer may transform into an electrical signal, i.e. radiant, mechanical, thermal, magnetic and chemical signals (Fig. 1.1, electrical-to-electrical signal conversion has been omitted). Another important distinction can be made between self-generating and modulating sensors. A self-generating sensor needs no auxiliary energy sources to produce the output; an example is the solar cell. As a contrast, in modulating transducers the input signal modulates an energy source to yield an output signal; Hall plates and magnetotransistors are good examples.

The second block in the measurement and the control-system chain is formed by the information-processing unit (digital and/or analog). At this stage the signal from the sensor is modified (filtered, amplified, etc.) in such a way that the appropriate signal can be sent to the third stage - the output transducer. The output transducer converts its input signal into a form that can be perceived by one of our senses (display), or it performs an action (actuator). In general, the output transducer is the inverse of the input transducer since it converts an electrical signal into one of the five possible signal forms [1.5], [1.6].

1.2 SILICON INTEGRATED SENSORS

As a result of the enormous efforts devoted to perfecting integrated circuits over the past thirty years, a dependable, diverse, commercially oriented, and sophisticated silicon IC technology is available nowadays. Silicon has shown itself to be the best material for this technology. Moreover, a wide range of modulating and self-generating effects is available in silicon which may be used for sensing purposes [1.6]. In Table 1.1 a few examples are given of physical and chemical effects in silicon which may be used in self-generating and modulating transducers. If an effect such as piezo-electricity cannot be found in silicon, a broad choice of compatible technologies exists in which it is possible to produce layers of materials having the desired properties on top of a silicon substrate (ZnO piezo-electric layers or NiFe magnetic layers, for instance). A good example of the versatility of silicon for sensor purposes is given in [1.7], which describes a multipurpose integrated-sensor chip that responds simultaneously to several different physical and chemical quantities.

Batch silicon technology fabrication offers the possibility of producing large quantities of sensors, thereby reducing their price. These sensors can

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have very small dimensions and they will not significantly disturb the measurand, their power consumption can be very small and their frequency response is good. Arrays of sensors are also possible for simultaneous, distributed measurements (CCD video cameras).

Silicon also has very good mechanical properties. For example, it has a higher limit of elasticity than steel. It also does not exhibit hysteresis when subjected to repeated stress. Micromachining of 3-dimensional structures is feasible [1.8], [1.9], and in terms of their chemistry, Si and Si02 are inert

in many hostile environments.

Table 1.1 Some physical and chemical effects in silicon which can be

used in transducers. SIGNAL DOMAIN Mechanical Thermal Radiant Chemical Magnetic

EFFECTS FOR SELF-GENERATING TRANSDUCERS -Seebeck effect Photovoltaic effect Galvano-electric effect

-EFFECTS FOR MODULATING TRANSDUCERS

Piezoresistance

Temperature dependence of resistance

Photoconductivity Ion-sensitive field effect Hall effect

Another very important advantage of using silicon as the material for sensors is that the sensor and the signal-processing electronics (or a part of them) may be integrated on the same chip (to obtain the so-called smart or intelligent sensor). In this way the problem of long connections which are susceptible to noise can be alleviated by amplifying the sensor signal or by encoding it on the chip, and subsequently transmitting it to the central signal processor. Power-supply stabilization and reference voltages can be integrated along with the circuitry to compensate for non-linearities and the temperature dependence of the sensor output. In this way the central signal processor can be somewhat relieved of its load. When an array of sensors is required, a multiplexing circuit may be integrated with the sensors, which would reduce the number of connections to the outside world.

But not all the problems have been solved yet. Drift of the sensor characteristics may be a problem in some sensors, in which case the sensor must be calibrated from time to time. Other sensors have the problem that the production yield is not high enough, which increases the sensor price. The present price of $ 5 to $ 10 for solid-state pressure transducers, for example, is still too high for the home appliances market [1.10]. The cost

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Magnetic-Field Sensors and Their Applications 5 of development and processing requires large quantities to be produced and sold. The production yield must be reasonably high to keep the price low. Packaging may present other difficulties. The sensor may have to operate in a hostile environment in which the usual IC encapsulation is inadequate.

In some cases the technological requirements for the sensor may be incompatible with the signal-conditioning circuit technology when a smart sensor is desirable. Where a smart sensor can be realized, unwanted feedback loops may be created (temperature feedback, for example). Silicon can also be used between - 5 0 ° C and +150 C only, and a smart sensor will always need a power supply.

These problems are the reason why integrated silicon sensors are not yet widely used and why at the present there is intensive research in that field. A comprehensive up-to-date review of solid-state sensors can be found in [1.11].

1.3 MAGNETIC-FIELD SENSORS AND THEIR APPLICATIONS

One of the five possible signal conversions to the electrical domain, as we have seen in the previous section, is the conversion of magnetic signals into electrical signals. A magnetic-field sensor is an input transducer which converts a magnetic signal into a useful electric signal. Magnetic phenomena can be described by a few fundamental quantities: the magnetic-field strength H, the magnetic-flux density B, and the magnetization M. Since this thesis deals with non-magnetic media only (just as the one in [1.12]) where the magnetization is zero, the relationship between the magnetic-flux density and the magnetic-field strength becomes:

B = n0H (1.1)

y,0 is the permeability of vacuum. Because the relationship between these

two quantities is so simple, an explicit distinction between the two will be made in this thesis only when it is essential. If it does not make a difference whether we are dealing with the magnetic-flux density B or the magnetic-field strength H, both will be referred to as the magnetic field.

The use of magnetic-field sensors can be classified into direct and indirect applications [1.12]. Direct applications implies that one is interested only in the magnitude and/or the direction of the magnetic-field vector itself. Examples of direct applications are [1.12], [1.13]:

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Field measurements (field mapping) on magnetic materials, devices or apparatus.

Earth magnetic-field measurements for navigational or geological purposes.

Readout of magnetic memories (disk, tape and bubble memories) [1.14].

Recognition of magnetic ink patterns of bank notes and credit cards.

sensor IC

coil spring

Fig. 1.2 A non-contact switch used in solid-state keyboards. A permanent

magnet on the plunger operates the switch.

