The influence of buffer layer thickness on the dc performance of GaAs/AlGaAs heterojunction bipolar transistors (HBT's) grown on silicon substrates by molecular beam epitaxy (MBE) was investigated. Current gain, collector-emitter breakdown voltage, emitter-base and collector-base diode ideality factors, and breakdown voltages were measured as the buffer layer thickness was varied between 0 and 5μm. The current gain steadily increases with increasing buffer layer thickness until the layer reaches 3 μm. However, the other dc parameters are relatively insensitive to the buffer layer thickness. A small-signal current gain of 60 is typically achieved for devices with 6 x 6-μm2 emitters at a current density of 6 x 104 A/cm2 when the buffer layer is a ≥3 μm.