Generalized analytical transport modeling of the DC characteristics of heterojunction bipolar transistors

The current transport mechanisms in double-heterojunction bipolar transistors, including the effects of conduction-band discontinuities of spikes, is analyzed. Two approaches, one based on the back-and-forth motion of electrons in the base between confining spikes and the other on the solution of the continuity equation in the base, are shown to be equivalent. The simplified derivation of the Ebers-Moll-like terminal current expressions ensures that the physical transport mechanisms have not been obscured. The general model is used to match the shape of device DC characteristics successfully by including separately measured parameters and adjusting other unknowns such as injection efficiency. A more complete model is possible by adding effects such as surface and bulk recombination through the emitter injection efficiency term and tunneling through an effective spike height. The physical effects resulting in collector-emitter offset voltages are also fully described and good agreement with experimental results is demonstrated