CALCULATION OF THE BASE CURRENT COMPONENTS AND DETERMINATION OF THEIR RELATIVE IMPORTANCE IN ALGAAS/GAAS AND INALAS/INGAAS HETEROJUNCTION BIPOLAR-TRANSISTORS

The base current density J(B) is an important parameter in determining the common-emitter current gain beta of heterojunction bipolar transistors (HBTs). To develop an analytical beta model with which a circuit designer can quickly estimate the current gain in a HBT, it is also important to identify the dominant component of J(B) so that minimum computations are required. Based on heterojunction device physics, the three components of J(B) have been calculated, namely, the recombination current density in the base J(RB), the recombination current density in the space-charge region J(SCR), and the injection current density from the base to the emitter J(RE), and have determined their relative importance to J(B) for abrupt AlGaAs/GaAs and InAlAs/InGaAs HBTs under normal bias conditions. It is found that relative importance of the three current densities depends strongly on the bias condition, strongly on the density of states N(tI) at the emitter-base heterointerface, but weakly on the density of trapping states N(tB) in the bulk of the emitter-base space-charge region. Also, J(B) is relatively insensitive to device makeup such as doping concentration and layer thickness. Depending on N(tI) and on the bias condition, either J(SCR) or J(RE) is the dominant component for AlGaAs/GaAs HBTs and either J(SCR) or J(RB) is the dominant component for InAlAs/InGaAs HBTs. Effects of base and heterojunction grading on the present findings are also addressed.