1422 IEEE Transactions on Nuclear Science, Vol. 35, No. 6, December 1988

COMPUTER SIMULATION OF IONIZING RADIATION f BURNOUT IN POWER MOSFETs

A. A. KESHAVARZ * , T. A. FISCHER * f; W. R. DAWES, JR.**, C. F. HAWKINS *

* Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, NM 87131. (505)277-2600.

** Sandia National Laboratories, Albuquerque, Nh4 87185. (505)844-8966

ABSTRACT

The first computer simulation of the +induced burnout in power MOSFETs is reported here. The modeling results support the current-induced avalanche burnout mechanism at the interface of the epitaxial layer and the substrate cou- pled with parasitic bipolar transistor action leading to secon- dary breakdown and thermal runaway. The simulations allow an evaluation of the effects of semiconductor parame ters, device geometry, doping profiles and bias voltage on the burnout dose rate threshold. The model provides a method for optimizing the radiation hardness of power MOSFETs.

INTRODUCTION

Power MOSFETs must operate in high radiation environments both in space and military applications, and in civilian situations such as nuclear power plants [2]. n-channel power MOSFETs are preferable to p-channel devices because of their superior electrical characteristics, however, these n- MOSFETs are much more susceptible to radiation-induced burnout under transient ionizing-radiation environments. The response of the internal parasitic bipolar transistor to radiation determines the power MOSFET response. Wrobel, et. al., experimentally determined the ionizing radiation (+) response and burnout of npn transistors [3], and the burnout mechanism was attributed to the high current injection that results in avalanche breakdown fields developed across the epi/n+ substrate. In 1985, a correlation between the photoionization-induced avalanche breakdown of the epitax- ial structures and high-current effects in these devices was presented [4,5]. Avalanche breakdowns were found even at low bias levels. The physical mechanisms responsible for burnout and the fabrication techniques that improve the sur- vivability of n-channel power MOSFETs were presented by Dawes et. al. [6]. Devices built on bulk silicon were more radiation-resistant than those built using an epitaxial struc- ture, which was a practical result supporting the current- induced-avalanche model. New circuit design techniques per- mitting the use of commercial power MOSFETs in severe ionizing radiation environments and new fabrication ?This work was performed at Sandia National Laboratories, which is operated for the U.S. Department of Energy under contract number D E AC04-76DP00789.

techniques were presented by Fischer et. al. [7]. The burnout and current-induced avalanche (CIA) mechanism are thoroughly discussed in references [4-71.

THE COMPUTER APPROACH TO RADIATION ANALYSIS

BAMBI is a semiconductor device simulator that can be used to analyze the electrical properties of a power MOSFET structure [I]. It solves the five major differential equations of solid-state devices in two dimensions to give steady-state or transient responses of the device. These are Poisson's equation, the electron and hole continuity equations, and current relations for both types of charge carriers. Subrou- tines supplied by the user specify the physical models for the program. BAMBI includes an avalanche multiplication mode1 that was of particular interest to this study because of its relevance to the burnout phenomenon [1,8]. Ionizing radi- ation was modeled by a constant generation term added to the calculated generation-recombination rate and was assumed to be 4.3~10'~ eh/cm3rad(Si) . The generic device structure shown in Figure 1 was simulated by BAMBI to find the steady-state solution for the power MOSFET device under no radiation using a drain-source voltage of 80 V.

S All dimensions in vm v A

y=o

x=o x 2 8

1) 1

A

1

Figure 1. n-channel power MOSFET structure.

0018-9499/88/1200-1422sO1.00 0 1988 IEEE

1423

- Doping Profile

1020

n++

0.0 1.0 6.0 9.0 12.0 15.0

y I =m x 1 ~ - 4

Figure 2. Doping profile at section AA.

The doping profile (Figure 2) and the device structure are somewhat generic and are very similar to the actual devices tested in reference 6. The breakdown voltage of the device under no radiation was approximately 280 V. The distribu- tion of the electric-field magnitude at section AA is plotted in Figure 3(a). The geometric base widths shown in Figures 1 and 2 were modified under applied bias voltage and the actual base width was taken as the region of zero electric field in the base region (Figure 3(a)).

