Extraction of RDS(ON) of n-Channel Power MOSFET by Numerical Simulation Model

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EXTRACTION

OF

_{Ri s(or}

OF n-CHANNEL

POWER

MOSFET

BY NUMERICAL

SIMULATION MODEL

C.-T. SALAME*

RadiationTechnology,InterfacultyReactorInstitute,

_Delft

University

of

Technology, Mekelweg15,2629JBDelft, TheNetherlands

(Received4September2000;In_finalform13September2000)

Inthispaperwepresentanoriginalmethod forn-channelpowerMOSFETresistance extractioninthe operation mode_(RDso).The

_IDs

f(VDs)electrical characteristics measurements for the transistor and the Body-Drain junction are realized for the experimental determination and the extraction (by numerical analysis) ofRDSO,

respectively. Values of this resistance areextracted for different positive biasapplied

between the gate and the source (+Vos). Physicals parameters obtained from the numerical analysis are inspected, and results shows that the numerically analysed junction characteristic is inverygoodcorrelation withthe electricalmeasurement Keywords: Power MOSFET; RDs(o;Numerical simulation; Physicalsparameters

1. INTRODUCTION

VDMOSFET operation (MOSFET with Vertical Diffusion) are

mainly controlled by the gate voltage consists in modulating the

channel conductivity resulting from theinversionlayer createdon the

SiO2_Siinterface [1-4].

Forthe n-channel powerMOSFETsdevices, Whenapositivebias is

appliedtothegatewithrespecttothesource(+ VGs),anelectric field

appears, createdacross the gateoxide region and into the Si surface

region immediately below the gate region (Fig. 1). If the gate bias is

sufficiently large and positive

(for

the n-channel operation), the

*Tel.: ₊31 15 278 3776,Fax:

+

31 15 278 6442, e-mail: salame@ieee.org 175

+Vs

(+) Drain direct diodeCurrent

FIGURE Cross-section forn-channelPowerMOSFETstructure.

majority carriers

(holes

in the p-body) are depleted in this surface

region, and the minority carriers

(electrons)

areattractedtothisregion

(for

Vs

_>

Vth) [5-8]. Thus, when a potential is applied between

the drain and the source contacts

(n+-doped

regions in Fig.

1),

the inversion layer provides a low resistance current channel easing the

electrons flow from thesourcetothedrain. Thedevice isthensaidto

beturned on, and the controlgate biaspotentialatwhich thechannel

begins to conductappreciable current, iscalled the threshold voltage

(Vth) ofthe device[9-12].

Reducing the gatevoltagetobelow

Vth

willcausethe MOSFETto

turn OFF. Then ifthedrain-source bias change to a negative values

(-VDS),the body-drain junction become forward biased (Thesource ispositivelybiased withrespecttothedrain)andadirect currentflows

through the source cell across the forward p-n junction

(see

Fig. 1)

results in minority carriers injection into the substrate.

At

low gate

voltage,atransistorreverse current can flowalong the smallinversion

layerandthe resultingcurrentis,generally, thesum ofthedirect diode current and of the reverse transistor current

[13]. As

the channel switchesOFF, for anull gate voltage,the resultingcurrent consists in the diode currentonly.

NUMERICAL ANALYSIS MODEL FOR JUNCTION

CHARAERISTIC

Current (1)

and voltage

(V)

values of two hundred points of

ex-perimenta

I-V

Junction current, are computer-driven via a data

acquisition board and stored for modelling analysis. The description

ofthe

I-V

characteristics ofsiliconp-njunctions have beenthoroughly

developed

[14,

15].The relatedimplicit

Eq. (1)

introduces the classical

parameters,series

(R),

shunt

(Rh)

resistances, theideality factor

(A),

the reverse diffusion current

(Ion)

and the reverse recombination

rrent

(I02).

(1)

The electronic diffusion-recombination phenomena in the

quasi-neutral region of the junction

(reverse

current

10)

is separated from

the surface and space-charge region recombination phenomena

(reverse

current

Io2).

This practice is recognized through its

imple-mentation in the Spice model ofthediode. The equivalent electrical

circuit for the junctionisrepresentedin Figure2, twodiodesD1 and

D2 in parallel taking into account the diffusion and recombination mechanisms to which are added series

(R)

and shunt resistances

(Rh).

A

specially conceived software [16],

PARADI,

extracts the

FIGURE2 Equivalentelectrical circuit for ajunctionstudiedbya model with two exponential.

values of I0, I02,

A,

Rs

and

Rsh

from the experimental I-V diode

measurements.

