(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.

Avalanche Modeling in Mextram 505 and Implications on Circuit Simulations

“…Modern SiGe HBTs typically have a thin and heavily doped collector to suppress Kirk's effect, which leads to a high electric field in the collector-base (CB) junction [1]. In NPN transistors, electrons injected from the emitter can gain enough energy from the electric field to create electron-hole pairs upon impact with the lattice, a process called "impact ionization."…”

“…• G EM closely relates to various transistor breakdown voltages. For instance, G EM = 1/β approximately determines the open-base breakdown voltage BV CEO [1]. In circuits, however, the base sees a low source impedance, and the transistor can operate well above BV CEO with careful modeling of I avl .…”