PERFORMANCE OF SOI POLYSILICON EMITTER
TRANSISTORS IN A RADIATION ENVIRONMENT

Onno W. Purbo
Inter University Center on Microelectronics
Institute of Technology Bandung
Bandung 40132
INDONESIA
Tel: 62-22 250-4256
FAX: 62-22 250-5442
yc1dav@itbgtw.itb.ac.id
onno-itb@kesemek.cs.ui.ac.id

ABSTRACT

		The effect of Gamma radiation up to 60 krad(Si) on ZMR-SOI and bulk-Silicon Polysilicon Emitter Transistors was investigated both experimentally and theoretically. MOS capacitors and Schottky diodes were used as vehicles for understanding the effect of Gamma radiation on Polysilicon Emitter Transistors. A positive charge accumulation, an increase in the surface recombination velocity, as well as a reduction in surface recombination lifetime in the oxide after Gamma radiation have been responsible for degrading the dc characteristics of both ZMR-SOI and bulk-Silicon Polysilicon Emitter Transistors in the same manner. To use SOI technology for bipolar applications, consideration has to be given to the quality of the field oxide in the base region which is responsible for the degradation of the bipolar transistor. In SOI bipolar applications, a better surface and field oxide is required to assure the radiation hardened properties of the transistors to tolerate higher total dose effects.

1.	INTRODUCTION.
	A more compact structure for Bipolar transistors can be achieved by using Polysilicon Emitter Transistor (PET) technology compared to conventional metal emitter transistors [1-3]. This leads to many advantages, such as, higher speed and decreased base width, as compared to metal contacted Bipolar transistor. In a Gamma radiation environment, a PET structure is known to be more resistant in terms of current gain degradation due to Gamma radiation compared to metal contacted bipolar transistors [4]. Moreover, in a neutron radiation environment, the thinner base width of PETs will help to limit the displacement damage [5,6].
	Silicon-on-Insulator (SOI) technology, especially in SOI Complementary Metal-Oxide-Semiconductor (CMOS) technology is reputed to be more resistant to a radiation environment [7]. The existence of the buried oxide will reduce the generated mobile carriers in the thin film silicon on top of the buried oxide. Thus, the effects of generated mobile carriers on the circuit in the thin film silicon of SOI ICs will be minimal. However, for bipolar transistor applications, earlier reports [4,5,8] indicate that the important factors which affect the characteristics of a bipolar transistor after Gamma radiation are the charge accumulation in the oxide [5,8] and the increased surface recombination velocity [4,5,8]. For SOI PETs with a narrow base width (0.2 μm) [9], the effect of bulk displacement damage is minimal [5,6].
	PETs normally have a higher base doping concentration at the base silicon/oxide interface and thinner base width. Therefore, the base current in PETs will flow closer to the base silicon/oxide interface. Thus, any changes in the base field oxide properties will affect the base current more severely in PETs. Furthermore, since the base oxide is normally thinner than that of the field-oxide, the higher electric field under metallization runs on the base will attract more trapped charges during Gamma radiation and thus, degrade the base current further [5].
	The importance of different field-oxide structures in a Gamma radiation environment has been described in [10]. Several different oxide structures, namely, fully thermally grown SiO2, P-doped double layer SiO2, and undoped double layer SiO2, were compared [10]. The thermally grown oxide was found to have more trapped charges and thus, degraded more severely under Gamma radiation. Neumeier et.al. [10] has found that a P-doped double layer SiO2 is most resistant to Gamma radiation. The existence of five valency phosphorous/arsenic atoms is suspected to act as electron traps and/or recombination centers within the oxide and thus, leads to less accumulation of positive charges at the Si-SiO2 interface [10]. In space applications, one should consider a high electron radiation environment [11], as well as Gamma radiation. It was found that there are no significant differences in the transistor's characteristics under Gamma or electron radiation. However, in integrated circuits, it was found that less dense oxides, such as, a Chemical Vapour Deposition (CVD) oxide, is more vulnerable to Gamma radiation. While more dense oxides, such as, a thermally grown oxide, is more vulnerable to electron radiation [11].
	The major focus in this work is to compare the total dose effects due to Gamma radiation on the dc characteristics of both bulk-Silicon and Zone-Melting Recrystallized Silicon-on-Insulator (ZMR-SOI) PETs. In addition, problems due to Gamma radiation, arising in Reactive Ion Etching (RIE) areas such as those used in trench isolation of SOI ICs are also investigated. Metal Oxide Semiconductor (MOS) capacitors and Schottky diodes are used as the vehicle to identify the major failure mechanisms in PETs due to Gamma radiation. Theoretical SUPREM 4 and PISCES 2B simulations are used to model the physical failure mechanisms in PETs.

