Current Gain Enhancement in Graded Base AlGaAs/GaAs HBTs Associated with Electron Drift Motion

The temperature dependence of current gain in AlGaAs/GaAs heterojunction bipolar transistors with a graded bandgap base (GB-HBTs) is described. High current gain up to 1100 has been achieved at a collector current density of 6×103 A/cm2 in MBE grown GB-HBTs with a 1×1019 cm-3 doped base. The current gain in the low temperature region does not depend on temperature. This tendency is clearly explained by a nearly constant base transit time due to electron drift motion caused by the built-in field, which contrasts with the electron diffusion.