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Experimental and comparative study of gamma radiation effects on Si-IGBTand SiC-JFET
B. Tala-Ighil a,⁎, J.-L. Trolet b, H. Gualous a, P. Mary b, S. Lefebvre c
a Laboratoire Universitaire des Sciences Appliquées de Cherbourg, ESIX Normandie, Université de Caen/Basse-Normandie, Rue Louis Aragon, BP 78, 50130 Cherbourg-Octeville, Franceb Ecole des Applications Militaires de l'Energie Atomique, Boulevard de la Bretonnière, BP 19, 50115 Cherbourg Armées, Francec SATIE, 61 Avenue du Président Wilson, 94235 Cachan Cedex, France
Please cite this article as: B. Tala-Ighil, et al., Etronics Reliability (2015), http://dx.doi.org/1
a b s t r a c t
a r t i c l e i n f o
Article history:Received 25 May 2015Received in revised form 27 June 2015Accepted 30 June 2015Available online xxxx
Keywords:Si-IGBTSiC-JFETGamma irradiation
This paper deals with an experimental study and a comparative study of the effects of total ionising dose of 60Cogamma radiation on Si-IGBT and SiC-JFET. The response of the threshold voltage and the turn-on switching pa-rameters are reported for both devices. Charge trapping in the gate oxide causes the decrease of the thresholdvoltage for Si-IGBT. The decrease of this parameter combined with the behaviour of Miller plateau during irradi-ation results in a decrease of the collector current rise-time, the collector-emitter voltage fall-time, and the turn-on switching energy and in an increase of the peak of the turn-on switching power and of the turn-on overshootcollector current. No changes in these parameters are observed for SiC-JFETs up to 2900 Gy with a dose rate of2.80 Gy/h. This indicates that those SiC-JFETs have extremely high radiation resistance with respect to the TIDeffects compared to the Si-IGBTs.
Because of their low cost, ready availability, high performances, theuse of COTS (Commercial-Off-The-Shelf) devices wherever and when-ever possible has evolved to become the main method of procurementin powering systems in nearly all military, aerospace and nuclearpower applications. The devices that make up the power bus such asdistribution units, circuit breakers, and power converters are COTSdevices.
However, the use of COTS devices raises a series of questionsconcerning their reliability in a radiation environment. Unfortunately,most often there is no alternative to the use of these commercialdevices, and system designers have to manage the risk associated totheir use. The most successful and higher performing systems will bethose that manage this risk the most successfully. The management ofthis risk requires a thorough understanding of the reliability and thefailure mechanisms associated with the selected device.
Until the first decade of the 2000s, in the field of power electronics,the IGBT (Insulated Gate Bipolar Transistor) was one of themost seriouscandidates for an evaluation with the aim of a use in radiation environ-ments. So, there has been an increasing interest in evaluating this devicein such environments.
In recent years, SiC (Silicon Carbide) controllable switching devicesbegin to be commercially available and are regarded as promisingcandidates for use in harsh radiation environments.
la-Ighil).
xperimental and comparative0.1016/j.microrel.2015.06.13
To make good use of these SiC-based devices and substitute forpresent Si systems properly, it is necessary to learn how they areimpacted by different types of radiation.
Three major radiation effects on semiconductor devices are known:SEEs (Single Event Effects), TID (total ionising dose) effect and DDD(displacement damage dose) effects.
The objective of this paper is to present an experimental and acomparative study of the effects of total ionising dose of gammaradiation on Si-IGBTs and SiC-JFETs. A particular interest is taken inthe switching parameters and, in the context of this paper we havechosen to focus specifically on the turn-on switching parameters.
2. Sample information and test setup
Commercially available punch-through Si-IGBTs with planar gatestructure and SiC-JFETs engineering samples were used in this study.Ratings information for the tested devices are shown in Table 1.
The gamma irradiation was performed using a 60Co (Cobalt-60)source, model CO60SGF01 manufactured by CERCA (French acronymfor Company for the Study and Creation of Atomic Fuels). This sourceis localised at “Ecole des Applications Militaires de l'Energie Atomique”in Cherbourg, France. It is a cylinder of radioactive 60Co enclosed in adouble-walled stainless steel capsule. It has an active diameter of3 mm. Its initial activity was 5 mCi. The device being irradiated isfixed at a distance of 16 mm from the source. At this distance, a doserate of 2.80 Gy/h was measured at the beginning of the irradiation. Be-cause of the long duration of the experiments, the known decay rateof the 60Co source was taken into account for TID calculations [1]. The
study of gamma radiation effects on Si-IGBT and SiC-JFET, Microelec-6
Table 1Ratings information for the tested devices.
