A study of frequency response in silicon heterojunction bipolar transistors with amorphous silicon emitters

A detailed physical model of amorphous silicon (a-Si:H) is incorporated into a two-dimensional device simulator to examine the frequency response limits of silicon heterojunction bipolar transistors (HBT's) with a-Si:H emitters. The cutoff frequency is severely limited by the transit time in the emitter space charge region, due to the low electron drift mobility in a-Si:H, to 98 MHz which compares poorly with the 37 GHz obtained for a silicon homojunction bipolar transistor with the same device structure. The effects of the amorphous heteroemitter material parameters (doping, electron drift mobility, defect density and interface state density) on frequency response are then examined to find the requirements for an amorphous heteroemitter material such that the HBT has better frequency response than the equivalent homojunction bipolar transistor, We find that an electron drift mobility of at least 100 cm/sup 2/ V/sup -1/ s/sup -1/ is required in the amorphous heteroemitter and at a heteroemitter drift mobility of 350 cm/sup 2/ V/sup -1/ s/sup -1/ and heteroemitter doping of 5/spl times/10/sup 17/ cm/sup -3/, a maximum cutoff frequency of 52 GHz can be expected.