An example of direct field measurements is the characterization of the fringe field generated by magnetic recorder heads [1.15]. The quantities of interest here include the field strength, orientation and its spatial variation. Although on a different scale, the same quantities are also of interest in high-energy and elementary-particle physics. Another illustration of direct field measurement application is the necessary

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Magnetic-Field Sensors and Their Applications 7

mapping of the magnetic field generated by magnetic devices in copying machines [1.16]. These devices may be used to apply toner having magnetic properties to the paper [1.17].

Measurement of the earth magnetic field for navigational reasons is very probably the earliest example of an application of a magnetic-field sensor. In the 12th century, mariners in China and Europe independently made the discovery that a piece of lodestone, a naturally occurring strongly magnetic variety of the mineral magnetite, floated on a stick in water, tends to align itself so as to point in the direction of the Pole Star [1.18]. In this case the compass magnetic needle is the sensor which transforms the earth magnetic signal (orientation) into mechanical rotation. Only the measurement of the direction of the magnetic-field vector is desired in this case, while in the case of geological and geophysical applications the measurement of the magnitude of the field vector is needed, too.

In contrast to the direct uses, where the magnetic field itself is the quantity of interest, in the indirect applications a non-magnetic quantity is measured, while the magnetic field serves as an intermediary. The non-magnetic measurand is magnetically encoded, and the magnetic signal becomes the information carrier of the original non-magnetic quantity [1.12]. Some examples are:

Linear displacement or position measurements. Angular displacement or position measurements. Metal detection.

Electrical current measurements.

Electrical power measurements (watt meters). Analog multiplication.

Modulator.

Galvanic isolation.

In the first two examples a mechanical quantity is being measured by using a magnetic signal as an information carrier. This transformation may be desirable if mechanical contact is to be avoided. A magnetic signal carrier may also be preferred to other possibilities such as an optical signal carrier, because of its higher resistance to contamination. In the case of the linear and angular position measurements the relative position between the moving parts is determined by attaching a magnetic source (a permanent magnet, for example) on one part, and the magnetic sensor on the other one [1.19], [1.20]. The output signal of the sensor will then depend on the relative position of the sensor and the magnet, which makes position

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sensing possible. An example of linear position sensing is portrayed in Fig. 1.2, which is one key of a solid-state keyboard [1.21], [1.22]. The magnetic sensor IC detects the position of the plunger by means of a permanent magnet that is affixed to the plunger. Compared to mechanical switches, the magnetic switch is less sensitive to contamination and does not wear out as fast.

Fig. 1.3 A Hall plate used to detect the position of a permanent-magnet

rotor. The switching of the current in the stator coils is performed electronically, without the commutators.

Another indirect application of magnetic-field sensors can be found in the brushless electromotors depicted in Fig. 1.3, which is an illustration of angular position sensing. In this case the position of the permanent magnet rotor is sensed by means of a magnetic-field sensor and not by commutators [1.23]-[1.25]. The signal from the sensor is subsequently used to time the switching of the current in the stator coils. As no wear-and-tear of the commutators is present, the life span of such a motor is longer, while at the same time better electronic control is possible, and contamination, in contrast to optical sensors, is less important. Brushless electromotors can nowadays be found in such items as video recorder heads and (floppy-) disk drives.

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Properties of Magnetic Sensors ₉ Similarly, magnetic-field sensors can be used to determine the position of a crankshaft and other moving parts in engines [1.26]-[1.28]. If the position of the shaft is encoded magnetically, non-contact sensing can take place with all the above-mentioned benefits, and the ignition point, for one thing, can be accurately determined. A magnetic encoder used in servo-motor systems is described in [1.29] and [1.30].

In traffic detection it is possible to measure the variation in the earth's magnetic field caused by a passing ferromagnetic vehicle. Another example of metal detection which is often used is the change in the LC product caused by the presence of a metallic body.

A current of 1 A produces a magnetic field of about 100 y.T at the surface of the conductor [1.13]. By measuring this field one can get an idea of the magnitude of the current through the conductor without interrupting the current flow. The field associated with a current in a conductor can also be used for power measurements [1.31].

Figure 1.4 depicts the last three possible applications of the magnetic sensors. The current /M in the coil is proportional to the magnetic-flux

density B which is generated by the coil. The output signal of the sensor

VH is, in the case of a Hall plate, which is used in the example of Fig. 1.4,

proportional to the product of B and the bias current of the sensor /s.

Consequently, the output signal is proportional to the /M/8 product - and

we have analog multiplication or modulation. At the same time, the two circuits are fully galvanically separated.

1.4 PROPERTIES OF MAGNETIC SENSORS

Each of the applications of magnetic sensors discussed in the preceding section imposes its own demands on the electronic, magnetic and mechanical properties of the transducer. The sensor should be able to cover the entire range of the field magnitudes for that particular application. A large magnitude of the sensor's output signal per unit of the measurand -high sensitivity - is one of the most usual requirements imposed on sensors in general. If the output signal of the sensor is too low to be useful by itself, which is often the case with solid-state sensors, it may be necessary to amplify the signal. A limit to a maximum practical amplification, however, is determined by the signal-to-noise ratio (magnetic resolution) of the sensor. There is not much sense in amplifying a signal (without filtering) which is drowned in noise.

Unlike the non-contact switch used in the solid-state keyboards, where only a two-state (on/off) output is required, most of the linear and angular position sensing applications require a high degree of linearity of the output signal as a function of the measurand. A low offset value is another characteristic which is also often desired.

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has to offer, the size and the weight of silicon solid-state magnetic sensors will usually not present a problem, but the mere possibility of realizing small structures, however, seems to lead to even more stringent demands on, for example, the spatial resolution of a magnetic measurement [1.32].

o

. I,

1

_)B

Hall plate

X

, ! f i/u

Fig. 1.4 The output signal of the Hall plate Vn is proportional to the /M/s product.

Last but not least, the problem of packaging will be mentioned. Unlike electronic systems and circuits which are usually kept in favorable and friendly environments, a sensor must by its very nature be in "contact" with the measurand. The sensor package, therefore, must satisfy two requirements:

It may not hinder or alter the measurand.

The package must shield the sensor from everything that could adversely influence the sensor's performance.