2.	EXPERIMENTAL PROCEDURE
	In this section, the fabrication steps will be briefly described. The bulk-Si and ZMR-SOI PETs were simultaneously fabricated using the same double implant process as described in [9]. MOS capacitors and Schottky diodes on 0.5 ohm-cm n-type wafer are used as vehicles to identify the major failure mechanisms in bipolar transistors as well as problems arising from the Reactive Ion Etching (RIE) areas exposed to Gamma radiation.
	We also used RIE-etched samples in an attempt to characterize the RIE process at a pressure of 10 mTorr with 400V RF voltage to be used in creating the trench isolation in ZMR-SOI bipolar ICs. Gas mixtures used in the RIE plasma are 9 sccm SF6 + 1 sccm O2 and 9 sccm SF6 + 1 sccm O2 + 0.5 sccm N2. In our recent work [12], we found that a small N2 additive in the plasma leads to higher etch-rate and selectivity while significantly lowering the reactive-ion damage.
	The MOS capacitors and Schottky diodes are fabricated using a fairly similar process. First, all the samples are cleaned using NH4OH clean (also known as "RCA solution" [25]). We prepared three different samples for both MOS capacitors and Schottky diodes. These samples were subjected to the following treatments (a) no RIE, (b) SF6+O2+N2 RIE and (c) SF6+O2 RIE etching. The sample which received no RIE exposure is used as reference sample. Subsequently, after cleaning, RIE process was performed. A 400 Angstrom dry oxide is grown at 1050oC for 25 minute to form MOS capacitors. Subsequently, Aluminum is deposited on the oxide. For the Schottky samples, the only difference with MOS process is no dry oxide process. In Schottky diode process, after the RIE process, the aluminium is deposited following a cleaning of the surface in "RCA solution" and HF etch to remove the oxides on the surface. Post-metal annealing is performed at 250oC in N2+H2 ambient to improve the characteristics of both MOS capacitors and Schottky diodes.
	The Gamma radiation exposure was performed at the Nuclear Reactor at McMaster University in Hamilton, Canada. It was done in a hot cell facility with an Co-60 Gamma Source at 5kCi. The dose rate was 2.5 krad(Si)/hour with a total dose of 20 krad(Si) for each run without any bias on the samples during exposure. For bulk-Si and ZMR-SOI PETs, the dc measurements were performed after a three step exposure of gamma radiation at 20 krad(Si) per step and thus, the total dose was 60 krad(Si). The devices were unbiased during exposure. This experiment was particularly designed to observe the effects of the total dose from long-term Gamma radiation exposure. The MOS capacitors and Schottky diodes were measured after one step of Gamma radiation exposure with a total dose of 20 krad(Si).
	The HP4145A Semiconductor Parameter Analyzer was used to measure the I-V characteristics. A Keithley 590 and 595 C-V system was used to measure C-V and C-t characteristics. A Signatone S-1060 hot-chuck system attached to the Keithley C-V system was utilized for performing Bias Thermal Stress (BTS) measurements [13] to deduce the mobile ion density and the surface state concentration in the MOS capacitors.
	SUPREM 4 and PISCES 2B simulations were used to aid theoretical studies of the effects of Gamma radiation on ZMR-SOI PETs. The parameters affected by Gamma radiation which were identified in the MOS C-V measurements were incorporated into the PISCES 2B simulations to study the effects of Gamma radiation. These affected parameters are surface recombination velocity, surface recombination lifetime and the accumulation of positive, fixed oxide charges.

3.	DISCUSSIONS
	In this section, we discuss the experimental results from the Gamma radiation exposure of PETs, MOS capacitors and Schottky diodes. A theoretical analysis based on a PISCES 2B simulation will be used to correlate these effects, which helps us to understand the performance of ZMR-SOI PETs in a Gamma radiation environment.