Ionizing radiation was then applied and its rate was increased in short time steps, reaching its peak value in 47 ns. The overall shape of the radiation pulse was representa- tive of the pulse shape of various flash x-ray facilities. Tran- sient analysis of the device current was done for the specific applied radiation rate at each time step and each time step was long enough so that the response could be considered the steady-state response for that radiation rate. A gradual increase in the radiation rate was necessary for convergence of the program.

RESULTS

Figures 3(a-f) show the distribution of the electric-field magnitude at section AA for different radiation rates. Fig- ure 3(b) shows the electric-field distribution for a radiation rate of 8.7~10" rad(Si)/s that induced a current density of 6.32~10' A /em2. Electron-hole pairs are uniformly generated throughout the device under ionizing radiation. Holes gen- erated within the base-collector depletion region surge to the base area and flow laterally across the base exiting the device at the base-emitter common contact. A voltage gradient forms across the lateral base resistance that effectively for- ward biases the base-emitter junction. As the radiation rate is increased, the base-emitter junction becomes more heavily forward biased and the bipolar action of the device is more

pronounced. Figure 3(c) shows the electric field distribution for a

radiation rate of 1.5~10" rad(Si)/s, where the base-emitter junction is heavily forward biased and the collector current density increases to 2.29~10' A/em2. The total space charge in the epi depletion region is now substantially modified by the arrival of electrons from the emitter into the base- collector depletion region, and the shape of the electric field changes. The total area under the curve is constant and equal to the applied drain-source voltage of 80 V. The peak of the electric field is transferred to the epi/substrate inter- face as predicted by the CIA theory. Figure 3(d) shows the electric field distribution for 1.75~10" rad(Si)/s with the peak increasing to a value slightly higher than 1 . 5 ~ 1 0 ~ V/cm. This value is still lower than the critical field in the substrate which is 3x105 V/cm.

When the radiation rate increased to 1.85~10" rad(Si)/s, the base-emitter potential barrier totally disappeared and a strong bipolar action occurred. The electric field peak was then close to the critical field for breakdown at the epi/substrate interface (Figure 3(e)) and the current density increased to 1.43~10' A/em'. The radiation dose was then reduced to zero in 0.17 ns. The field distribution at this time is shown in Figure 3(f) and the current continued to increase to very high levels even though the radiation source was removed. The peak value of the electric field remained high at the critical level and sustained breakdown was observed.

This runaway current phenomenon was not observed for any radiation rate lower than this critical value ( 1.85~10" rad(Si)/s). At subthreshold radiation rates, irrespective of any initial high value of current, the current density decayed to zero with the removal of radiation and the field distribu- tion returned to the steady-state case for no applied radia- tion (Figure 3(a)). The threshold radiation rate was 1.85~10" rad(Si)/s for this particular device structure at a drain-source voltage of 80 V. At this radiation rate, a strong avalanche multiplication occurs at the epi/substrate interface producing many electron-hole pairs, and a positive feedback action with a loop gain of one is maintained [3,5].

Figure 4 shows the collector current density at different radiation rates. The curve is linear at lower radiation rates when the base-emitter junction is not yet strongly forward biased. However, at radiation rates higher than about 10" rad(Si)/s the curve becomes nonlinear because of the device bipolar action. Figure 5 shows the shape of the applied radi- ation pulse and how it was increased in time steps up to the threshold rate and then back to zero. Figure 6 shows the significant part of the device transient response including the breakdown, with the last three points corresponding to the time when the radiation was reduced to zero.

BAMBI was also used to simulate the independent effects of drain-source bias voltage and base width on the radiation rate burnout threshold. The results are shown in Figures 7 and 8. Higher applied voltages caused the device to break down at a lower radiation rate, while the base width was kept constant at 1.8 p m and p was calculated to be 2.1.