A magnetic-field sensor used, for instance, for crankshaft angle position sensing in combustion engines has to operate in an exceptionally hostile environment. The sensor would be subjected to chemical contamination (humidity, oil, etc.), large temperature swings, and

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Properties of Magnetic Sensors 11

mechanical vibrations. Good encapsulation of the sensor is in this case of the utmost importance [1.33].

In the brushless electromotor application the size and the shape of the sensor's package may clash with the small dimensions of contemporary motors [1.34]. The packaging aspect of sensor research in general has not received its due attention, which is evidenced by the practically non-existent literature on this subject.

In the following a list of noteworthy criteria is compiled, which could be used to match an application with a potential sensor:

Magnetic range. Magnetic sensitivity.

Orientation of the sensitivity with respect to.the plane of the chip. Magnetic resolution.

Spatial resolution of the measurements.

Capability of measuring more than one component of the magnetic vector.

Linearity.

Frequency response. Offset.

Temperature dependence of the characteristics. Temperature range in which the sensor functions. Drift of the characteristics.

Size of the sensor.

Weight of the sensor and the package. Power consumption and dissipation. Reliability.

Lifespan.

Manufacturing and packaging cost.

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REFERENCES

1.1] A.Gupta and H.-M. D. Toong, The special issue on personal computers, Proc.

IEEE, vol. 72, 1984, pp. 243-245.

1.2] S.Middelhoek, Integrated sensors, Proc. 3rd Sensor Symp., Japan, 1983, pp. 1 -10.

1.3] A. Dumbs and J.Hesse, Sensor problems in robotics, Sensors and Actuators, vol.4, 1983, pp. 629-639.

1.4] S. Middelhoek, S. Kordic and D. W. de Bruin, Silicon: a promising material for sensors, SEV-Bulletin, vol. 5, 1985, pp. 253-257.

1.5] S. Middelhoek and D. J. W. Noorlag, Three-dimensional representation of input and output transducers, Sensors and Actuators, vol. 2, 1981/82, pp. 29-41. 1.6] S. Middelhoek and D. J. W. Noorlag, Silicon micro-transducers, J. Phys. E: Sci.

Instrum., vol. 14, 1981, pp. 1343-1352.

1.7] D. L. Polla, R. S. Muller and R. M. White, Integrated multisensor chip, IEEE

Electron Device Lett., vol. EDL-7, 1986, pp. 254-256.

1.8] S. Middelhoek, J. B. Angell and D. J. W. Noorlag, Microprocessors get integrated sensors, IEEE Spectrum, vol. 17, 1980, pp. 42-46.

1.9] J. B. Angell, S.C.Terry and P. W. Barth, Silicon micromechanical devices,

Scientific American, vol. 248, 1983, pp. 36-47.

1.10] R.Allen, Sensors in silicon, High Technology, vol.4, 1984, pp. 43-50.

1.11] Solid-State Sensors: State-of-the-Art Reviews 1986, Parts I and II, S. Middelhoek (ed.), Sensors and Actuators, vol. 10, 1986.

1.12] V. Zieren, Integrated silicon multicollector magnetotransistors, Ph.D. thesis, Delft University of Technology, 1983.

1.13] H. P. Baltes and R.S. Popovic, Integrated semiconductor magnetic field sensors,

Proc. IEEE, vol.74, 1986, pp. 1107-1132.

1.14] A. W. Vinal, Considerations for applying solid state sensors to high density magnetic disk recording, IEEE Trans. Magn., vol. MAG-20, 1984, pp. 681-686.

1.15] V. Zieren, W. G. M. van den Hoek and J. de Wilde, Submicron magnetoresistive sensors for the measurement of magnetic recording head fields, Proc. Sensors

and Actuators Symp., Enschede, The Netherlands, 1984, pp. 61-66.

1.16] S. Kordic, P. J. A. Munter and P.J.French, Simultaneous measurements of 3-D magnetic fields in close proximity to the sample with a single silicon sensor,

4th Int. Conf. on Solid-State Sensors and Actuators, Tokyo, Japan, 1987,

Dig. Techn. Papers, pp. 527-531.

1.17] Th. Siebers, Océ - The Netherlands, private communication.

1.18] The New Encyclopaedia Britannica, Encyclopaedia Britannica, Inc., Chicago, IL, USA, 1985.

1.19] W. D. McCall Jr. and E.J.Rohan, A linear position transducer using a magnet and Hall effect devices, IEEE Trans. Instrum. Measur., vol. IM-26, 1977, pp. 133-136.

1.20] Y. Netzer, A very linear noncontact displacement measurement with a Hall-element magnetic sensor, Proc. IEEE, vol.69, 1981, pp. 491-492.

1.21] J. T. Maupin and M. L. Geske, The Hall effect in silicon circuits, in C. L. Chien and R. Westgate (eds.), The Hall Effect and its Applications, Proc. of the

Commemorative Symp., Baltimore, MD, USA, 1979, Plenum Press, New York,

1980, pp. 421-445.

1.22] J.A.Blackburn and S. Vik, Computer menu tablet employing Hall-effect switches, Rev. Sci. Instrum., vol. 55, 1984, pp. 1358-1359.

1.23] T. Kenjo and S. Nagamori, Permanent-magnet and Brushless DC Motors, Clarendon Press, Oxford, 1985.

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References ₁₃

1.24] J.F. Wise Jr. and F. O.Simons Jr., A brushless Hall generator dc servomotor,

IEEE Trans. Industrial Elec. Control Instrum., vol. IECI-21, 1974, pp. 75-77.

1.25] Y. Kanda, M. Migitaka, H. Yamamoto, H.Morozumi, T. Okabe and S. Okazaki, Silicon Hall-effect power ICs for brushless motors, IEEE Trans. Electron

Devices, vol. ED-29, 1982, pp. 151-154.

1.26] R. W. Holmes, Improved crankshaft sensing techniques, Proc. 2nd Int. Conf. on

Automotive Electronics, London, UK, 1979, pp. 155-158.

1.27] L. Halbo and J. Haraldsen, The magnetic field sensitive transistor: a new sensor for crankshaft angle position, Congress and Exposition of the Society of

Automotive Engineers, Detroit, MI, USA, 1980.

