Two-dimensional simulations of AlGaAs/GaAs HBTs with various collector structures were performed. The introduction of a semi-insulating external collector improves cutoff frequency fT in the low current region because the base-collector capacitance decreases. However, it leads to an earlier fall of fT in the high current region, because a high injection effect is enhanced. This high injection effect has two features: (1) expansion of the collector depletion layer is remarkable near the interface between the intrinsic collector and the semi-insulating external collector, resulting in a longer transit time in this region; and (2) the effective channel in the intrinsic collector becomes narrow by introducing the semi-insulating external collector and so the effective current density there becomes high, leading to an earlier appearance of the high injection situation. It is shown that to minimize these unfavourable high injection effects, the semi-insulating layer should be slightly away from the intrinsic collector region so that it may not affect electron transport in the intrinsic collector region.
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