High Frequency Characteristics of InAlAs/InGaAs HBT's

InAlAs/InGaAs heterojunction bipolar transistors (HBT's) grown by molecular beam epitaxy are fabricated. A cutoff frequency ( f T ) of 52 GHz is achieved in the HBT's with a base thickness of 1000 Å. A decrease in f T under a high collector current density condition is observed. This is attributed both to the base widening and base-collector capacitance increase caused by the electron space charge in the collector depletion layer.