1.28] J. D. Rickman Jr., Magnetic methods of sensing shielded part motion, Congress

and Exposition of the Society of Automotive Engineers, Detroit, MI, USA,

1982, Proc. pp. 1-9.

1.29] S. Kawamata, T. Takahashi, K.Miyashita and K. Tamura, Magnetic rotary encoder with high resolution, Proc. 4th Sensor Symp., Japan, 1984, pp. 277-280.

1.30] T. Takahashi, S. Kawamata, K.Miyashita and H. Kanai, Magnetic rotary encoder with zero reference detection function, Proc. 4th Sensor Symp., Japan, 1984, pp. 281-284.

1.31] K. Matsui, S. Tanaka and T. Kobayashi, GaAs Hall generator application to a current and watt meter, Proc. 1st Sensor Symp., Japan, 1981, pp. 37-40. 1.32] S. Kordic, Sensitivity of the silicon high-resolution 3-dimensional

magnetic-field vector sensor, Int. Electron Devices Meet., Los Angeles, CA, USA, 1986, IEDM Techn. Dig., pp. 188-191.

1.33] J. M. Giachino, Ford Motor Co., private communication.

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D INTRODUCTION

D THE BASIC PHYSICAL PRINCIPLES D HALL PLATES AND RELATED

DEVICES D MAGNETOTRANSISTORS D CARRIER-DOMAIN MAGNETIC-FIELD SENSORS D MAGNETODIODES D CONCLUSIONS

There is only one thing in the world worse than being talked about, and that is not being talked about.

Oscar Wilde

2 SILICON MAGNETIC-FIELD SENSORS

2.1 INTRODUCTION

The birth of solid-state magnetic sensors can be traced all the way back to 1879 to the discovery of the Hall effect by E.H. Hall [2.1], [2.2] and the resistance change of a material in a magnetic field discovered in 1856 by W. Thomson. Hall, then a graduate student at Johns Hopkins University in Baltimore, was able to measure a cross-current in a thin gold layer on glass under the influence of a magnetic field. This was the proof that the magnetic field exerts a force on the electric current in a conductor and not on a conductor itself, as was claimed by Maxwell. Since then there was almost no activity in the solid-state magnetic-field sensor field until some 20 to 30 years ago. Silicon became an interesting material for

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magnetic sensors somewhat later when silicon-based IC technology became of age. The reason for the increasing importance of silicon as a material for magnetic sensors compared to materials such as GaAs and InSb, which have a much higher Hall mobility, is the ease of integration which makes it possible to not only integrate the sensor but to place signal-conditioning circuitry on the same chip. (This advantage may not be long lived as quite recently there appeared a report on a GaAs Hall sensor integrated on the same chip with an amplifier and other circuitry [2.3].)

e

k

r

Fig. 2.1 The trajectory of an electron in vacuum in crossed electric and

magnetic fields. The average electron motion is 90 to both the Bz and £x vectors.

Applications of magnetic-field sensors [2.4]-[2.11] have been discussed in the previous chapter. In this chapter a review of integrated silicon-based magnetic-field sensors will be presented [2.12]-[2.16]. A review of magnetoresistive sensors can be found in [2.17].

There are quite a few criteria which could be used for the classification of magnetic-field sensors. One possibility is classification according to the type of the output signal, which yields three classes of silicon magnetic sensors: voltage, current and frequency output sensors. Another possibility is a classification based on the underlying mechanism which determines the sensitivity. In this case possible classification could be: Hall devices, carrier deflection devices, injection modulation devices, etc. The classification which has been chosen in this review, the one most

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The Basic Physical Principles 17 frequently encountered in the literature, is according to the type of the device which constitutes the sensor. We have bulk Hall plates as a contrast to FET Hall devices, and magnetotransistors have been divided into two classes, lateral and vertical magnetotransistors, while carrier-domain magnetometers are mainly based on npnp structures. There is also a section dealing with magnetodiodes. (There are, as always, exceptions to the rule. JFET magnetic sensors have been mentioned, for example, in the section on FET Hall devices, even though a JFET is a bulk device.)

This review deals only with silicon magnetic sensors because of the importance of the silicon-based IC technology, but a few devices fabricated in other materials have also been mentioned. Sometimes the reasons for this are historical, so that the more recent developments are placed into a better perspective, while some non-silicon examples are included because even though a silicon based device does not yet exist, there is no fundamental reason for not using silicon.

In a review of such broad scope, different problems can only be discussed superficially. Subjects like the numerical simulation of magnetic sensors, for example, have only been broached in passing. To compensate somewhat for the lack of depth, an extensive reference list (with the accent on recent work) has been included, also in the hope that a novice starting in this field will have a useful source of initial information.

2.2 THE BASIC PHYSICAL PRINCIPLES

The magnetic-field sensors presented here are all based on the interaction between the moving charge carriers and the magnetic field. The interaction is described by the well-known Lorentz force:

FL = q(vxB) (2.1)

where FL is the Lorentz force experienced by a charge carrier of charge q

when it is moving at velocity v in a magnetic-flux density (magnetic induction) B. If one also takes into account the effect of the electric field, the equation of motion in vacuum becomes

w ^ f = q(E + vxB) (2.2)

where m is the mass of the charge carrier. If we assume that we are dealing with electrons and that all components of E and B fields except £x

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j2 d X At j 2 2 At'

£( + *>.%)**

■■dt e n dx '""•dt (2.3) -y

e

a

Fig. 2.2 In a solid, collisions will determine the amount of deflection.

The Hall angle ©H, which is the angle between the average trajectory of

the electron and the applied electric field, is smaller than 90°.

