ABSTRACT
About twenty years ago, anomalies in the current gain of bipolar transistors resulting from a heavily doped emitter were measured. The mechanisms responsible for these supplementary differences, called heavy doping effects(HDE), found an increasing interest. Heavy doping effects change the bandstructure of a semiconductor in two particular aspects. First, the bandgap which is a characteristic quantity for each crystalline material, narrows. This effect is called bandgap narrowing (BGN). Second, HDE introduce energy levels in the forbidden energy zone resulting in a modification of the density of states (DOS). The latter effect is known as bandtailing. The electrical effect - correlation effect - mainly consists in a screening of the charges in the plasma. The exchange effect distinguishes the one-electron approximation from the many-body description: non-interacting identical particles can be interchanged without altering the system while interacting identical fermions modify the system’s energy by the exchange energy.