1424

{=O r/s,I=O A/cm 2 r 09

x 1 ~ - 4 Y , cm

(a)

?=1.75Ell r/s, 1=4.33EO! 2 A/cm

Y , cm x10-4

(d)

f=8.7E10 r/s, 1=6.32E02 "00- A/cm 4 2 X

2

0 & =- 5- 2 9- -

9 L , 0.0 3.0 G.0 9.0 12.0 15.0

Y , cm x 1 ~ - 4

?=1.85Ell r/s, I=1.43E04 2 A/cm

f=1.5E11 r/s, 1=2.29E03

09 * A/cm2

I+=O r/s, 1=1.37E05 A/cm 2

Figure 3(a-f). The electric-field distribution versus device depth as a function of radiation-dose rate ( VDs = 80 V ).

The p-type doping profile in the base was then modified to change the base width from a minimum of 0.96 pm to a maximum of 1.8 p m . Consequently, the p and lateral base ,resistance were simultaneously reduced as the base width was increased. Both changes reduce the bipolar action and increase the radiation rate threshold as observed in Figure 8. These results agree with the CIA theory. In addition, the linearity of the curve in Figure 4 at low radiation rate and the nonlinearity at high radiation rates agree with experi- mental results. The sustained breakdown phenomenon predicted by the BAMBI model is qualitatively in agreement with those observed in experimental tests [2]. However, the radiation rate burnout thresholds predicted by the model may be different from the experimental rates because of a

number of differences between the generic structures tested by BAMBI and those devices tested by others [4,6,7]. The important factors are the horizontal dimension of the device (Figure I), the gain of the parasitic bipolar transistor, and the device contact resistances (not calculated by BAMBI).

The radiation responses of n- and p-channel devices were compared by applying identical stimuli to an n- and a p-MOSFET having the same structure and doping profiles with a base width of 1.8 p m . A drain-source voltage of 120 V was used for the n-channel and -120 V for the p-channel device. The threshold level for the n-channel device was found to be 2.27~10" rad(Si)/s. The results for the p-channel device, however, were totally different. Figures 9(a-c) show

'

1425

4 2 -

2 0

v) x

Figure 4. Collector current versus radiation rate.

!I, Figure 5. Radiation profile.

Figure 6 . Transient current response of the irradiated device.

B a s e width=1.8 urn 0 0 4 , * 00 rf m- X

m , - N -

O ::- .m

*P * N rJ- 0

0 N i

80.0 90.0 100.0 110.0 120.0 V,,I v

Figure 7 . Radiation-rate threshold versus V,, .

Figure 8. Radiation-rate threshold versus base width.

the electric field distributions for this device with no radia- tion and very high radiation rates of 3.00~10~~ rad(Si)/s and 4.65~10" rad(Si)/s respectively. Figure 9(c) shows that even at a rate of 4.65~10" rad(Si)/s the base-emitter junction is not yet fully forward biased and the p-channel device does not break down. The peak of the electric field is not transferred to the epi/substrate interface, contrary to the case for the n-channel device.

Figure 10 shows that the current density versus radiation rate varies linearly up to the highest radiation rates moni- tored indicating no significant bipolar action.

The simulation shows that the p-channel MOSFET is inherently radiation-hardened and no burnout was observed which is in agreement with other published results [7] . The reason for this behavior is that the parasitic bipolar transis- tor in p-channel devices is a pnp type. The majority carriers for pnp devices are holes and the avalanche multiplication factor for holes is significantly lower than that for electrons

1426

i=o r/s,I=O A/Cm 2

%s=-120 "

1 15.0

Yl c m X l o - '

If=3.00~11 r/s, 13.05E03

0.0 4.0 6.0 12.0 16.0

Y, c m x 1 ~ - 4

A/cmL

0.0 4.0 8.0 12.0 16.0

y, cm x10-'

(a) ( C )

Figure 9 (a-c). p-channel electricfield distribution for different radiation-dose rates.

Figure 10. p-channel device collector current versus radiation rate.

[SI. In a pnp device, the electrons generated as a result of avalanche multiplication of the hole current in the base- collector depletion region form the lateral base current. But because of the low multiplication factor for the hole current, the electron current in the base is never sufficient to turn the baseemitter junction on. As a result of the low bipolar action the hole current entering the baseemitter depletion region is not strong enough to modify the net space charge in this area, hence the peak of the electric field is not moved to the epi/substrate interface. So, neither strong bipolar

action, nor large avalanche multiplication factor exist and the p-channel device does not burn out.