Type Rating Manufacturer
Si-IGBT 600 V/16 A@25 °C IRSiC-JFET 600 V/10 A@25 °C SiCED
0 500 1000 1500 2000 2500 3000-3,5
-3,0
-2,5
-2,0
-1,5
-1,0
-0,5
0,0
Thr
esho
ld v
olta
ge s
hift
V
th (
V)
Total Ionising Dose (Gy)
SiC JFET (Vth0=-16.95V)
Si IGBT (Vth0=4.69V)
Rate dose : 2,8 Gy/hIn situ gate bias : V
GE=0
Fig. 2. Shift of the threshold voltage for Si-IGBT and SiC-JFET as a function of total ionisingdose.
2 B. Tala-Ighil et al. / Microelectronics Reliability xxx (2015) xxx–xxx
samples were electrically tested and verified as functional beforeshipping them to the irradiation site. Parts were irradiated under 0 Vin situ gate bias while the collector and the emitter terminals weregrounded. Regularly, the devices were removed from the irradiationsite to be immediately and completely characterised. The characterisa-tion tests included measurement of all key static and turn-on switchingparameters.
3. Switching test circuit and instrumentation
Tomeasure the various switching parameters, a series chopper withan inductive load, as encountered in most applications, was used. Aftereach step of irradiation, the transistors were removed from the irradia-tion site and introduced into the test circuit shown in Fig. 1. The deviceunder test (DUT) was paralleled with an identical and not irradiateddevice called “Pilot Device”.
A driver circuit delivers a series of adjustable frequency and dutycycle pulses. One of 1024 pulses is inhibited on the gate of the pilotdevice and switched to the gate of the DUT. While the pilot device isswitched at a frequency of 2 kHz, the DUT is only approximatelyswitched at 2 Hz: it is turned on during only 200 μs periodically every500 ms. Thus, we avoid the heating of the device under test and it canbe assumed that the junction of the DUT remains at room temperatureduring measurements.
The use of such a pair transistor system permits: (1) to have a wellsmoothed load current. This is ensured by the high switching frequencyof the “Pilot Device” and (2) to keep the DUT at room temperature bythe fact that this later switches at a very low frequency.
In order to correctly acquire the switching parameters, no snubbercircuit was used. The various switching times, and the switching losseswere measured using a Tektronix oscilloscope model TDS754D in asso-ciation with Tektronix voltage probes model Tek P6139A and a currentprobe model TCP202.
DUTGateDrive
Circuit
GateDrive
Circuit
RG RG
Pilote
Load DF
E
DUT drivepulses
Pilot Devicedrive pulses
Dri
ve
vo
ltag
e v
D
Gat
e volt
age
vG
Coll
ecto
r cu
rren
t i C
Collector-
voltage vCE
to-Emitter
Device
Fig. 1. Switching test circuit.
Please cite this article as: B. Tala-Ighil, et al., Experimental and comparativtronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.13
4. Experimental results and discussion
4.1. Effect of the total ionising dose upon the threshold voltage
There are several methods to extract the gate threshold voltage VTh
[2]. In the IGBTs manufacturer manual [3], VTh is defined as the valueof the gate-emitter voltage VGE corresponding to collector currentIC = 250 μA with the collector-emitter voltage equal to the gate-emitter voltage (VCE=VGE). However, once the threshold voltage shiftsbelow zero, this definition cannot be used. So, for the extraction of thisparameter, we used thewell knownELR (Extrapolation in the Linear Re-gion)method [2] for the Si-IGBT as well as for the SiC-JFET. This methodconsists of finding the gate-emitter voltage axis intercept (i.e. IC = 0) ofthe linear extrapolation of the transfer characteristic IC(VGE) at itsmaximum first derivative point for a low value of VCE (VCE = 0.9 V inour case). All measurements were performed at a constant temperatureof 30 °C.
