W.E. Combs

Physically based comparison of hot-carrier-induced and ionizing-radiation-induced degradation in BJTs

A physically based comparison between hot-carrier and ionizing radiation stress in BJTs 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. >

Hardness-assurance and testing issues for bipolar/BiCMOS devices

The dose-rate dependence of bipolar current-gain degradation is mapped over a wide range of dose rates. This dependence 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 MOSFETs, irradiation and annealing tests cannot be used to predict the low-dose-rate response of bipolar devices. A comparison of X-ray-induced and /sup 60/Co gamma-ray-induced gain degradation for bipolar transistors is reported. The role of the emitter bias during irradiation is also examined. Preliminary field-oxide capacitor studies suggest that the mechanism for the dose-rate effect may be related to charge yield in the basic surface oxides. Recommendations for hardness-assurance testing of bipolar devices include testing at dose rates below 10 rad(SiO/sub 2/)/s and applying safety factors to estimate the space-environment response. >

Comparison of ionizing-radiation-induced gain degradation in lateral, substrate, and vertical PNP BJTs

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.

Hardness-assurance issues for lateral PNP bipolar junction transistors

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/spl deg/C). The high-temperature degradation is nearly as great as that observed at very low dose rates, and is even greater when differences between /sup 60/Co 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.

Dose‐rate effects on radiation‐induced bipolar junction transistor gain degradation

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.

Gain degradation of lateral and substrate pnp bipolar junction transistors

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/spl deg/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.

Excess collector current due to an oxide-trapped-charge-induced emitter in irradiated NPN BJT's

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. >

Comparison of hot-carrier and radiation induced increases in base current in bipolar transistors

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. >