Solid State Electronic Devices, 7th Edition

7.9 Heterojunction Bipolar Transistors

In Section 7.4.4 we saw that the emitter injection efficiency of a bipolar transistor is limited by the fact that carriers can flow from the base into the emitter region, over the emitter junction barrier, which is reduced by the forward bias. According to Eq. (7–25) it is necessary to use lightly doped material for the base region and heavily doped material for the emitter to maintain a high value of γ and, therefore, α and β. Unfortunately, the requirement of light base doping results in undesirably high base resistance. This resistance is particularly noticeable in transistors with very narrow base regions. Furthermore, degenerate doping can led to a slight shrinkage of E_g in the emitter as the donor ...