3.1	ZMR-SOI Polysilicon Emitter Transistors.
	In this section, we will describe the experimental results from our ZMR-SOI PETs. The known basic damage mechanisms which affect bulk-Si bipolar devices are (1) inversion at the surface due to charge trapping in the oxide, (2) changes in the surface recombination velocity which linearly depends on the interface trap density Dit [14], and (3) bulk-damage which occurs at high radiation, for example, a significant bulk-damage can be produced by 1-2 MeV electron or Co-60 Gamma radiation [5,6]. As summarized in [8], there are three important mechanisms, namely, surface recombination velocity, oxide trapped charge and bulk damage, which affect a bulk-Silicon bipolar transistor under Gamma radiation. We found that these failure mechanisms play an important role in degrading the dc characteristics of the ZMR-SOI PETs (as well as bulk-Si PETs).
	Figure 1a and 1b show the measured Gummel plots of bulk-Silicon and ZMR-SOI PETs, respectively, after 0, 20, 40 and 60 krad(Si) Gamma radiation exposure. Consistent with previous studies [5,8,15], our experimental observation of bulk-Si PETs indicates that the current gain degradation begins to become significant at low base currents. The increase in base currents at low Vbe suggests an increase in the recombination current in the bipolar transistor after Gamma radiation.
	Figure 2 shows the dc current gain as a function of Collector current for both bulk-Silicon (Fig. 2a) and ZMR-SOI PETs (Fig. 2b). Significant changes in the current gain occur after the first 20 krad(Si) of Gamma radiation exposure. Compared to the first 20krad(Si) Gamma radiation, no significant degradation in current gain is observed after exposure to 20-60 krad(Si). The dc current gain degradation of bulk-Si PETs (Fig. 2a) becomes noticeable at low levels of collector current.
	Figure 3 shows the collector-base (CB) and emitter-base (EB) leakage currents for both bulk-Silicon (Fig. 3a, 3c) and ZMR-SOI PETs (Fig. 3b, 3d) for 0, 20, 40, 60 krad(Si) Gamma radiation exposure. It is clearly seen in Fig. 3 that Collector-Base (CB) and Emitter-Base (EB) leakage currents of a ZMR-SOI PET was found to be significantly affected by the radiation. In contrast, the effect on the CB and EB leakage current is relatively small for bulk-Si PETs. From Iceo measurements on our bulk-Si and ZMR-SOI PETs, Gamma radiation exposure of up-to 60 krad(Si) reduces Iceo slightly for both bulk-Si and ZMR-SOI PETs. Only the ZMR-SOI PET's EB forward characteristics were affected by Gamma radiation. However, this is not the case for bulk-Si PETs as no significant effect in bulk-Si PET's EB and CB forward characteristics was observed after Gamma radiation. The CB breakdown voltage and the output characteristics at peak current gain are not affected by Gamma radiation exposure of up-to 60 krad(Si). To summarize, the experimental results indicate that Gamma radiation exposure up-to 60 krad(Si), appears to degrade the dc characteristics of ZMR-SOI PETs more than bulk-Si PETs. The degradation in the dc characteristics manifests itself in an increase in the recombination / leakage current.

