Introduction

Epitaxial-layer, bipolar transistors are intrinsically well suited to high-frequency applications because their critical, physical dimensions are mainly in the direction of the semiconductor film growth, which can be controlled on a near-atomic scale. This is in contrast to field-effect transistors (FETs), where the critical dimension of gate length must be determined lithographically.

Among the family of bipolar transistors, heterojunction bipolar transistors (HBTs) are particularly attractive for operation at high frequencies because their employment of a wide-bandgap emitter allows a highly doped base region to be used without compromising the current gain [1]. With a highly doped base, the base width can be reduced while still maintaining an acceptable base resistance. A short base width leads directly to an improved cut-off frequency, fT, which, when coupled with the lower base resistance, leads to an improved oscillation frequency, fmax. These, and other, attributes of HBTs have been reviewed [2].

In this chapter, some important aspects of modeling high-performance HBTs are discussed. The aims are twofold: (i) to gain some insight into the workings of an HBT at the microscopic level; (ii) to use this insight to examine, or develop, analytical expressions which may be useful in the engineering design of high-frequency and high-speed devices.

At the microscopic level, the emphasis is on the collector current density, JC.