Physical processes of current gain in InAs bipolar junction transistors

InAs bipolar junction transistors (BJTs), grown by molecular beam epitaxy, are reported with common emitter current gains (βʹs) as large as 400. The factors affecting the common emitter current gain have been studied by estimating the magnitudes of the base transport factor (αT) and emitter injection efficiency (γ). This has been accomplished by studying a sequence of InAs BJTs with varying emitter doping densities, NE. Minority carrier diffusion length in the base (LB), αT, and γ have been extracted from measured electrical characteristics. The results of the study of these InAs BJTs are as follows: View the MathML source, αT≈98% and γ ranges from 92% to nearly 100% depending on NE. This knowledge of the magnitudes of the injection efficiencies suggests when it would be useful to move from the simple BJT structure to the more advanced heterojunction bipolar transistor (HBT) structure. Lower γ BJTs would be improved, however high-γ BJTs would benefit little, by the use of the widegap emitters of HBTs. The method developed here to estimate γ, αT and LB is not specific to InAs BJTs, but should be useful for study of BJTs and HBTs in any material system.