Fig. 2 shows the shift ΔVTh = VTh,ir − VTh,0 of this parameter. VTh,0
and VTh,ir are respectively the pre-radiation value of VTh and its valuefor each total ionising dose. The values of VTh,0 are indicated in brackets.As it can be seen, the threshold voltage decreases with increasing totalionising dose for Si-IGBT and remains unchanged for SiC-JFET.
As for MOSFETs, this negative shift of VTh for Si-IGBT is attributed toincreasing positive oxide-trapped charges [4–7]. Contrary to what is re-ported in certain publications, one can notice that this shift does notevolve linearly according to the total ionising dose [8] and doesnot saturate to 0.5 V [9] For higher TID, VTh can shift below zero for
-1 0 1 2 3 4 5-2
0
2
4
6
8
10
12
14Rate dose : 2,8 Gy/hIn situ gate bias : V
GE=0
vegatlov
re tt ime-eta g
no -n ruT
GE
-on(
t) (
V)
Time (µs)
Pre-RadiationIrradited at 2894
Increase ofthe rate-of-rise dVGE/dt
Decrease of the level and thewidth of « Miller plateau»
Fig. 3. Effects of the total ionising dose upon the turn-on gate emitter voltage for Si-IGBT.
e study of gamma radiation effects on Si-IGBT and SiC-JFET, Microelec-6
Fig. 4. Effects of the total ionising dose upon the turn-on gate emitter voltage for SiC-JFET.
0 500 1000 1500 2000 2500 3000-700
-600
-500
-400
-300
-200
-100
0
100
Si IGBT (tMil-on0=1520ns)
SiC JFET (tMil-on0=804ns)
ttf ihshtdi
wuae talp
relliM
no-nr uT
Mil
-on
(ns)
Total Ionising Dose (Gy)
Fig. 6. Shift of the Miller plateau width for Si-IGBT and SiC-JFET as a function of totalionising dose.
3B. Tala-Ighil et al. / Microelectronics Reliability xxx (2015) xxx–xxx
Si-IGBT [4,5]. A threshold voltage which shifts below zeromeans thatthe device, which is a normally off device, changes to a normally ondevice and will require a negative gate voltage to be turned-off.
4.2. Effects of the total ionising dose upon turn-on gate-emitter voltageVGE-on (t)
Figs. 3 and 4 show the turn-on waveforms of the gate-emittervoltage VGE-on (t) for Si-IGBT and SiC-JFET before irradiation and afterirradiation at 2894 Gy and 2827 Gy respectively. The observations thatcan be made from these figures are the decrease of the level and thewidth of the “Miller plateau” and the increase of rate-of-rise dVGE (t)/dtfor Si-IGBT. These parameters are not affected in the case of SiC-JFET.
Figs. 5 and 6 show the effects of the total ionising dose on the shifts ofthe turn-on Miller plateau level ΔVMil − on = VMil − on(ir) − VMil − on(0)
and its width ΔtMil − on = tMil − on(ir) − tMil − on(0) for Si-IGBT andSiC-JFET. VMil-on(0) and tMil-on(0) are the pre-radiation values of respec-tively the level and the width of the Miller plateau, while VMil-on(ir)
and tMil-on(ir) denote the values of these two parameters for each totalionising radiation dose. The values of tMil-on(0) and VMil-on(0) are givenin brackets.
It is well known that the behaviour of the turn-on gate-emitter volt-age VGE-on (t) (such as the turn-off voltage VGE-off (t)) is strongly depen-dent on the gate-emitter capacitance CGE and the gate-collectorcapacitance CGD [10–12]. One can reasonably assume that the changesin the “Miller plateau” could be related to a possible change in thesecapacitances due to the radiation induced charge in the gate oxide.
0 500 1000 1500 2000 2500 3000-2,5
-2,0
-1,5
-1,0
-0,5
0,0
Si IGBT (VMil-on0=8,15V)
SiC JFET (VMil-on0=-9,2V)tf ih sle veluae ta lp
r ell iM
no-n ruT
VM
il-o
n (V
)
Total Ionising Dose (Gy)
Fig. 5. Shift of theMiller plateau level for Si-IGBT and SiC-JFET as a function of total ionisingdose.