From the point of view ofthe diode direct current measurement,

any tiny subthreshold channel current would appear as a current

flowing throughthe shuntresistance(Eq. 1).

3. RESULTS ANDDISCUSSION

The electrical characteristics ID$

f(VDs)

for the transistor and the

Body-Drain junction of the n-channel VDMOSFET (IRF 130,

International Rectifier) were determined using the setup represented

in Figure 3. The Body-Drain junction current

(I-V)

measured for

different gate voltage

(Vos)

values is represented in Figure 4.

Experimental values of

Rrs(oN)

for the different

_Vos

are obtained

from the transistor current expression

Ios=

f(VDs) which have an

expressioninthelinearregiongivenby:

W

los I-teff

Cox

"

Vas

Vth

Vos

(2)

The conductance havean expression given by:

Otos

w

(Vs-

vh)

(3)

andthetransistorresistance

(RDs(ON))

can be written by: 1

R

(4)

g

u,fcox

-

(VGs

Vth)

in this expression _laeerindicates the carriers effectivemobility, Cox the gateoxidecapacity,Wthechannel width,Zthechannellength and

Vth

thethresholdvoltage.

The parameter

_Vth

in Eq. (5) must be determined to separate the

junction direct current studied by numerical analysis from the transistor reverse current.

_Vth

was obtained in the saturation region where theVDMOSFETcurrent isgiven [17] by:

KEITHLY2420

Drain

Gate Source

IEEE488

KEITHLY220 ProgrammableSourceVoltage

FIGURE3 Experimentalset-upfor electrical characteristics measurements.

Vth

can be deduced from the linearly extrapolated value at the

_Vs

axis,

Vth

haveavalues about3Vfor the studied devices(IRF 130).

By plotting the transistor current Ir)s versus

Vrs,

the transistor

conductance values (gd), for different

Vs,

can be experimentally

deduced from the linearly extrapolated value at the Vr)s axis

(Fig. 5). Then

_RDS(ON)

have the reverse value ofgd obtained from

0,10 o,o

%,5v

0,06

Vs--2’8

s=l’

,

,1 0,0 0,1 O O 0,4 O,S 0,6 0,7

FIGURE4 Body-Drainjunctioncurrent(I-V)measuredfor differentgatebias(Vs).

0,070 0,065 0,060 0,055 0,050 0,045

-:

0,040 0,035 0,030 0,025 0,020 0,015 0,010 0,005 0,000 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9

V s(V)

FIGURE 5 Transistor electricalIDs f(Vrs)usedfor_Rts(oN)determination.

RDS(ON)

experimentally obtained from above and

Rsh

(shunt

resistance) numerically extracted from the junction characteristic

40 35 30 25

’

2o 10 ---.--Rh(shu,0-Numerialanalysis 2,8 2,9 3’0

_VG3’I

soy) 3,2 3,3

FIGURE6 Comparison between the transistor resistance experimentally measured

Rrs(oN)andtheshunt resistance

_R0h

numericalextracted.

I-V electricalmeasurement

0o00 I-Vnumericalanalysis

11I/

qO ql Q2 Q3 Q4 q5 q6

V.s(V)

FIGURE7 Comparison between the junction electrical characteristic and it is descriptionbythe modelwith twoexponential.

tothe threshold voltage (Vth) to study the surfacepotential influence

onthe channel conductivity. Foragatevoltage values towards0Vthe

voltage become comparableto

_Vth

the

_RDS(ON)

toward0 f. Thisresult

(Fig. 6) shows the good correlation between the electrical

measure-mentand numerical analysis. Thereforeitpossibletoidentify the

_Rsh

parameter, used in the two exponential model, with the transistor

resistance.

Figure 7 givesacomparisonbetweentheexperimental characteristic (junction current I-V) and its description using the extracted

parameters from Eq. (7). The good agreement between these two

curves shows the validity of the model.

4. CONCLUSION

A

numerical analysis model forthe junctionBody-Drain characteristic

allows to extract the MOSFET structure resistance in the operation

mode

_(RDs(o).

Shunt resistance (Rsh) in the model with two

exponential is identified as the transistor resistance

(RDs(ON))

and

extracted values are in good agreement the experimentally values measured.

Thismethodcan bevery useful tostudy the degradation properties

ofMOSFETs structure after irradiation or electrical stress, specially

when this degradationarerelatedto the surfacepotential.

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