Numerical simulations of AlGaAs/GaAs HBT's with various collector parameters are made to investigate the cutoff frequency characteristics by 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 fT. It is found that the importance of energy transport effects arises from the fact that the actual electron energy deviates largely from the field-determined one. The velocity overshoot can occur in a graded band-gap base and in the collector depletion layer, resulting in much higher fT than that predicted by the conventional model. The energy relaxation time is found to be an important parameter to determine an achievable fT. It is also found that the thickness of the velocity overshoot region existing in the collector depletion layer is rather thin (− 1000 A). Therefore, the enhancement of predicted fT by including the energy transport effects is more pronounced for thinner n-layer. A value of fT higher than 140 GHz is expected for an HBT with an n -layer thickness of 1000 A.
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