Effect of emitter design on the dc characteristics of InP-based double-heterojunction bipolar transistors

In this paper we compare the characteristics of InP-based double-heterojunc tion bipolar transistors (DHBTs) with different emitter layer designs. The offset and saturation voltages can be significantly reduced by inserting an InP layer between a large-bandgap InAlAs emitter and a small-bandgap InGaA s base layer. The current gains of the composite-emitter (InAlAs/InP) DHBTs are higher than those of conventional InAlAs and InP DHBTs, especially at low cut-rents. However, if the InP layer is too thick, the current gain dec reases, presumably due to the suppression of quasi-ballistic transport.