Reduced sidewall effects in SiGe-base bipolar transistors

In small Si bipolar transistors (BJTs) the sidewall effects play an important role. Extensive device simulations on Si/SiGe/Si heterojunction bipolar transistors (HBTs) and Si BJTs show that in the HBTs the sidewall current injection is strongly suppressed. Consequently, the current gain in the HBT keeps constant down to a much smaller emitter width (We ¿ 0.20 ¿m) than for a conventional Si transistor (We ¿ 0.60 ¿m). Although the ratio of the charge storage in the sidewall emitter-base region and charge storage in the intrinsic region is also much less in the HBT than it is in the BJT, the effect on the cutoff frequency is about the same just because of the suppressed current spreading.