Please cite this article as: B. Tala-Ighil, et al., Experimental and comparativetronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.13
This behaviour of VGE-on (t) combined with the behaviour of thethreshold voltage VTh can explain the evolution of the switching param-eters that will be presented below.
4.3. Effects of the total ionising dose upon turn-on switching times
The switching times were obtained as usual: the turn-on delay timetdon is the time it takes for the collector current to reach 10% of nominalcurrent value starting from the time a drive signal is applied to the gate.The current rise-time trc is the time requested to reach 90% of the collec-tor nominal current starting from 10% of its value. The voltage fall-timetfv is the time requested to reach 10% of the collector-emitter nominalvoltage from 90% of its value.
In Figs. 7 and 8 are illustrated the turn-onwaveforms of the collectorcurrent iC-on (t) and the collector-emitter voltage VCE-on (t) for Si-IGBTand SiC-JFET before irradiation and after irradiation at respectively2894 Gy and 2827 Gy. One can notice on these curves a shift of thewaveforms to the left and an increase of the turn-on overshoot collectorcurrent during the irradiation for Si-IGBT. No changes are observed forSiC-JFET.
Figs. 9–11 show the shift of the turn-on delay time Δtdon =tdon,ir − tdon,0, the shift of the collector current rise-time Δtrc =trc,ir − trc,0 and the shift of the collector-emitter voltage fall-timeΔtfv = tfv,ir − tfv,0, where tdon,0, trc,0 and tfv,0 are respectively thepre-radiation values of tdon, trc and tfv while tdon,ir, trc,ir tfv,ir denote the
-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0-2
0
2
4
6
8
10
12
14
16Rate dose : 2,8 Gy/hIn situ gate bias : VGE=0
Tur
n-on
col
lect
or c
urre
nt i
C-o
n(t)
(A
)
Time (µs)
Pre-RadiationIrradiated at 2894 Gy
0
25
50
75
100
125
150
Tur
n-on
col
lect
or v
olta
ge v
CE
-on(
t) (
V)
vCE-on(t)Shift to the left
Increase of the turn-onovershoot current
iC-on(t)Shift to the left
Fig. 7. Effects of the total ionising dose upon the turn-on waveforms for Si-IGBT.
study of gamma radiation effects on Si-IGBT and SiC-JFET, Microelec-6
Fig. 8. Effects of the total ionising dose upon the turn-on waveforms for SiC-JEFET.
0 500 1000 1500 2000 2500 3000-250
-200
-150
-100
-50
0
Si IGBT (td-on0=738ns)
SiC JFET (td-on0=308ns)
Rate dose : 2,8 Gy/hIn situ gate bias : V
GE=0
Tur
n-on
del
ay ti
me
Shi
ft
t don (
ns)
Total Ionising Dose (Gy)
Fig. 9. Shift of the turn-on delay time for Si-IGBT and SiC-JFET as a function of total ionisingdose.
0 500 1000 1500 2000 2500 3000
-100
-80
-60
-40
-20
0
Si IGBT (tf-v0=360ns)
SiC JFET (tf-v0=578ns)
Rate dose : 2,8 Gy/hIn situ gate bias : V
GE=0
tfihse
mitllafegatlov
rettime-rocello
Ct fv
(ns
)
Total Ionising Dose (Gy)
Fig. 11. Shift of the collector-emitter voltage fall time for Si-IGBT and SiC-JFET as a functionof total ionising dose.
4 B. Tala-Ighil et al. / Microelectronics Reliability xxx (2015) xxx–xxx
values of these parameters for each total ionising dose. The pre-radiation values of these parameters are given in brackets.
For Si-IGBT, the decrease of the turn-on delay time is attributed tothe decrease of the threshold voltage whereas the decreases of thecollector current rise-time and the collector-emitter voltage fall-time
0 500 1000 1500 2000 2500 3000-80
-70
-60
-50
-40
-30
-20
-10
0
10
Si IGBT (tr-c0=345ns)
SiC JFET (tr-c0=249ns)
Rate dose : 2,8 Gy/hIn situ gate bias : V
GE=0
Col
lect
or c
urre
nt r
ise
tim
e sh
ift
t rc (
ns)
Total Ionising Dose (Gy)
Fig. 10. Shift of the collector current rise time for Si-IGBT and SiC-JFET as a function of totalionising dose.
