The importance of bandgap narrowing distribution between the conduction and valence bands in abrupt HBTs

Abrupt heterojunction bipolar transistors (HBTs) show interfaces where discontinuities in the energy levels appear. Currents through these interfaces are controlled by tunneling and thermionic emission. The values of these currents depend on the form and height of the energy barriers, which are disturbed by the heavy doping effects on semiconductor energy band structure. In this work, the real bandgap narrowing is distributed between the conduction and valence bands according to Jain-Roulston model, and its effect on the base and collector currents of Si/SiGe and InP/InGaAs HBTs is analyzed. This analysis is carried out through a numerical model which combines the drift-diffusion transport in the bulk of transistor with the thermionic emission and tunneling at the base-emitter interface, and an empirically determined surface recombination current.