Design of a-Si:H/GaAs heterojunction bipolar transistors with improved DC and AC characteristics

Results from a detailed simulation study are presented to help evaluate the pros and cons of a wide-gap hydrogenated amorphous silicon emitter (a-Si:H) introduced to improve the injection efficiency at the emitter-base junction of a GaAs bipolar transistor. For a set of devices withstanding the same maximum emitter-collector voltage, it is shown that the current gain, besides a net dependence on the defect state concentration at the emitter-base interface, is also strongly influenced by the base thickness and doping. The base thickness, however, has a weak impact on the cut-off frequency, which in turn shows a clear dependence on the emitter electron mobility. The study predicts that the current thin film silicon technology would allow the fabrication of a transistor performing a DC current gain close to 3000 and a cut-off frequency in excess of 15 GHz. Owing to the simplicity of fabrication, such a device could represent an effective way of adding a bipolar stage to a GaAs MESFET IC without resorting to AlGaAs/GaAs heterostructures.