Numerical simulation of AlGaAs/GaAs heterojunction bipolar transistors with various collector parameters

Numerical simulations of AlGaAs/GaAs HBTs (heterojunction bipolar transistors) with various collector parameters are carried out to investigate the cutoff frequency characteristics, using the conventional static model and the energy transport model. It is shown that the transit time in the collector depletion layer is an intrinsically more important factor than the collector charging time. Therefore, a thinner n^--layer with higher doping density is desirable to achieve higher cutoff frequency, f_T. It is found that the importance of energy transport effects arises from the fact that the actual electron energy deviates strongly from the field determined energy. The velocity overshoot can occur in a graded bandgap base and in the collector depletion layer, resulting in much higher f_T than that predicted by the conventional model. A value of F _T higher than 140 GHz is expected for an HBT with an n ^--layer thickness of 1000 Å