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

In this paper, we investigate an npn silicon heterojunction bipolar transis tor (HBT) with an n-doped single crystalline emitter region between the p-d oped base and the n-doped heteroemitter. In a previous paper [Garner DM, Am aratunga GAJ. A study of frequency response in silicon heterojunction bipol ar transistors with amorphous silicon emitters. IEEE Transactions on Electr on Devices 1996;43(11):1890-1899] we have demonstrated how a high heteroemi tter 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 sil icon as the heteroemitter, we show how the insertion of a crystalline silic on region between the base and the heteroemitter can increase the cutoff fr equency from 2.6 GHz for an ordinary silicon HBT with an a-Si:H emitter, to 6.3 GHz for the new structure and! mure importantly, how a heteroemitter m aterial with mobility of only 20 cm(2) V-1 s(-1) is required with the new s tructure for it to outperform a comparable silicon BJT. (C) 1999 Elsevier S cience Ltd. All rights reserved.