ABSTRACT

At present, SiGe technology development is almost exclusively centered on npn SiGe HBTs. For high-

speed analog and mixed-signal circuit applications, however, it is well known that a complementary

(npn þ pnp) bipolar technology offers significant performance advantages over an npn-only technology [1]. Push-pull circuits, for instance, ideally require a high-speed vertical pnp transistor with comparable

performance to the npn transistor [2]. The historical bias in favor of npn Si BJTs is due to the

significantly larger minority electron mobility in the p-type base of an npn Si BJT, compared to

the lower minority hole mobility in the n-type base of a pnp Si BJT. In addition, the valence band

offset in SiGe strained layers is generally more conducive to npn SiGe HBT designs, because it translates

into an induced conduction band offset and band grading that greatly enhance minority electron

transport in the device, thereby significantly boosting transistor performance over a similarly con-

structed npn Si BJT. It has been shown that this band alignment is not as restrictive, however, as has been

commonly assumed [3]. For a pnp SiGe HBT, on the other hand, the valence band offset directly results

in a valence band barrier, even at low injection, which strongly degrades minority hole transport and thus

limits the frequency response. Careful optimization to minimize these hole barriers in pnp SiGe HBTs

is thus required, and has in fact yielded impressive device performance compared to Si pnp BJTs, as

demonstrated in the pioneering work reported in Refs. [4-6].