TABLE OF CONTENTS The SiGe HBTs are double heterojunction bipolar transistors (DHBTs) as the SiGe material is used as a narrow bandgap material in the p-type base. The emitter and the collector are silicon and have larger bandgap. The AlGaAs/GaAs and InGaP/GaAs HBTs benefit from a single heterojunction formed between the AlGaAs wide bandgap emitter and the GaAs p-type base. InP/InGaAs and InAlAs/InGaAs grown on InP substrate give double heterojunction devices as both emitter and collector regions include wide bandgap materials. HBTs based on III-V semiconductors are currently the fastest devices.
During the last decade, there have been a lot of debates and discussions on the competition between SiGe and GaAs HBTs for high-frequency RF applications. An early review by Cressler [16] gives an excellent description of the advantages of SiGe HBT over Si bipolar for high-frequency analog applications. It is clear that reported data show a large overlap in the performance of GaAs and SiGe HBTs [17].
Some of the key physical properties of Si and GaAs relevant to the electrical design of SiGe and GaAs HBTs are listed in Table 10.1 [18]. It shows that the dielectric constant, e, for Si and for GaAs are about the same. Therefore, the depletion layer capacitance of a p-n junction in Si or in GaAs is a function of the junction depletion layer width only. The electron diffusion coefficient D, which is related to mobility, is much larger in GaAs than in Si. For fields less than about 3 10
