Electron velocity overshoot in the collector depletion layer of AlGaAs/GaAs HBT's

Electron velocity in the collector depletion layer of AlGaAs/ GaAs heterojunction bipolar transistors (HBT's) is characterized by analyzing the cutoff frequencies. The f
t
value of tested HBT's was 40 GHz at a collector voltage V
CE
of 2 V, and it significantly decreased to 27 GHz at V
CE
= 5 V. A conventional model assuming a constant electron velocity in the collector depletion region does not account for this f
t
variation. A new simple model composed of a velocity overshoot region and a saturation velocity region is proposed and it successfully explains the variation.