Bipolar devices, most notably circuits fabricated with lateral PNP transistors (LPNP) and substrate PNP transistors (SPNP), have been observed to exhibit an enhanced low dose rate sensitivity when exposed to ionizing radiation. These dose rate sensitive bipolar devices exhibited enhanced degradation of base current in transistors and of input bias current, offset current, and/or offset voltage in linear circuits at dose rates less than 0.1 rd(Si)/s compared to devices irradiated at dose rates greater than 1 rd(Si)/s. The total dose responses of several bipolar transistors and linear circuits in a space environment are demonstrated to exhibit enhanced degradation comparable, in magnitude, to ground-based data irradiated at a dose rate of 10 mrd(Si)/s indicating that enhanced low dose rate sensitivities (ELDRS) do indeed exist in space.

Many conventional bipolar linear microcircuits used in space systems have shown a dose rate sensitivity to total dose degradation. Dose rate data, taken by several agencies, have been combined and presented in terms of sensitive parameter shifts at a fixed dose and dose rate enhancements factors

The effect of dose rate on radiation-induced current gain degradation at 20 krad(Si) was quantified for lateral and substrate pnp bipolar transistors over the range of 0.001 to 294 rad(Si)/s. Degradation increases monotonically with decreasing dose rate, such that, at an emitter-to-base voltage of 0.7 V, radiation-induced excess base current differs by a factor of approximately, eight at the extreme dose rates. Degradation shows little dependence on dose rate below 0.005 rad(Si)/s, suggesting that further degradation enhancement at space-like dose rates may be negligible. In addition, the effect of ambient temperature on radiation-induced gain degradation at 294 rad(Si)/s was thoroughly investigated over the range of 25 to 240 degrees C. Degradation is enhanced with increasing temperature while simultaneously being moderated by in situ annealing such that, for a given total dose, an optimum irradiation temperature for maximum degradation results. Optimum irradiation temperature decreases logarithmically with total dose and is larger and more sensitive to dose in the substrate device than in the lateral device. Based on the measurement of midgap interface trap density in the base oxide, enhancement in transistor gain degradation due to elevated temperature is explained as an increase in surface recombination velocity in the base. Maximum high dose rate degradation at elevated temperature closely approaches low dose rate degradation for both devices. Based on high-temperature irradiations, a flexible procedure for the accelerated prediction of low dose rate gain degradation at 20 krad(Si) is developed for each of the devices studied.

The dose-rate dependence of gain degradation in lateral PNP transistors is even stronger than the dependence previously reported for NPN BJTs. In this work, several hardness-assurance approaches are examined and compared to experimental results. obtained at low dose rates. The approaches considered include irradiation at high dose rates while at elevated temperature and high-dose-rate irradiation followed by annealing. The lateral PNP transistors continue to degrade during post-irradiation annealing, in sharp contrast to NPN devices studied previously. High-temperature conditions significantly increase the degradation during high-dose-rate irradiation, with the amount of degradation continuing to increase with temperature throughout the range studied here (up to 125 degrees C). The high-temperature degradation is nearly as great as that observed at very low dose rates, and is even greater when differences between Co-60 and x-ray irradiation are accounted for. Since high-temperature irradiation has previously been shown to enhance the degradation in NPN transistors, this appears to be a promising hardness-assurance approach for bipolar integrated circuits. Based on these results, preliminary testing recommendations are discussed.

Excess collector current in irradiated NPN BJTs is linked to an oxide-trapped-charge-induced inversion layer acting as an additional emitter. Excess collector current is modeled by interpreting the inversion layer as an extension of the emitter.

A comparison is presented of ionizing-radiation-induced gain degradation in lateral, substrate, and vertical PNPs. The dose-rate dependence of current gain degradation in lateral PNP BJTs is even stronger than the dependence previously reported for NPN BJTs. Various mechanisms are presented and their relative significance for gain degradation in the lateral, substrate, and vertical PNPs is discussed. A detailed comparison of the lateral and substrate PNP devices is given. The specific lateral and substrate devices considered here are fabricated in the same process and possess identical emitters. Even though these devices have identical emitters and undergo the same processing steps, the lateral devices degrade significantly more than the substrate devices.

A physically based comparison between hot-carrier and ionizing radiation stress in BJT's is presented, Although both types of stress lead to qualitatively similar changes in the current gain of the device, the physical mechanisms responsible for the degradation are quite different, In the case of hot-carrier stress the damage is localized near the emitter-base junction, which causes the excess base current to have an ideality factor of two, For ionizing radiation stress, the damage occurs along all oxide-silicon interfaces, which causes the excess base current to have an ideality factor between one and two for low total doses of ionizing radiation, but an ideality factor of two for large total doses, The different physical mechanisms that apply for each type of stress imply that improvement in resistance to one type of stress does not necessarily imply improvement in resistance to the other type of stress, Based on the physical model, implications for correlating and comparing hot-carrier-induced and ionizing-radiation-induced damage are discussed.

The excess base current in an irradiated BJT increases superlinearly with total dose at low-total-dose levels. In this regime, the excess base current depends on the particular charge-trapping properties of the oxide that covers the emitter-base junction. The device response is dose-rate-, irradiation-bias-, and technology-dependent in this regime. However, once a critical amount of charge has accumulated in the oxide, the excess base current saturates at a value that is independent of how the charge accumulated. This saturated excess base current depends on the device layout, bulk lifetime in the base region, and the measurement bias. In addition to providing important insight into the physics of bipolar-transistor total-dose response, these results have significant circuit-level implications. For example, in some circuits, the transistor gain that corresponds to the saturated excess base current is sufficient to allow reliable circuit operation. For cases in which the saturated value of current gain is acceptable, and where other circuit elements permit such over-testing, this can greatly simplify hardness assurance for space applications.