Assuming that the electron is at rest in the origin at / = 0, the solution of the above set of equations becomes [2.18]

% c

Ex 1

y(t) = -£-(öTsinu;c/ - O

D„ C

wc = mBz (2.4)

where wc is the cyclotron frequency. The trajectory of the electron in

vacuum is depicted in Fig. 2.1. We can see that the average motion of the electron is perpendicular to both the electric and the magnetic fields, and parallel to the y axis. Therefore, under the influence of the magnetic field the average direction of motion of the electron deviates by 90° from the

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The Basic Physical Principles ₁₉ case in which Bt = 0 (electron moves parallel to E). We can introduce a new

quantity 6H (Hall angle) defined in Eq. (2.5), which is the angle between

the applied electric field E and the average current-density vector J [2.18]. In vacuum 6H becomes 90° for all values of the magnetic field

larger than zero.

tan 0H = -f (2.5)

•'x

In a solid (semi-) conductor the electron (hole) cannot move indefinitely and undisturbed in the electric and magnetic fields. After a certain time the electron will collide with a lattice atom, and if the scattering mechanism is isotropic, we can assume that on the average, taken over the whole electron cloud, the electron loses all its energy to the lattice and its velocity just after the collision is zero [2.19]. The acceleration of the particle starts all over again continuing until the next collision. If we assume that all collisions take place after a mean collision-free time <r> the resulting trajectory is the one depicted in Fig. 2.2. The Hall angle can be written as [2.20]

V(<T>) ÖT s i n (wc <r> ) " <T>

t a n 0

H = x7^r7) " ~f

( 2 6 )

t £ [ COS (WC<T>) - 1 ]

For small values of 6H and wc<r> the Hall angle becomes proportional to

" c = l *E (2-7)

e H =jn<T>

Finally, with the expression for electron mobility n as a function of the mean collision-free time <r>, the Hall angle becomes

GH a nBz (2.8)

Of course, all electrons do not have the same velocity, nor is the collision-free time <T> a constant. However, if these effects are taken into

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account [2.19], only a slightly different result for 6H is obtained, namely:

6H - fiHBz

(2.9)

The Hall factor r generally has different values for electrons and holes and is dependent on the scattering mechanism involved. /zH is the Hall

mobility. For electrons r lies between 0.97 and 1.15 (see Fig. 2.3 [2.81]), depending on the donor concentration [2.21 ], and it is somewhat lower for holes [2.22].

The product /iH5 is a dimensionless quantity. In M.K.S. units n has

dimensions m2V"1s"1, while B, measured in teslas (T) has dimensions

m~2Vs. The factor r is dimensionless. 1.20 0) a. - 1.10 v. *- 1.00 0.90 I 1 I 1 1 _ T = 3 0 0 K , 1 . 1 1 1 1 1 1 ^ n - S i i ' ' i 1.00 0.90 ai a. 4— I - 0.80 £ 0.70 10u 10" 10° 10" 10" 10"

Dopant density A/(crrT3)

Fig. 2.3 Theoretical Hall factors for n - and p-type silicon.

2.3 HALL PLATES AND RELATED DEVICES

The theory in the previous section is valid only if there are no boundaries to obstruct the current flow caused by the applied magnetic field (flow in the -y direction in Fig. 2.2). These boundaries are present in Hall plates and the theory of the previous section must be somewhat modified.

Since the charge carriers are forced to one side cf the plate (Fig. 2.4) by the Lorentz force, their accumulation on one side will create an electric field Ey which counteracts the Lorentz force. This opposing electric field,

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Hall Plates and Related Devices 2 1

called the Hall field, resulting from the redistribution of the charge carriers in the Hall plate under the influence of the magnetic field, generates the Hall voltage VH which can be picked up by electrodes placed

at the opposite edges of the device. To a first approximation no current deflection will take place after a very short transition period because the Lorentz force is canceled by the force generated by the Hall field, and if the definition of the Hall angle given in Eq. (2.5) of the previous section is applied to this case, 6H would be zero ( Jy = 0). In the section on the basic

physical principles we saw that the current-density vector changed its orientation under the influence of the magnetic field. The electric-field vector remained unaltered. In the Hall plate case, however, in which the current flow in the y direction is obstructed by the device boundaries, the current density vector remains the same while the electric-field vector changes orientation under the influence of the magnetic field. For Hall plates and magnetic sensors based on Hall plates, a different definition of 0H must be used [2.19]:

t a n 6H = Fy (210)

The assumption that no current deflection takes place in a Hall plate is valid only if all electrons have the same velocity, but, of course, in reality this is not the case. The current flow is, in fact, not parallel to the x-axis for another reason, too. Short-circuiting of the accumulated charge takes place around the conducting bias contacts of the platelet, and especially in the vicinity of the bias contacts the current-density vector will have both

x and y components [2.23]. Far from the contacts, however, the assumption

of Jy = 0 is a good approximation if the velocity distribution of the

electrons is neglected. Using the condition that Jy = 0 the Hall angle can be

calculated according to Eq. (2.10). Finally, except for the sign, the same result is obtained as in Eq. (2.9).

The Hall voltage VH can be quite easily calculated by integrating the

Hall field Ey:

constant voltage bias

(negative for n-type material) constant current bias

(2.11) (n-type) (p-type) = K

_T

r

^/(—)

'H WH "H WH R„ = - r ne H pe

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Here we see the Hall angle (rnBz), the bias voltage VB across the

contacts ba and b2, the width-length ratio wH//H, and a correction factor

/(/H/wH) which depends on the geometry of the device. The correction

factor takes into account the shorting effect of the bias contacts [2.18]. The optimum rectangular IC Hall elements have wH//H ratios slightly less than

unity [2.4]. Is is the bias current of the Hall plate, dH is its thickness, and Rn is the Hall coefficient. R^ is a complicated function of the carrier

densities and mobilities, but in the case in which one of the carriers predominates it takes the simpler form given in Eq. (2.11).

From Eq. (2.11) we see the importance of the carrier mobility of the material and its influence on the sensitivity of the device. As mentioned earlier, the ease of integration and the possibilities that silicon IC technology has to offer have made this material very interesting for magnetic sensors. b., o ■•

y\

vv + + + + + + W u Is . -»—°b, ■o h?

Fig. 2.4 The presence of boundaries in a Hall plate (p-type in this case)

results in the accumulation of charge carriers on one side of the Hall plate (and a depletion on the other side) under the influence of the magnetic field. This manifests itself as a potential difference (Hall voltage Vn) between the Hall contacts hj and h2.

