Study on collector design of AlGaAs/GaAs heterojunction bipolar transistors by two-dimensional simulation

The cutoff frequency (f_T) characteristics of NPN^-n⁺ AlGaAs/GaAs HBTs (heterojunction bipolar transistors) with various n^- collector structures are studied by two-dimensional simulation. It is shown that the transit time in the collection depletion layer becomes a more important factor than the collector charging time in the high current region. Therefore, a thinner n^- collector layer with higher doping density is desirable for achieving higher cutoff frequency. The introduction of a semi-insulating external collector is effective in improving cutoff frequency characteristics in the relatively low current region. However, it is noted that it may lead to the earlier fall of cutoff frequency due to a high injection effect