3.2	MOS Capacitors and Schottky diodes.
	To gain further understanding of the failure mechanisms in ZMR-SOI PETs under Gamma radiation, Schottky diodes and MOS capacitors were used as vehicles. Changes in the parameters of MOS capacitors and Schottky diodes due to Gamma radiation exposure have been identified. The change in these parameters was correlated with the effects of Gamma radiation on bulk-Si and ZMR-SOI PETs characteristics.
	The I-V characteristics of the Schottky diodes, for both before and after Gamma radiation, are shown in Figs. 4a and 4b, respectively. These figures show the d.c. characteristics of Schottky diodes made on three differently processed wafers, namely, without RIE (solid lines), with SF6+O2+N2 plasma RIE (long dashed lines) and with SF6+O2 plasma RIE (short dashed lines). No significant degradation in the leakage current of the RIE Schottky diodes is observed after Gamma radiation. The major degradation is in the leakage current of a Schottky diode which was not exposed to RIE. The ideal reverse Schottky current which increases gradually with the applied reverse voltages is qualitatively affected after Gamma radiation. The nearly flat reverse current indicates that the recombination-generation process is dominating the reverse current characteristics instead of the Schottky barrier [16].
	In Fig. 5a, the Schottky barrier heights extracted from the extrapolation of 1/C2 characteristics (open symbols) are shown for samples prepared without RIE (1), with SF6+O2+N2 (2), and with the SF6+O2 RIE process (3). Both before (solid lines) and after 20 krad(Si) of Gamma radiation exposure (dashed lines) are shown. The extracted Schottky barrier heights of the samples which were not subjected to the RIE process (before Gamma radiation) is in a good agreement with published results [16]. No significant differences in the extracted barrier heights are observed between the samples etched with two different plasmas in the RIE-processes. Gamma radiation is found to degrade the Schottky barrier heights and quite severely for non-RIE samples, as seen from the extraction of the barrier heights by the 1/C2 method (open symbols) (see Fig. 5). This is consistent with the observation of severe degradation in the leakage currents of non-RIE samples. The reduction of the Schottky barrier height due to Gamma radiation is known and is due to the fact that the Fermi level is pinned by the surface states since all the states at the surface are occupied by electrons [15]. This in turn, renders the material impervious to damaging Gamma radiation effects.
	We found the difference in the extracted Schottky barrier heights between Js and 1/C2 measurement is quite high. These differences arise from the non-ideality of the Schottky diode currents (with >60 mV/decade gradient at room temperature) especially in RIE-processed samples. This leads to a higher uncertainty in determining the Schottky barrier height based on Js measurements even if it was performed at low current at which less influence from the parasitic resistances. In contrast, the 1/C2 curve is fairly linear and, thus, leads to a less ambiguous extraction of the barrier height with less uncertainty. Thus, the reliability of the extracted Schottky barrier heights from 1/C2 measurements appear to be better than those from Js measurements.
	Figure 5b shows the ideality factors observed in the forward characteristics of Schottky diodes for both before (solid line) and after Gamma radiation (dash line). The SF6+O2+N2 RIE processed Schottky diodes give an ideality factor close to unity. As expected, the ideality factor of the Schottky diodes increases (to about 1.3-1.4) after Gamma radiation exposure.
	Figure 6 shows the surface recombination velocity S (Fig. 6a) and the surface recombination lifetime (Fig. 6b) obtained from MOS capacitor measurements, respectively for cases, without RIE (1), with a SF6+O2+N2 plasma (2) and with a SF6+O2 plasma (3). Both before, (solid lines) and after, Gamma radiation (dashed lines) characteristics are shown. In Fig. 6, one of the important results shown is an increase of three orders of magnitude in S after Gamma radiation in Fig. 6a. Fig. 6b indicates that the surface recombination life-time is shortened by about one order of magnitude after exposure to 20 krad(Si) Gamma radiation. A relatively more severe degradation is observed on the samples that are not exposed to RIE processes.
	Figure 7 shows the BTS measurement results. The concentrations of positively charged mobile ions and the negatively charged surface states [24] are shown in Fig. 7a and 7b, respectively, for three different MOS capacitor processes, namely, (1) without RIE, (2) with SF6+O2+N2 and (3) with a SF6+O2 plasma exposure. As shown in Fig. 7a, the BTS measurement reveals an increase in the positive mobile ions by a factor of 2-3 higher than that observed before Gamma radiation, which is consistent with previous reports [10,17,18,20-23] of an accumulation of positive charge occurring in the oxide due to Gamma radiation. The increased mobile ion concentration is quite consistent for all MOS capacitor samples both with, and without RIE processing. The accumulation of positive charge in the oxide creates an inversion layer in the p-material. This inversion layer can give rise to a non-ideal base current, an increase in leakage currents, as well as causing large leakage current to flow through in a trench isolation [19]. Considering the more severe accumulation of the positive fixed charges in the non-RIE samples (see Fig. 7a), an isolation process which does not rely on the RIE process, such as a wet etch (for mesa isolation), is likely to exhibit more severe degradation than that of trench isolation with RIE process.
	It has been suggested in [20-23] that the positive changes near the Si-SiO2 interface become trapped and are than slowly converted into surface states. This in turn, adds more positive charge to the negatively charged surface states and in effect, reduces the concentration of negatively charged surface states after 20 krad(Si) Gamma radiation. The differences in the characteristics of the two different RIE-processed MOS capacitors observed for before and after Gamma radiation is not significant.
	The major conclusion from these experiments is Gamma radiation was found to affect the Silicon surface properties and the oxide properties which include (1) an increase in the positive charge in the oxide, (2) a significant increase in the surface recombination velocity (S), and (3) a decrease in the surface recombination lifetime. Considering the severe degradation observed for our MOS capacitors as well as that noted in previous reports [15] and since at low Vbe, a significant part of the base current flows at the surface close to the Si-SiO2 interface [5,8,15], the increased surface recombination velocity due to Gamma radiation causes an increase in the base leakage current at low Vbe and thus, should degrade the dc current gain [5,15].

