Non-ideal base current in bipolar transistors at low temperatures

Abstract

Bipolar transistors havetraditionally been considered not useful in low-temperature applications. This assumption, however, is based upon an incomplete physical understanding of bipolar device physics at low temperatures. This paper shows experimentally that recombination mechanisms play a substantially larger role in determining base current at low temperatures than at room temperature. The results are explained and quantitatively modeled using conventional Shockley-Read-Hall theory, with the addition of the Poole-Frenkel high field effect. It is concluded that trap levels in the silicon bandgap due to bulk traps or interface states are very important in determining bipolar transistor base currents at low temperatures. Non-ideality factors larger than 2 are often observed. Such trap levels will have to be carefully controlled if low-temperature operation of bipolar transistors is to be considered.