Characterization of InGaP/GaAs heterojunction bipolar transistors with a heavily doped base

Characteristics of InGaP/GaAs heterojunction bipolar transistors (HBTs) with a heavily doped base are examined at the base doping level N-B ranging from 5 x 10(19) to 5 x 10(20) cm(-3). At N-B of less than 3 X 10(20) cm(-3), the current gain is mainly determined by Auger recombination in the intrinsic base region and is inversely proportional to the square of N-B. In contrast, the current gain at N-B above 3 x 1020 cm(-3) is significantly decreased. We evaluated the effective barrier height of holes between the emitter and the base by measuring temperature dependence of current gain, and found that the effective hole barrier is reduced as N-B increases. This result is explained by the large energy shift of the Fermi level inside the valence band due to heavy doping, causing the increase in the back injection of holes into the emitter, and thus reducing the current gain.