This paper presents the results of a simulation study focused on the evaluation of the fundamental DC and high-frequency characteristics of a GaAs-based heterojunction bipolar transistor employing a wide gap emitter made of hydrogenated amorphous silicon (a-Si:H). The role of the fundamental geometric design parameters on the device performance is analysed, showing in particular that the emitter thickness has a strong impact on the DC current gain, while the base thickness weakly affects the cut-off frequency. The role of the electronic properties of the thin film amorphous emitter is also discussed, leading to the conclusion that the device AC characteristics are in Fact principally limited by the poor carrier mobility typical of a-Si:H. However, a DC current gain of 4000 and a cut-off frequency close to 10 GHz can be predicted for an optimised device if standard values for the electronic parameters of the materials are assumed.