3.3	Theoretical studies based on PISCES 2B simulation.
	To understand the effects of the accumulation of positive charge and the increase of S due to Gamma radiation, we used a SUPREM4 coupled with a PISCES 2B simulation to model these effects. To model the degradation due to Gamma radiation exposure, several parameters, namely, the surface recombination velocity (S) and the positive fixed-oxide charge (Qf), were varied in the PISCES 2B simulation. Both bulk-Si and ZMR-SOI Polysilicon Emitter Transistors were simulated.
	Figure 8 shows the simulated Gummel plot of (a) bulk-Si and (b) SOI PET in the presence of a positive Qf of the order of 1010-1012 cm-3, as indicated by the MOS capacitor measurements after Gamma radiation. Consistent with the experimentally observed Gummel plots (Fig. 1), the base current at low Vbe increases with positive Qf. Similar simulation results were also obtained when considering the increase in S at the silicon/oxide interface. A more drastic increase in the low Vbe base current can be shown when both positive Qf and S exist simultaneously in the oxide. No significant differences in the simulated results are found between bulk-Si and SOI PETs.
	The simulated EB reverse characteristics are shown in Fig. 9 for both bulk-Si (Fig. 9a) and SOI PETs (Fig. 9b) with a positive Qf of the order of 1010 cm-3 up to 1012 cm-3 and an S of 100 up to 104 cm/s as indicated by the experimentally measured MOS capacitor characteristics after Gamma radiation. As shown in Fig. 9, the simultaneous existence of a positive Qf and S increases the EB reverse current. In the actual measurement, we observed a soft breakdown in the EB reverse characteristics. No gradual increase (soft breakdown) in the simulated EB reverse current is possibly due to a lack of impact ionization option during simulation. Convergence problems have occurred in attempting to simulate the EB reverse characteristics with the impact ionization option. A similar behaviour was observed in the simulated CB reverse characteristics. Consistent with the experimentally measured forward characteristics, no significant change is observed in the simulated forward characteristics for both the EB and CB junction due to an increase in a positive Qf and S.
	To study further the physical consequences of the increase in a positive Qf and S in the base oxide, we simulated the situation using PISCES-2B device simulator. Two major differences are apparent with the existence of positive Qf, namely, the appearance of a depletion layer under the base oxide and the relocation of the peak net recombination (cm-3/sec) to the silicon/oxide interface in the base. The appearance of a depletion layer and the relocation of the contour of the net recombination (cm3/sec) under the base oxide will affect the current flow. A similar behaviour is observed with an increase in S. Since Qf is positively charged, no depletion layer is created in the n-type collector, nor in the n+ buried layer on the buried oxide. Thus, in a Gamma radiation environment, mostly the area under the base oxide is affected due to the accumulation of positive charges in the oxide.
	The consequences of the appearance of a parasitic depletion layer and the relocation of the contour of the net recombination (cm-3/sec) under the base oxide can be seen in the simulated, two-dimensional current vector, shown in Fig. 10. In normal SOI PET (Fig. 10a), the minority hole current in the base flows parallel with the oxide interface. However, the introduction of Qf in the oxide will shift some of the minority hole current close to the oxide interface and supply the recombination at the silicon/oxide interface in the base. This deviation manifests itself as an increased in the hole current vectors pointing towards the oxide. Thus, this shows clearly that the increase in the experimentally observed EB current after Gamma radiation is mainly due to the increase in hole current to supply the recombination centers created at the silicon/oxide interface in the base. Similar results can be shown for simulated EB and CB reverse characteristics. No difference is observed in simulated current vector flow close to the buried oxide with the existence of Qf and S, which indicate that the buried oxide has little to do with the total dose effects in a low term Gamma radiation exposure.

4.	SUMMARY
	We have demonstrated that the degradation of the dc characteristics of ZMR-SOI PETs due to Gamma radiation depends on similar processes to those in bulk-Si PETs, namely, an accumulation of a positive Qf in the oxide, an increase in S, and a decrease in the surface recombination lifetime.
	Confirmed by theoretical PISCES 2B simulation studies, the accumulation of a positive Qf in the oxide, an increase in S and a decrease in the surface recombination lifetime at the silicon/oxide interface will lead to the creation of a parasitic depletion layer under the base oxide with an increase recombination within the parasitic depletion layer at the Silicon/oxide interface. This leads to an increase in the current supplying the parasitic recombination centers created in the base. Less severe accumulation of a positive fixed charge in the RIE oxide indicates that the trench isolation may have a higher tolerance to Gamma radiation than that of mesa isolation.
	As confirmed in PISCES 2B simulation studies, the positive fixed charge accumulation in the buried oxide layer in ZMR-SOI PETs has no effect on the n+ buried layer nor the collector current flow and thus, has little to do with the degradation dc characteristics after Gamma radiation. Total dose effects in ZMR-SOI PETs are more pronounced than those for bulk-Si PETs, which indicates that more consideration has to be given to the quality of the field oxide as well as the silicon surface of the ZMR-SOI materials. 

