Radiation-induced degradation of bipolar transistors
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The radiation-induced degradation of high frequency npn bipolar transistors with low stress LPCVD silicon nitride as the surface isolation is investigated for the first time. The results are compared with those for devices with silicon dioxide as the isolation. The total dose degradation of the devices is observed to be a reduction in the current gain I-C/I-B because of increase in the base current I-B and no change in the collector current I-C. Devices with lower perimeter-to-area ratio are observed to be more rad-hard compared to devices with higher perimeter-to-area ratio. The degradation of both type of devices is found to be qualitatively similar, but the degradation is more severe for devices with nitride as the surface isolation. The results presented in this paper indicate that radiation-induced degradation of bipolar transistors is primarily because of damage to the surface isolation layer over the emitter-base junction region due to irradiation. The larger degradation of bipolar devices with nitride as surface isolation compared to devices with oxide as isolation suggests the presence of more traps in the nitride layer.
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