The base current densityJBis an important parameter in determining the common‐emitter current gain &bgr; of heterojunction bipolar transistors (HBTs). To develop an analytical &bgr; model with which a circuit designer can quickly estimate the current gain in a HBT, it is also important to identify the dominant component ofJBso that minimum computations are required. Based on heterojunction device physics, the three components ofJBhave been calculated, namely, the recombination current density in the baseJRB, the recombination current density in the space‐charge regionJSCR, and the injection current density from the base to the emitterJRE, and have determined their relative importance toJBfor 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 statesNtIat the emitter‐base heterointerface, but weakly on the density of trapping statesNtBin the bulk of the emitter‐base space‐charge region. Also,JBis relatively insensitive to device makeup such as doping concentration and layer thickness. Depending onNtIand on the bias condition, eitherJSCRorJREis the dominant component for AlGaAs/GaAs HBTs and eitherJSCRorJRBis the dominant component for InAlAs/InGaAs HBTs. Effects of base and heterojunction grading on the present findings are also addressed.