IEEE Transactions on Nuclear Science, Vol. NS-29, No. 6, December 1982

RADIATION EFFECTS ON MOS POWER TRANSISTORS*

H. Volmerange and A. A. WittelesTRW, Inc.

One Space ParkRedondo Beach, California 90278

TOTAL-DOSE RESPONSE

The use of power MOS transistors in spacecraftand military equipment is limited by their vulner-ability to ionizing radiation (total-dose and dose-rate). The total-dose hardness of four types of powerMOS is examined in this paper. All devices experiencecomparable shifts of VGS(th) following total-doseexposure. Other functional parameters are not affectedby dose up to half a megarad. Thus, an effectivehardening technique consists of extending the range ofgate bias and gate drive to allow operation at highdose levels.

The results of dose-rate testing of three types ofpower MOS are presented and analyzed. Photocurrentburnouts are tentatively attributed to thermally 'inducedsecond breakdown of the drain-source diode. Decreasingthe carrier lifetime of the junction material is anti-cipated to decrease this vulnerability.

INTRODUCTI ON

The high level of interest in the application ofpower MOSFET devices, also known as VDMOS (VerticalDouble diffused MOS), in switching power supplies stemsfrom their efficient high frequency switching of up toseveral kilowatts of power. This intrinsic efficiencyat high frequencies is directly due to the MOS struc-ture operating as a majority carrier device withoutthe time delays associated with minority carrier chargestorage effects of comparable bipolar transistors.This intrinsic advantage of MOSFETs versus bipolars hasenabled airborne and spaceborne equipment designers togain major weight, volume, and power advantages. Theuse of power MOS in linear power amplifiert from audiofrequencies to 1OOMHz is also increasing, where theadvantages of low power drive requirements, safetyfrom second breakdown and improved li-nearity areexploited.

This paper presents the results of a study tocharacterize the radiation response of a representa-tive group of state-of-the-art power MOSFET transistors.Since MOS technology is generally vulnerable to rela-tively low levels of total ionizi'ng dose the projectconcentrated on supplementing previousl,2 total doseresponse data. In addition, three transistor typeswere exposed to a prompt dose rate environment in orderto characterize their transient photoresponse and toinvestigate their vul nerabi 1 ity to bipol ar-junction-induced second breakdown, a condition previouslyreported3. Neutron irradiation was not performed,since as majority-carrier devices, MOS transistors arenot susceptible to neutron induced degradation mechan-ism at moderate level (1012 n/cm2). A study of neutroneffects on MOS power transistors was conducted byD. L. Blackburn et al.4 at higher fluence levels.

Four types of power MOS transistors from fourmanufacturers were tested to total-dose maxima of 100Krad (one type) and 500 Krad (three types). Theoperating characteristics of these devices are listedin Table 1. Total-dose exposures were performed in

Table 1 - DESCRIPTION OF TEST DEVICES

Part Type HPWR6501 IRF9131 HTM1224 TA9192Manufacturer HPA INR MOTA RCADate Code 8036 8037 8133 noneChannel N P N N

B ADSSlon (Y) 450 -60 80 100

BVGS Mn (V) +40 +20 +20 +20

ID Max (A) 6 -8 12 16

IGSS max (nA) 100 -100 500 100

V8sth Min/eax (Y) 3/7 -2/4 1.5/4 1/4at 1 mAR0s0N max at 6A (o) .85 .30 .25 .80

Sample Size 5 5 5 5

TRW's Gammacell 220 Co-60 source with the devicesbiased at 28V VOD and switched ON/OFF at a 1OKHz rate(50%'duty cycle). The dose-rate was 50 rad/s.

All the devices had the cellular configurationexampl'ified by International Rectifier CorporationHEXFETs (described by Abare & Martindale3) and by theMotorola TMOS structure.

The mean total-dose responses of the four samplesare listed in Table 2.

Since data presented in Reference 2 indicate nosignificant change in VGS(th)Versus dose curve at highID levels, no data was taken at intermediate dose pointsfor high ID levels.

Total-dose responses was similar for all samplesand can be summarized as follows:

o The only significant effect of dose exposureon power MOS transistors is a shift of VGS{th)toward a more negative value (Figure 1). This

5

N-CHANNEL (TA9192) / I 1]EXPOSURE (krcO) 500 500

D(A)

-5* This effort was supported by TRW Independent

Research and Development project 80114742.-5

VGS (V)

Figure 1 . ID versus VGS Shift for IRF 9131 and TA 9192

001 8-9499/82/1200-1565$00.75©) 1982 IEEE

ABSTRACT

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Table 2 - PARAMETER VALUE VERSUS EXPOSURE FOR POWER MOSFETs

shift of threshold voltage is proportional todose at low dos-e levels and tends to saturateat high dose levels as shown in Figure 2.