2.3.1 Bulk Hall Devices

In Fig. 2.5 a top view and a cross-section of a rectangular, n-type silicon Hall plate are depicted. The plate is defined by a deep-p diffusion making an n epilayer island. The Hall and the bias contacts are formed by

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Hall Plates and Related Devices ₂₃ a shallow n+ diffusion. The advantage of this structure is that it is fully

compatible with an IC bipolar process which uses a low impurity epilayer (high electron mobility), so that bipolar signal conditioning circuitry can be integrated on the same chip with the Hall plate without any extra processing steps. There are quite a few ideas and variations on the basic device structure and layout. One is to use an n implanted layer for the active Hall region [2.8]. This approach is used to get a more uniform distribution of impurity concentration over the wafer which is, in turn, advantageous for a lower value of the offset voltage of the device.

B

n+ Hall contacts

I

5SZ

l ï Z

n - epilayer p-substrate

Fig. 2.5 Top view and a cross-section of an n-type Hall plate. Contacts

1 and 2 are the bias contacts, while contacts 3 and 4 are the Hall contacts.

The Hall plate of the geometry of Fig. 2.5 is sensitive to magnetic fields perpendicular to the plane of the chip. In [2.24] and [2.25] a vertical Hall device is proposed that is sensitive to one component of the

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magnetic field in the plane of the chip (Fig. 2.6). One bias contact is between the Hall contacts on the surface of the device. The other one is placed far away, so that the main current flow in the vicinity of the Hall contacts is perpendicular to the surface of the chip. This sensor was fabricated using a standard bulk CMOS technology. The unusual geometry does not seem to affect the sensitivity of the device.

Fig. 2.6 The vertical Hall plate.

In [2.26] a device is discussed which has both Hall contacts on the same side of the platelet. Such a layout lends itself to placing the device on the edge of the substrate if measurements in the close proximity of a magnetic surface are desired. Although the sensor fabricated in this case consisted of an evaporated bismuth film, this structure could also be implemented in silicon.

The additional control electrodes proposed in [2.27]-[2.29] allow variable offset voltage control and magnetic field simulation. This also allows threshold and gain adjustments.

Finally, in [2.30] a multielectrode structure is proposed which yields a more uniform sensitivity over the wafer, reduces piezoresistive effects on the offset, and reduces the input and output resistance of the Hall plate.

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Hall Plates and Related Devices 25

Numerical simulations of Hall and related devices can be found in [2.23] and [2.31 ]-[2.41 ], and the response of Hall devices to non-homogeneous magnetic fields is discussed in [2.42]-[2.49].

Fig. 2.7 An n-channel MOS Hall plate.

2.3.2 FET Hall Devices

With a few modifications a MOSFET structure can also be used as a Hall sensor [2.50]-[2.60]. An n-channel MOS Hall plate is depicted in Fig. 2.7. If the gate of the device is biased above the threshold voltage a thin n-type inversion layer is formed on the p-type substrate. This inversion layer can be used as the body of a Hall plate. The bias contacts of the Hall plate are the n+ source and drain, while additional n+ regions are

used as Hall contacts. The advantage of a thin Hall layer (if the platelet is under constant current bias) formed by inversion is somewhat decreased by the lowered Hall mobility of the carriers compared to the bulk devices. In [2.51] a p-channel device was tested and the mobility of the holes in the channel was found to be about 150 cm2/Vs. The Hall voltage was found to

rise linearly with the drain-source voltage Vda when the device is operated

below saturation, while no increase in the Hall voltage with Vds is observed

above saturation. A maximum in the Hall voltage vs. gate-source bias is observed at voltages close to the turn-on voltage of the MAGFET. The position of the Hall contacts with respect to the drain of the device is quite

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important for the sensitivity of the device to magnetic fields [2.52]. An optimal configuration is the one in which the Hall contacts are close to the drain (ratio y/lH close to 1). Because of the higher mobility of the electrons

in the inversion layer, an n-channel device is more sensitive to magnetic fields than its p-channel counterpart [2.51], [2.57]. Moreover, the Hall voltage of a MAGFET does not depend on the geometry of the gate of the device [2.56]. A non-contact keyboard switch based on a MOS Hall device is reported in [2.58]. A low power consumption MOS amplifier was integrated along with the Hall plate to boost the output signal. In [2.60] a lumped element simulation of MOS Hall devices is presented.

Fig. 2.8 A JFET Hall plate. The thickness of the channel is modulated

by the Hall voltage VH.

A Hall sensor based on a junction field effect transistor (JFET) has been mentioned in [2.61] for germanium, and in [2.62] for silicon. These devices are mainly sensitive to magnetic fields in the plane of the device and perpendicular to the current flow, but there are also reports of a JFET being parasitically sensitive to the other two field components [2.63]. Unlike the other Hall effect devices, in which a potential difference across the Hall contacts is a measure for the applied magnetic field, in JFET devices the magnetic field modulates the junction bias, which in turn influences the drain current. The change in the drain current is a measure for the magnetic field (Fig. 2.8).

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Hall Plates and Related Devices 27

source of non-linearity in junction-isolated, integrated Hall devices. This is caused by the modulation of the plate thickness by the Hall voltage.

2.3.3 Hall Plates Incorporated in an IC

The output signal of a silicon Hall plate is too small to be useful by itself and some amplification of the signal is necessary. This can be quite easily achieved by integrating an amplifier on the same chip with the Hall plate. An elegant implementation of this idea can be found in [2.65] and [2.66]. In Fig. 2.9 [2.81] the cross-section is given of a Hall plate incorporated in the shared base region of a differential amplification magnetic sensor (DAMS) [2.65]. We can see that instead of n+ Hall

contacts, two p+ diffusions can be found on the sides of the n-type Hall

plate. Under the shared n-type base (Hall) region two separate p-type collectors can be found. The top view and the equivalent circuit of this structure are illustrated in Figs. 2.10 and 2.11, respectively. From the equivalent circuit we can see that the voltage generated by the Hall plate is directly converted into a difference in the injected emitter currents of the two pnp transistors. The collector-current difference is finally converted into a voltage difference by the two load resistors.

emitter

B

>'.

4-

Al emitter

V \i_E*/ ® Jb n-Hall (base) region \$Ï±J J

p- collector J \^ p-collector

n - substrate

Fig. 2.9 Cross-section of a Hall plate that forms a shared base of a

differential amplifier (DAMS).

In [2.8] a circuit is described which controls the period of current flow in stator coils of brushless electromotors. The signal from the n-type Hall plate is amplified by an npn bipolar transistor differential stage. Finally, the signal is supplied to a power transistor switching circuit which controls the current in the stator coils.