5.	ACKNOWLEDGMENTS
      We wish to thank Prof. C.R.Selvakumar at University of Waterloo, Canada for his encouragements and support during the course of the work. We wish to thank the Government of Indonesia and the Canadian International Development Agency (CIDA) for granting the overseas scholarship.

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FIGURE CAPTIONS

Figure 1.	Experimentally measured Gummel plots of (a) a bulk-Si and (b) a ZMR-SOI Polysilicon Emitter Transistor after 0, 20, 40, 60 krad(Si) Gamma radiation.

Figure 2.	The experimentally measured DC current gain as a function of collector current for (a) bulk-Silicon and (b) ZMR-SOI PETs after 0, 20, 40, 60 krad(Si) Gamma radiation.

Figure 3.	Experimentally measured Collector-Base leakage currents for (a) bulk-Silicon and (b) ZMR-SOI PETs. While the experimentally measured Base-Emitter leakage currents are shown in Figs. (c) and (d) for bulk-Silicon and ZMR-SOI PETs, respectively.

Figure 4.	Both reverse and forward current-voltage characteristics of the Schottky diode after 20 krad(Si) Gamma radiation for diodes with no RIE exposure (solid lines), with an SF6+O2+N2 RIE exposure (long dashed lines) and with a SF6+O2 exposure (short dashed lines). Characteristics before and after 20 krad(Si) of Gamma radiation are shown in (a) and (b), respectively.

Figure 5.	(a) The extracted Schottky barrier height barrier (eV) for the three different processes, namely, (1) without RIE, (2) with a SF6+O2+N2 RIE process and (3) with a SF6+O2 RIE process. Solid symbols were obtained from interpolation of the saturation current (Js). While the open symbols were obtained from the extrapolation of the 1/C2 curve. (b) is the ideality factor of the Schottky diode forward characteristics for the three different processes for both before (solid lines) and after Gamma radiation (dashed lines).

Figure 6.	(a) The surface recombination velocity and (b) the surface recombination lifetime plotted as function of three different MOS capacitor processes, namely, (1) no RIE, (2) after SF6+O2+N2 RIE processed and (3) after SF6+O2 RIE processed, for both before (solid lines) and after Gamma radiation exposure (dashed lines).

Figure 7.	(a) The concentration of positively charged mobile ions and (b) the negatively charged surface states are plotted for three different MOS capacitor processes, namely, (1) no RIE, (2) after a SF6+O2+N2 RIE process and (3) after a SF6+O2 RIE process, for both before, (solid lines) and after, Gamma radiation exposure (dashed lines).

Figure 8.	PISCES 2B simulated Gummel plots of (a) a bulk-Si and (b) a SOI Polysilicon Emitter Transistor with the existence of a positive fixed oxide charge of 1010-1012 cm-3 due to Gamma radiation.

Figure 9.	PISCES 2B simulated Emitter-Base reverse current of (a) a bulk-Si and (b) a SOI Polysilicon Emitter Transistor with the existence of positive fixed oxide charge of 1010-1012 cm-3 and a surface recombination velocity of 104 cm/s, due to Gamma radiation.

Figure 10.	A PISCES 2B simulated two dimensional cross-section of the current vectors in a Polysilicon Emitter Transistor for (a) a normal transistor, without any charge accumulation or surface recombination velocity, and (b) with the existence of a positive fixed charge 1012 cm-3 in the oxide due to Gamma radiation. The junctions are shown as long dashed lines.

Table 1. Summary of Total Dose Mechanisms
for bulk-Silicon bipolar devices [8]

Mechanisms	Energy
Dependence	Bias
Dependence	Effect on
Device	
Surface
recombination
velocity	Slight	Strong	Gain
reduction	
Oxide trapped
charge	Slight	Strong	Inversion
Gain reduction
Inc. leakage
Dec. breakdown	
Bulk
damage	Strong	None	Gain reduction