(Invited) Strained Silicon Heterojunction Bipolar Transistors

Abstract

Strained Si HBTs have been demonstrated with a maximum current gain of 3700 using a relaxed Si0.85Ge0.15 virtual substrate, Si0.7Ge0.3 base and strained Si emitter. This represents 10x and 27x larger gain compared with pseudomorphic SiGe HBTs and Si control BJTs that were manufactured in parallel and had current gains of 334 and 135, respectively. The key idea in SiGe Heterojunction Bipolar Transistors (HBTs) is the incorporation of Ge in the base. This reduces the bandgap Eg, and increases the current gain. However, the amount of Ge that can be incorporated in the base is limited by material constraints such as critical thickness. Epitaxial growth of SiGe layers beyond a certain thickness can lead to strain relaxation by the generation of defects. Therefore, it is interesting to increase the Ge concentration while keeping the strain at a tolerable level.