We have performed capacitance-voltage (C-V) and thermally-stimulated-current (TSC) measurements on non-radiation-hard MOS capacitors simulating screen oxides of modern bipolar technologies. For 0-V irradiation at similar to 25 degrees C, the net trapped-positive-charge density (N-ox) inferred from midgap C-V shifts is similar to 25-40% greater for low-dose-rate (< 10 rad(SiO2)/s) than for high-dose-rate (> 100 rad(SiO2)/s) exposure. Device modeling shows that such a difference in screen-oxide N-ox is enough to account for the enhanced low-rate gain degradation often observed in bipolar devices, due to the similar to exp (N-ox(2)) dependence of the excess base current. At the higher rates, TSC measurements reveal a similar to 10% decrease in trapped-hole density over low rates. Also, at high rates, up to similar to 2.5-times as many trapped holes are compensated by electrons in border traps than at low rates for these devices and irradiation conditions. Both the reduction in trapped-hole density and increased charge compensation reduce the high-rate midgap shift. A physical model is developed which suggests that both effects are caused by time-dependent space charge in the bulk of these soft oxides associated with slowly transporting and/or metastably trapped holes (e. g., in E(delta)' centers). On the basis of this model, bipolar transistors and screen-oxide capacitors were irradiated at 60 degrees C at 200 rad(SiO2)/s in a successful effort to match low-rate damage. These surprising results provide insight into enhanced low-rate bipolar gain degradation and suggest potentially promising new approaches to bipolar and BICMOS hardness assurance for space applications.

The correlation of hot carrier stress and ionization induced gain degradation in npn BJTs was studied to determine if hot-carrier stress could be used as a hardness assurance tool for total dose. The correlation was measured at the wafer level and for several hardening variations for a single process technology. Additional experiments are planned and will be presented in the full paper. Based on a detailed physical analysis of the mechanisms for hot-carrier stress and ionization no correlation was expected. The results demonstrated the lack of correlation and indicate that hot-carrier stress degradation is not a predictor of total dose response.

Analysis of radiation damage in modern NPN bipolar transistors at various dose rates is performed with a recently introduced charge separation method and pisces simulations. The charge separation method is verified with measurements on metal-oxide-semiconductor capacitors. Gain degradation is more pronounced at lower dose rates. The charge separation technique reveals that depletion-region spreading and effective recombination velocity are both greater for devices irradiated at lower dose rates.

Transistors and commercial operational amplifiers from the AT&T CBIC-R, CBIC-U2 and CBIC-V2 bipolar processes were tested to Co60 ionizing radiation. High and low rate Co60 dose rates were used along with various bias conditions. The data shows the CBIC-U2 process amplifiers to be tolerant to total ionizing dose of 10 Mrad(SiO2). The CBIC-V2 amplifiers show minor effects at 100 krad(SiO2). The CBIC-R amplifiers show significant loss of transresistance by 1 Mrad. Bandwidth and total harmonic distortion were checked pre-rad and post-rad and no degradation was found.

A comparison was made between hot-carrier stress induced and ionizing-radiation induced increases in the base current of bipolar linear microcircuit transistors from two process technologies. The comparison was made on the basis of a failure stress in seconds and a failure dose in rad(SiO/sub 2/) for a failure criterion of /spl Delta/I/sub B/=2 nA measured at an I/sub C/ of 1 /spl mu/A and V/sub CE/ of 5 V. Comparisons were made for several die on a single wafer, die from different wafers in a process lot, and die from split lots with various base oxide (also called spacer of screen oxide) hardening techniques applied. For each of these cases no correlation was found between stress-induced failure and ionizing-radiation induced failure. This result is consistent with modeling that shows different mechanisms for the degradation from hot-carriers and ionization. Hot-carrier stress induced damage is dominated by interface traps near the emitter-base junction periphery; whereas, ionizing-radiation induced damage is dominated by trapped positive charge in the base oxide over the extrinsic base region.<>

The effects of the midgap-level interface trap density and net oxide charge on the total-dose gain degradation of a bipolar transistor are separately identified. The superlinear dose dependence of the excess base current is explained.

The role of net positive oxide trapped charge and surface recombination velocity on excess base current in bipolar junction transistors (BJTs) is identified. The effects of the two types of damage can be detected by plotting the excess base current versus base-emitter voltage. Differences and similarities between ionizing-radiation-induced and hot electron-induced degradation are discussed.

Different hardness-assurance tests are often required for advanced bipolar devices than for CMOS devices. In this work, the dose-rate dependence of bipolar current-gain degradation is mapped over a wide range of dose rates for the first time, and it is very different from analogous MOSFET curves. Annealing experiments following irradiation show negligible change in base current at room temperature, but significant recovery at temperatures of 100 degrees C and above. In contrast to what is observed in MOSFET's, irradiation and annealing tests cannot be used to predict the low-dose-rate response of bipolar devices. A comparison of x-ray-induced and Co-60 gamma-ray-induced gain degradation is reported for the first time for bipolar transistors. The role of the emitter bias during irradiation is also examined. Implications fdr hardening and hardness assurance are discussed.