Please cite this article as: B. Tala-Ighil, et al., Experimental and comparativtronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.13
are attributed to the decreases of the threshold voltage and the levelof Miller plateau. No changes are observed for SiC-JFET.
4.4. Effects of the total ionising dose upon turn-on switching losses
In Figs. 12 and 13 are represented the pre-radiation waveforms ofthe turn-on switching power pon (t) for Si-IGBT and SiC-JFET before irra-diation and after irradiation at 2894 Gy and 2827 Gy respectively. Theturn-on switching power is the product of the turn-on collector currentVC-on (t) and the turn-on collector-emitter voltage VCE-on (t). We ob-serve an increase of the peak of turn-on switching power for Si-IGBTwhile it remains unchanged in the case of SiC-JFET.
Fig. 14 shows the observed changes in turn-on switching energy forthe two devices. It represents the shift ΔWon = Won,ir − Won,0 whereWon,0 and Won,ir are respectively the pre-radiation value of won and itsvalue for each total ionising dose. The values of Won,0 are indicated inbrackets.
Turn-on switching energy is the integral of the product of collectorcurrent and collector-emitter voltage over the interval from when thecollector current rises past 5% of the test current to when the voltagefalls below 5% of the test voltage. This definition is in accordance withJEDEC standard 24–1 for measuring turn-on energy [13].
The decrease of Won in the case of Si-IGBT can be explained by thedecrease of the collector current rise-time and of the collector-emittervoltage fall time.
-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,00
200
400
600
800
1000
1200
1400
1600 Rate dose : 2,8 Gy/hIn situ gate bias : V
GE=0
Tur
n-on
sw
itch
ing
pow
er P
on(t
) (W
)
Time (µs)
Pre-RadiationIrradiated at 2894 Gy
Increase of the turn-onswitching power peak
Shift to the left
Fig. 12. Effects of the total ionising dose upon the turn-on switching power for Si-IGBT.
e study of gamma radiation effects on Si-IGBT and SiC-JFET, Microelec-6
Fig. 13. Effects of the total ionising dose upon the turn-on switching power for SiC-JEFET.
0 500 1000 1500 2000 2500 3000
-120
-100
-80
-60
-40
-20
0
Si IGBT (Won0=627µJ)
SiC JFET (Won0=274µJ)
Rate dose : 2,8 Gy/hIn situ gate bias : V
GE=0
Tur
n-on
sw
itch
ing
ener
gy s
hift
Won
(µ
J)
Total Ionising Dose (Gy)
Fig. 14. Shift of the turn-on switching energy for Si-IGBT and SiC-JFET as a function of totalionising dose.
5B. Tala-Ighil et al. / Microelectronics Reliability xxx (2015) xxx–xxx
5. Conclusion
In this paper, the effects of radiation on the threshold voltage and onthe turn-on electrical characteristics of Si-IGBT and SiC-JFET devices aredescribed with respect to the TID effects. The radiation resistance ofSiC-JFET was compared to Si-IGBTs. The main conclusions which can bemade from the present study are: For Si-IGBT, gamma irradiation causesdecrease of the threshold voltage and theMiller plateau level andwidth.
Please cite this article as: B. Tala-Ighil, et al., Experimental and comparativetronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.13
This leads to the decrease of the turn-on delay time, the current rise-time, the voltage fall-time, the turn on switching losses and the increaseof the peak of turn-on switching power and the turn-on overshoot col-lector current. This last consequence could be harmful for the device.The decrease of the threshold voltage reduces the drive capability ofthe device. To prolong its lifetime in radiation environments a ±15 Vgate drive should be preferred to 0/+15 V. For SiC-JFET, no changesare observed in the examined parameters up to 2900 Gy. SiC-JFETsshowed higher radiation tolerance than Si-IGBTs with respect to theTID effects and under a dose rate of 2.80 Gy/h, they could be operatedup to 2900 Gy at least. Finally, one can notice that irradiation has notbeen achieved in the worst conditions. The presented results are only afirst step of a work in progress on TID effects on SiC devices. Furtherwork needs to be carried out particularly the study of the case whereSiC-JFET is positive or negative gate biased.
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