Commercial general-purpose Hall-effect devices are available from various companies including Texas Instruments, Siemens, Honeywell and Sprague. These silicon Hall ICs contain, besides the Hall sensor, on-chip amplification and stabilization circuitry [2.67]-[2.70]. The same

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manufacturers produce a series of Hall-effect switches [2.71] for applications where only a high-low output is needed. Besides the on-chip amplification and stabilization circuitry, these switches contain a Schmitt trigger and an output stage controlled by the trigger.

emitter

Hall region

^ e m i t t e r i f l

Fig. 2.10 Top view of a DAMS.

dams L C i -R, R, H1

■e-R H2

Ï I

I

Rr

_J

-°C, ^ L

K

Fig. 2.11 Equivalent circuit of a DAMS.

Since Hall plates are modulating transducers [2.72], they are somewhat plagued by offset, which is an additive error in the output of the sensor

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Hall Plates and Related Devices ₂₉ responsible for a non-zero output signal when the measurand is zero:

VH = SmHB + V0

(2.12)

VH is the output signal of the sensor and 5m I I is the sensitivity. One can

make a distinction between the offset signal V0, which is the signal (in

volts for a Hall plate) at the output of the sensor when the measurand B is zero, and the equivalent offset B0 expressed in the units of the measurand.

If the application permits repetitive calibration (because of drift) of the sensor by setting B to zero or a known value, the offset signal can be easily eliminated electronically. If, however, B remains an unknown at all times, as in the brushless electromotor application, other methods of offset reduction have to be sought.

Fig. 2.12 Equivalent circuit of a Hall plate. Contacts bx and bg are the bias contacts. \ and hg are the Hall contacts. Offset signals are a result of imbalances in the resistor bridge.

Offset in Hall plates is mainly caused by mask misalignment of the Hall contacts (with respect to the bias contacts, isolation walls and each

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other) and by piezoresistive effects [2.8], [2.41], [2.73], [2.74]. Other less important causes of offset are discussed in [2.18]. Both the misalignment and piezoresistive causes of offset can be explained by the resistor bridge circuit of Fig. 2.12. If the bridge is perfectly symmetrical the voltage difference between the Hall contacts will be zero if B = 0. If the Hall contacts are not well aligned with respect to the bias contacts, isolation walls or each other, the bridge of Fig. 2.12 will no longer be balanced: an offset signal will appear on the Hall contacts. In the same manner thermally and mechanically induced stress and strain which are exerted on the chip when it is attached to a carrier and molded with resin will cause changes in the resistors of the bridge, and consequently offset. In [2.75] it is reported that under certain conditions a Hall-effect device can even be used as a strain and pressure sensor.

The value of offset in Hall plates can be reduced by decreasing the misalignment errors or, when a limit is reached, one can design the Hall structure in such a way that the influence of misalignment becomes less pronounced [2.30], [2.77]. Another approach is presented in [2.4], [2.78] and [2.79]. The Hall effect non-reciprocity can be used to reduce offset. If the Hall and bias contacts of a symmetrical Hall plate are alternatively switched and the results of the magnetic field measurements added, the value of offset is reduced. Solid-state technology offers the possibility of implementing this idea in a different manner [2.4], [2.79]. It is assumed that the bridge imbalances of the two neighboring Hall plates are similar. A combination of the outputs of these two platelets reduces the value of offset.

The crystallographic orientation of the Hall device and the current flow greatly influence the offset caused by the piezoresistive effects [2.8], [2.73]-[2.76]. Hall devices with a <100> direction of current flow in the (110) plane have the minimum offset voltage caused by stress and strain, while devices with a <110> current flow in the (100) plane are most sensitive to piezoresistive effects.

The characteristics of some Hall-plate-based sensors are given in Table 2.1.

2.4 MAGNETOTRANSISTORS

In contrast to Hall plates, where the signal which is proportional to the magnetic field is a potential difference between Hall contacts, the output signal of a magnetotransistor (magnistor) is a current difference. The basic ingredients necessary to make a magnetotransistor are a current source in the form of a pn-junction and a few electrodes (collectors) to pick up the current. The magnetic field disturbs the currents drawn by the collectors and the difference between the collector currents is a measure for the magnetic field. Two-collector structures are most frequently encountered,

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Magnetotransistora ₃₁ and these are sensitive to only one component of B. Sensitivity to other components is usually parasitic. Quite recently reports of multi-collector devices which are sensitive to two components of the magnetic-flux density or even the full B vector have been published.

Table 2.1 Characteristics of some Hall-plate-based sensors. The

sensitivity is calculated according to 100% VH/VSB. The sensitivity of devices with an on-chip amplification circuit is expressed as Vout/B, where Vout is the output signal after amplification (in this case the bias of the sensor itself is not important). In the column on directional sensitivity Bz means that the sensor is sensitive to a component of the magnetic-flux density perpendicular to the chip surface. Bx indicates a sensitivity to one of the in-plane field components.

Kanda [2.8] Popov ic [2.24] Randhawa [2.30] DAMS [2.65] Siemens [2.68] Texas Ins. [2.67] Honeywell [2.70] Sprague [2.71] Hemmert [2.56] Yagi [2.57] Hirata [2.58] Popovic [2.54] SENSITIV. 7.6 %/T 4.9 %/T 7.2 %/T 120 V / T 100 V / T 15 V / T 75 V/T 7 V / T 3.1 % / T 3.7 %/T 7.6 % / T 1.2 V / T DDL SENS. B* Bx B* Bt B, Bt B, Bt B, B* Bt B, DIMENSIONS L x W (/im) 2 0 0 x 2 0 0 — 2 2 0 x 2 2 0 -— — — 2 2 0 x 5 0 0 1 6 0 x 160 6 0 0 x 7 0 0 -OFFSET (mT) < 10 — < 10 -< 3 5 — — " -— — -TYPE bulk n-epi bulk n-epi bulk n - e p i bulk n - t y p e epilayer SAS 231 epilayer TL 173 LOHET U G N 3 5 0 1 n-channel MOS-FET n-channel MOS-FET n-channel MOS-FET CMOS dif f. amp. ON-CHIP AMP. no no no yes yes yes yes yes no no no yes

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As early as 1950, it was noticed that the magnetic field influences the common-base current gain of a germanium bipolar transistor by deflecting the minority carriers in the base [2.80]. One, therefore, does not necessarily need more than one collector for a magnetically sensitive device. Multicollector devices are preferred because of the absence of a very large quiescent signal and the non-linearity. Nor does one need a structure as complicated as a transistor to make a sensor with an electrode current difference as the output signal. A simple resistive slab of silicon with one current source contact and two collector contacts will also yield a collector current difference as a response to an applied magnetic field, but because of concentrated current injection, a comparable magnetotransistor structure has a higher magnetic sensitivity [2.81].