The three N-channel device types tested shiftedfrom an enhancement to a depletion mode opera-tion (negative bias necessary to hold ID(OFF)at or under lOA) at mean dose levels between8 and 40 Krad(Si). When small-sample statis-ti:cal factors are taken into account, theoperation of these devices with a positive gatevoltage is limited to less than 2 Krad in thewrst-case (MTM 1224) for 90% of the lot at a90% confidence level.Operation of these devices at dose levels abovethe enhancement-depletion mode shift requiresa negative gate bias (to turn OFF the draincurrent)'and a positive gate drive (to turn ONthe drain current). Application of negativebias is limited to the gate-to-source breakdownvoltage (BVGS). The three N-channel typestested could be used at dose levels of morethan 500 Krad(Si) within the BVGS limitation.The P-channel devices become more enhanced withradiation exposure and accordingly, the IRF9131 devices exhibit more negative thresholdvoltages with increasing exposure. Thisincrease is moderate up to 100 Krad(Si), but in

EXPOSURE LEVEL

NUMBLER PARAMETER & TEST CONDITIONS 0 10 20 50 100 200 50 UNITSNUMBER PARAMTER &TEST ONDIT ONS krad krad krad krad krad krad krad

MTM VGSTH at IDl1OiA, VDS=28V 1.4 -.68 -.98 -4.0 -6.5 -6.5 -6.3 V1224 lmA 2.4 1.1 .13 -2.0 -4.1 -5.1 -5.0

.1 A 3.4 -3.7lA 4.2 -2.65 A 5.4 -1 .1

IGSS at VGS=-2OV -.1 -6 pA

RDSON at ID=5 A, VGS=lOV .14 .12 Ohm

TA VGSTH at ID=lOlA, VDS=28V 1.0 .71 -.14 -2.02 -4.1 -6.0 -6.6 V9192 lmA 2.4 1.3 .45 -1.4 -3.4 -5.3 -5.8

.1 A 3.1 -4.71A 3.9 -3.55 A 5.7 -1.5

IGSS at VGS= 20V .18 .64 pA

RDSON at ID=5 A, VGS=lOV .58 .56 Ohm

IRF VGSTH at I D=lO4A, V I=-28V -2.6 -3.1 -3.6 -4.8 -6.4 -8.8 -15 V9131 lmA -3.0 -3.5 -4.2 -5.2 -6.9 -9.4 -16

.1 A -3.5 -1 91 A -4.2 -205 A -5.5 -23

1GSS at VGS=-20V -16 -21 pA

HP5WR | VGSTH at ID=lmA, VDS=40V 4.8 2.8 1.1 -2.6 -10.5 V6501

oDSS at VGS=OV, VDS=40V .15 .15 23 >lOmA PA

IGSS at VGs=-20V -.70 -.87 -1.1 -.62 -.46 pA

RDSON at ID=20, VGS=20V .75 .75 .75 .75 .74 Ohm

VGS o('/)

DOSE (kred)

Figure 2. VGS Shift versus Dose for TA 9192

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higher dose environments, the usage of thisdevice is limited by the ability to deliver IDwithout exceeding the gate-to-source breakdownvoltage (BVGS). For example, the IRF 9131devices require a gate bias of 20 volts todeliver a drain current (ID) of 5 A followinga 400 Krad(Si) exposure (Figure 3). Small sam-

2 5 10 20 50 100 200

ID(A

CURVE 1: PHOTOCURRENT RESPONSETO 3ps PULSE VDS 40 V

CURVE 2: PHOTOCURRENT RESPONSE22 na PULSE VDS 24 V

VGS 0 RGS K

CURVE 3: PHOTOCURRENT RESPONSEPULSEWIDTH (STORAGEEFFECTS) VS. DOSE RATE

100

50 RESPONSEP.W. (ns)

5 io8 2 5 109 2 S5oIo2DOSE - RATE (rod/ s)

I

10 20 50 100 200 500 1 kPROMPT - DOSE (rod (Si))

Figure 4. Average Photocurrent Response for IRF 130Transi stors

500

DOSE (krod)

Figure 3. BVGS Limitation for the IRF 9131

ple statistics reduce the dose level to 300 Krad(Si) for a lot level acceptance of 90% at a 90%confidence.

* The saturation characteristics RDS(ON), de-creased with increasing dose. This decreaseis, however, small: less than 15% following a500 Krad(Si) exposure. Dispersion is charac-terized by the fractional standard deviationwhich remains small at a mean value of 6% for9 measurements.

e The gate leakage current (IGSS) increases withdose but remains negligible (less than 50 pAincluding small -sample stati stical adjustmentfor 90% of the lot at 90% confidence level).

e The transconductance (gfs) remains constant asshown by the constant slope of ID versus VGSat increasing exposure levels (Figure 1).The dependence of dose induced parametric degra-dation on bias condition during irradiation wasnot investigated. Previous2 work indicatedlittle or no dependence of degradation on biasfor IRF 150 samples which were biased continu-ously when compared to samples which operatedat 50% duty cycle.

