A silicon heterojunction bipolar transistor with an amorphous silicon emitter and a crystalline silicon emitter region

Abstract

In this paper, we investigate an npn silicon heterojunction bipolar transistor (HBT) with an n-doped single crystalline emitter region between the p-doped base and the n-doped heteroemitter. In a previous paper [Garner DM, Amaratunga GAJ. A study of frequency response in silicon heterojunction bipolar transistors with amorphous silicon emitters. IEEE Transactions on Electron Devices 1996;43(11):1890–1899] we have demonstrated how a high heteroemitter material mobility (of the order of 100 cm 2 V −1 s −1) is required to make a traditional silicon HBT with performance better than an equivalent homojunction bipolar transistor (BJT). Here, with emphasis on amorphous silicon as the heteroemitter, we show how the insertion of a crystalline silicon region between the base and the heteroemitter can increase the cutoff frequency from 2.6 GHz for an ordinary silicon HBT with an a-Si:H emitter, to 6.4 GHz for the new structure and, more importantly, how a heteroemitter material with mobility of only 20 cm 2 V −1 s −1 is required with the new structure for it to outperform a comparable silicon BJT.