B

B^n+J B2(n+)

Fig. 2.13 Drift-aided lateral magnetic-field sensitive pnp transistor.

Two mechanisms can be responsible for the magnetic sensitivity of the magnetotransitors: carrier deflection as described in the section on the physical principles, and modulation of emitter injection [2.82]-[2.84]. Which mechanism will dominate depends on geometrical factors and operating conditions [2.83], [2.85]. Carrier deflection as described in the section on the Hall angle will dominate in devices in which the Hall field is prevented from building up. The absence of the Hall field will enable the full deflection of the carriers to take place. The deflection will cause a collector current imbalance. If, however, a portion of the transistor base can be regarded as a Hall plate a device reminiscent of DAMS will emerge. Just like DAMS a Hall voltage built up in the base will modulate current injection across the emitter length, which also causes a difference in the collector currents.

One could use the two mechanisms as a basis for a classification of magnetotransistors, but it is more convenient to make a distinction based on the direction of the main current flow with respect to the chip surface. We can distinguish between lateral magnetotransistors: the main current flow is parallel to the chip surface, and the sensor is usually sensitive to fields

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Magnetotransistors 33 perpendicular to the surface; and the vertical magnetotransistors: the main current flow is perpendicular to the chip surface, and the sensor is sensitive to fields in the plane of the chip.

2.4.1 Lateral Magnetotransistors

Figure 2.13 depicts the structure of a lateral magnetotransistor which was first reported in [2.86] and [2.87] (a Ge version is presented in [2.88]). This pnp magnistor can be fabricated in standard bipolar technology, where the n-type base region is an epilayer island. If we momentarily disregard the influence of the base bias, we can see that the emitter current (density) /e which is injected into the base will be deflected by the

hole Hall angle 0H from its original path, resulting in a collector-current

difference A/c. If, in addition, a positive voltage bias is applied between

base contacts B1 and B2 the base region will behave as an n-type Hall plate

and charge will accumulate on the sides, as indicated in Fig. 2.13; this charge will generate the Hall field. The generated Hall field will, in turn, alter the Je current path by an additional Hall angle 0H n. The total

deflection of the current-density vector Je becomes 0H + 6H n and the

collector current difference A/c = /cl - /c2 is:

Arc = K(nHp + nHn)BIe (2.13)

where K is a constant dependent on the bias conditions, geometry and electrical properties of the device. /xHs are the hole and electron Hall

mobilities and /e is the emitter current. No emitter injection modulation has

been reported. A pnp magnistor was used in [2.89] to magnetically trigger an on-chip SCR.

A device with only one base contact, which is, therefore, not drift-aided, is presented in [2.90]. In this pnp double-collector transistor only deflection of the holes which are injected into an elongated base plays a role in the magnetic sensitivity.

In [2.91] it was reported that a drift-aided double collector magnistor is sensitive mainly to the magnetic field component perpendicular to the chip surface Bz, as is to be expected. Still, the output signal exhibits some

dependence on the magnetic field in the plane of the chip and parallel to the collectors (SJ. Quite contrary to these results, in [2.92] it was experimentally determined that the lateral magnistor in question was 10 times more sensitive to Bx than to Bz. These differences can be explained

by the presence of a current injected in the z direction (perpendicular to the surface of the chip). This current is susceptible to deflection by the Bx

fields. In [2.92] the authors were dealing with strong vertical injection, while lateral currents contributed only modestly to the total sensitivity.

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Nevertheless, lateral magnetotransistors remain sensitive to both the z and the x component of the magnetic field. This may introduce errors into the measurements. Other reports on the structures similar to the device of Fig. 2.13 are given in [2.93]-[2.100]. In Fig. 2.14 a lateral magnetotransistor is depicted. Deflection in this device takes place in the collector region, and not in the base.

Fig. 2.14 A lateral npn magnetotransistor based on current deflection.

In Fig. 2.15 a structure is presented which resembles the one in Fig. 2.13 except that the dominant mechanism responsible for the magneto-sensitivity is emitter injection modulation [2.82], [2.83], [2.101]. The functioning of this device is very similar to that of DAMS [2.65] described earlier. Due to the lateral base confinement the interaction between the base current and the magnetic field will result in a Hall field across the width of the base region. This will cause the emitter to inject more on one side, while the current injection on the other side is supressed [2.83], causing a collector-current imbalance. The sign of the response depends not only on the field direction but also on the biasing of the base contacts which determines the direction of the Hall field. Which of the two mechanisms, i.e. carrier deflection or injection modulation, dominates depends on the specific structure and operating conditions of the device

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Magnetotranaistors 35 [2.83], [2.85]. Other accounts of structures similar to the one in Fig. 2.15 are given in [2.102]. ' 'Jci E B,

®fi

' - n+^ . }

ht

P P n+ P

1

Fig. 2.15 Lateral magnetotransistor with two base contacts. The base

region acts as a Hall plate. The polarity of the response to magnetic fields depends on the biasing of the base.

Fig. 2.16 Lateral magnetotransistor with collectors and base contacts

Offset reduction and three-dimensional field sensing wigh magnetotransistors

X i l «*.«'» '

l -

OFFSET REDUCTION

AND THREE-DIMENSIONAL FIELD

OFFSET REDUCTION

AND THREE-DIMENSIONAL FIELD

SENSING WITH MAGNETOTRANSISTORS

Offsetreductie en driedimensionale veldmetin

met magnetotransistoren

PREFACE

CONTENTS

1

INTRODUCTION

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Hall plate

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