DOSE-RATE/PROMPT DOSE RESPONSE

Two types of power MOS transistors (IRF 130,2N6661) were exposed to a prompt dose rate environmentin TRW's Febetron 705 FXR (Flash X-Ray) facility andthree IRF 150s were exposed to the TRW Vulcan FXRfacility in an investigation of the photocurrent vul-nerability of power MOS transistors.

Photocurrent versus dose-rate and prompt dose forthe IRF 130 is presented at two radiation pulsewidths(curves 1 and 2) in Figure 4. Curve 3 shows the rela-tionship between the photocurrent pulsewidth and theprompt dose. The radiation storage time is about 80nanoseconds at 1 Krad(Si) (22 nanosecond FXR pulse).This storage time is due, in part, to the presence ofa current sampling resistance of 0.3 ohm in the photo-current path. This storage time is consistent withthe prompt dose response of a diode characterized byreverse recovery time of 500 ns and is traceable tothe drain-source diode (also known as body diode) ofpower MOS transistors. The majority of the photocurrentis generated in the depletion layer of this diode sinceits area is significantly larger than the channel areaof conduction as shown in Figure 5.

Figure 5. Photocurrent Paths in Power MOS Transistors

The peak photocurrent for the 2N6661 transistorwas substantially lower (3.8A) than the IRF 130. Thisis consistent with the smaller geometry of the 2N6661which has a maximum dc current rating of 2A (versus12A for the IRF 130). The shape of the photocurrentcurve was very similar to Figure 4. None of the IRF130 or 2N6661 transistors was damaged during the FXRtesting.

Photocurrent burnout resulting in drain sourceshort-circuit was observed to occur during the FXRtesting of three IRF 150 HEXFET transistors. Thefailure prompt doses ranged from 1 Krad to 5.2 Krad(45 ns pulsewidth). The peak failure photocurrentsranged from 80 to 180A. Comparable photocurrent burn-outs have been reported in IRF 351 HEXFET transistorsby Abare & MartindaleO at prompt doses of only 250rad(Si). In both cases the burnout is most likelydue to a second breakdown (S/B) of the drain-sourcediode triggered by the photocurrent pulse (S/B is a

frequent failure mode for overstressed reverse-biaseddiodes). The onset of S/B current was delayed 0.2,safter the photocurrent peak (Figure 6) in the Abare &

S/B CURRENT(IRF 251)

PHOTO-;/DRAIN CURRENT S/B CURRENTCURRENT (IRFCU1 R0)

(RELATIVE X/X I IRF150/

S/B (IRF 351)

0 1 2 3 4TIME (*s)

Figure 6. Photocurrent Induced Burnout in HEXFET MOSTransi stors

Martindale experiment and more than lhis in the TRWexperiments. This type of photocurrent failure hasbeen observed by the authors in several bipolar tran-

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I 4I

:

-7-~

.5A 1ai .

- -- -BV

_ ~~~~~~~~~~I I

-10

VGS

-20

-30

0 --

L-

sistor experiments. The susceptibility of HEXFETstructure power MOS transistors thus appears to bedue to photocurrent-induced S/B failure originatingin the drain-source junction diode. The failure levelis well below the pulse Safe Operating Area (SOA)levels of the device channels in the Abare & Martindaleexperiment and comparable to the pulsed SOA of thedevice channels in the TRW experiments. By loweringthe carrier lifetime, reduction in photocurrentresponse (amplitude and storage time) could be expected,thereby lowering the device sensitivity to photocurrentinduced failure. Investigation of the photocurrentsusceptibility in non-HEXFET structures is planned.

CONCLUSI ON

Current state-of-the-art MOS power transistorscan be used in most applications with moderate radia-tion environment. For applications in more severeenvironments, MOS power transistors are not applicableabove a few Krad(Si) without modification to the gatecircuitry. With a negative gate bias, the operationof N-channel transistors can be extended to exposuresof more than 500 Krad(Si). P-channel devices requirea higher gate drive after dose exposure. Theirdrain current capability may be limited followingexposures to 250 Krad(Si).

Burnout of HEXFET transistors in FXR testing isattributed to photocurrent-induced second-breakdownof the source-drain diode. The failure occurred atlow-to-medium levels of pulsed power dissipation.Decreasing the carrier lifetime in the junctionmaterial is expected to decrease the photocurrentsusceptibility.

REFERENCES

1. W. E. Baker, Jr., "The Effects of Radiation onthe Characteristics of Power MOSFETs," Proc.POWERCON 7, Pg. D3-1, 1980.

2. International Rectifier, "Radiation Testing ofHEXFETs, Preliminary Report," March 1981.

3. W. E. Abare and W. K. Martindale, "Dose RateTolerant HEXFET Power Supply," IEEE TransactionNuclear Science Volume NS-28, Pg. 4380, December1981.

4. D. L. Blackburn, T. C. Robbins, and K. F. Galloway,"VDMOS Power Transistor On-Resistance RadiationDependence," IEEE Transaction Nuclear ScienceVolume NS-28, Pg. 4354, December 1981.

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