The design, analysis and characterization of high-performance heterostructure bipolar transistors

Abstract

Described in this thesis is an investigation of design issues concerning the heterostructure bipolar transistor. The use of semiconductors with differing energy gaps in the bipolar transistor offers advantages over homojunction bipolar transistors in the ability to construct these bandgap differences such that carrier flow can be either impeded or assisted. At the emitter-base heterointerface, a wide bandgap emitter presents a barrier to injection from the base, thereby permitting higher doping of the base and lower doping of the emitter for improved high-speed performance. In the base, the formation of an accelerating field through doping or compositional grading can also improve high-speed performance, as well as reduce emitter-size effects. This thesis examines the advantages of these characteristics of the heterostructure bipolar transistor, both experimentally and theoretically.

With the design of a base-doping grading in an AlGaAs/GaAs heterojunction bipolar transistor (HBT), reductions in base recombination current were demonstrated experimentally. However, the effects of bandgap narrowing were shown to limit the accelerating field formed by doping-concentration grading. In addition, the breakdown behavior of the base-collector junction was also studied. These experimental results were duly considered in developing a model for characterizing the high-frequency performance of the bipolar transistor. Applying this model to compute cutoff frequencies for the AlGaAs/GaAs HBT, the first derived high-frequency output characteristics were reported for this device. The high-frequency performance of the Si/SiGe HBT was also examined, revealing a limited advantage of the SiGe base over a Si base for the state of the art device geometries and structures investigated.