Abstract
The effects of extrinsic base surface passivation and base quasi‐electric field on the current gain of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) grown upon GaAs and Si substrates are compared. The degree of improvement in the current gain realized by either the base field and/or passivation depends strongly on which substrate is used. It is also shown that base‐emitter junction space‐charge recombination current is a significant base current component for all devices, even at high current levels. The current gain for passivated devices with a base field of 1.25×104 V/cm achieves a maximum value of 1100 for a HBT‐on‐GaAs, and 100 for a HBT‐on‐Si.