DISCUSSION There is a pressing need for power MOSFETs in elec-

tronic systems that can function after a transient radiation exposure(+). Historically, n-channel power MOSFETs have failed in high .4. environments making them unsuitable for such use. The work described here is intended to model the transient failure mechanisms and to determine the relative importance of the device parameters in improving its tran- sient radiation response. This work represents the first modeling of the transient +induced breakdown in power MOSFETs.

The simulation results shown here are consistent with experimental data [2,4,6,7] and the CIA breakdown model for power MOSFETs. The nonlinear increase in 1, above a certain radiation rate indicates the onset of significant bipo- lar transistor action (Figure 4). A positive feedback mechan- ism is established when the electric field reaches the critical value for avalanche breakdown (Figure 3(e)). This permits significant injection of majority carriers into the base. At this point the radiation stimulus can be removed and collec- tor current runaway occurs, thus establishing that burnout has been achieved. The p-MOSFET did not show break- down characteristics even for 4 intensities that were three times higher than those applied to a simulated n-MOSFET.

Figures 7 and 8 demonstrate how the model can predict the radiation response of various geometrical structures and operating voltages. The results indicate that increasing the

1427

base width can only realistically be used to obtain a factor of two improvement in burnout threshold. This model can be used to determine the relative improvement in transient hardness as a function of the semiconductor properties (such as minority carrier lifetime), impurity profiles, and device geometry.

CONCLUSIONS The transient response of a power MOSFET device to

ionizing radiation was examined using a modified BAMBI device simulator. The radiation rate threshold for burnout was determined for several different geometries and biasing voltages. The burnout mechanism was attributed to current-induced avalanche accompanied the bipolar transis- tor action. The effects of the applied drain-source voltage and the base width of the parasitic bipolar device on the burnout dose rate were simulated. It was found that the threshold is lower at higher drain-source voltages or nar- rower bases. This work can be extended to quantify the effects of device architecture, semiconductor parameters, impurity profiles, and bias voltages on the burnout thres- hold.

REFERENCES [l] A. F. Franz, and G. A. Franz, "l3AMEiI- A Design Model for Power MOSFETs," IEEE Trans. on Computer-Aided Design, Vol. CAD-4, No. 3, pp. 177-189, July 1985.

[a] R. Sevens, "F'ower MOSFETs and Radiation Environ- ments," Technical Article TA84- 3, Siliconix, April 1984. [3] T. F. Wrobel and J. L. Azarewicz, "High Dose Burnout in Silicon Epitaxial Transistors," IEEE Trans. on Nuclear Sci- ence, Vol. NS-27, No. 6, pp. 1411-1415, December 1980. [4] T. F. Wrobel, F. N. Coppage and A. J. Smith, "Current Induced Avalanche in Epitaxial Structures," IEEE Trans. on Nuclear Science, Vol. Ns-32, No. 6, pp. 3991-3995, December 1985.

151 H. C. Poon, H. K. Gummel, H. C. Scharfetter, 'Righ Injection in Epitaxial Transistors," IEEE Trans. on Electron Devices, Vol. ED-16, No. 5, pp. 455-457, May 1969. [6] W. R. Dawes, Jr., T. A. Fischer, C. C. Huang, W. J. Meyer, C. S. Smith, R. A. Blanchard and T. J. Fortier, "Transient Hardened Power FETs," IEEE Trans. on Nuclear Science, Vol. NS-33, No. 6, pp. 1425-1427, December 1986. [7] T. A. Fischer, W. R. Dawes, Jr., C. C. Huang, R. A. Blanchard, T. J. Fortier, W. J. Meyer, C. S. Smith and S. Yoon, "Power MOSFET Usage in Radiation Environments : Circuit Design Techniques and Improved Fabrication Methods," Government Microcircuit Applications Conference, Digest of Papers, Vol. 12, November 1986.

[8] S. Selberherr, "Analysis and Simulation of Semiconductor Devices," Wien, Springer-Verlog, 1984, Chapter 4.