ON THE THERMIONIC-DIFFUSION THEORY OF MINORITY TRANSPORT IN HETEROSTRUCTURE BIPOLAR-TRANSISTORS

Theory of the minority-carrier transport in heterostructure bipolar tr ansistors (HBT) is reconsidered with a particular emphasis on the diff erence between the cases of abrupt and graded emitter-base junctions a nd the role in the former case of the quasi-Fermi level discontinuity at the interface. Exact analytical formulas are derived for the curren t-voltage characteristics of a double-heterojunction HBT, valid for ar bitrary levels of injection and base doping, including the degenerate case. The theory is applied to the static characterization of HBT whic h compares the forward and reverse dependences I(C)(V(EB)) and I(E)(V( CB)). It is shown that these characteristics coincide in the low-injec tion limit, if both the emitter-base and the collector-base diodes hav e ideality factors close to unity. The ratio of base cur rents in the reverse and forward modes of operation can be used to determine the ab rupt emitter-base conduction band discontinuity and estimate the scatt ering length in the base.