On the thermionic-diffusion theory of minority transport in heterostructure bipolar transistors

The theory of the minority-carrier transport in heterostructure bipolar transistors (HBT) is reconsidered with a particular emphasis on the difference between the cases of abrupt and graded emitter-base junctions and the role in the former case of the quasi-Fermi level discontinuity at the interface. Exact analytical formulas are derived for the current-voltage characteristics of a double-heterojunction HBT, valid for arbitrary levels of injection and base doping, including the degenerate case. The theory is applied to the static characterization of HBT which 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-injection limit if both the emitter-base and the collector-base diodes have ideality factors close to unity. The ratio of base currents in the reverse and forward modes of operation can be used to determine the abrupt emitter-base conduction band discontinuity and estimate the scattering length in the base