The effect of base doping on the performance of Si bipolar transistors at low temperatures

The possibility of using Si bipolar transistors at low temperatures (77 to 200 K) is examined. Estimates of the de-current gain β and the switching performance are obtained from results on the variation of the effective band gap with the concentration of impurities in the emitter region and from the variation of the transport properties with the impurity concentration in the base. The reduction in β at low temperatures cannot be eliminated by using an optimum As concentration in transistors with As doped emitters. There is, however, a significant improvement in the low-temperature from eliminating the compensating acceptors from the emitter region. It is important to also reduce the number of compensating donor traps from the base region at low temperatures. The effect of varying the base doping is calculated using the data on Si of Morin and Maita, replotted to obtain the dependence on impurity concentration. The switching speed goes through a maximum close to 150 K. At even lower temperatures majority-carrier freeze out is responsible for a decreased performance. At low temperatures, the de-current gain can be increased by reducing the base doping. The resulting performance decrease is less than at room temperature and the improvement in β is greater, because, at low temperatures, the decreased impurity scattering at a lighter doping significantly improves the electron and hole mobilities in the base region.