High-frequency characteristics of AlGaAs/GaAs heterojunction bipolar transistors

The fabrication and high-frequency performance of MBE-grown AlGaAs/GaAs heterojunction bipolar transistors (HBT's) is described. The achieved gain-bandwidth product f
T
is 25 GHz for a collector current density J
c
of 1 × 10
⁴
A/cm
²
and a collector-emitter voltage V
CE
of 3 V.f
T
continues to increase with the collector current in the high current density region over 1 × 10
⁴
A/cm
²
with no emitter crowding effect nor Kirk effect. The limitation on f
T
in fabricated devices is found to be caused mainly by the emitter series resistance.