In this study, various N-p-n heterojunction bipolar transistors (HBT) structures with high C-doped bases, grown by metal-organic chemical vapour deposition (MOCVD), have been fabricated with identical geometry and processing steps. Measured results show that in addition to the intrinsic heterojunction emitter injection efficiency, the base bulk recombination plays an important role in maintaining the current gain at high temperature. Furthermore, it is shown that emitter/base space-charge region recombination has a significant detrimental effect on the variation of current gain with increasing temperature. A theoretical model is presented which predicts well the current gain variation with temperature for both AlGaAs/GaAs and InGaP/GaAs HBTs.

The temperature dependencies of current gain are investigated in both AlGaAs/GaAs and InGaP/GaAs HBTs. Various Npn HBT structures with high C-doped bases, grown by MOCVD, have been fabricated with identical processing steps. Measured results show that base bulk recombination current plays an important role in maintaining the current gain at high temperature. In addition it is shown that both space-charge and surface recombination currents are the cause of current gain reduction with temperature.

The improvement in the emitter-base leakage current of HBTs has been investigated by the use of an InGaP emitter InGaP/GaAs npn HBT structures with high C-doped bases. grown by MOCVD, have been fabricated and these devices show Gummel plots with near ideal I-V characteristics (n(c) = 1.00 and n(b) = 1.09). Measured current gain remains relatively flat over five decades of collector current and its magnitude is greater than unity at collector current as low as 0.1 muA. The characteristics of these HBTs were compared with fabricated AlGaAs/GaAs HBTs having similar device structure. The superior performance of the InGaP emitter HBT is demonstrated.