Optimization of bipolar transistors for low temperature operation

Most bipolar transistors designed for room temperature operation suffer serious current gain degradation at liquid nitrogen temperature. This is mainly due to bandgap narrowing in the heavily doped emitter. Switching performance also suffers because of carrier freezeout at lower temperatures. In this paper, bipolar transistors designed specifically for operation at liquid nitrogen temperature are discussed. It is concluded that for high gain, high performance LN2bipolar transistors, the emitter concentration should be around 5 × 10¹⁸cm^-3. Compensating impurities in the base should be kept to minimum. Test bipolar transistors with polysilicon emitter contacts were fabricated using these criteria. The devices show very little current degradation between room temperature and 77K. Polysilicon emitter contacts are also shown to be somewhat more effective at lower temperatures.