In this work the properties of an a-Si:H/GaAs heterojunction are discussed and the analysis of advantages that may result from its use in bipolar devices compatible with the GaAs homojunction technology is performed. Experimental and theoretical results are presented concerning the application of a wide gap amorphous silicon layer to improve the injection efficiency into GaAs regions. Fundamental DC and high frequency characteristics of an a-Si:H/GaAs heterojunction bipolar transistor (HBT) are investigated through detailed numerical simulations. The electronic properties of the a-Si:H layer, as the distributed density of states typical of amorphous materials, have been carefully considered. The tuning of the simulator and the reliability test of its output have been performed on experimental results obtained through the fabrication of a-Si:H/GaAs p-i-n diodes. The study shows that limiting the number of defects located at the amorphous/crystalline interface below a critical level would dramatically improve the minority carrier injection ratio at the heterojunction. 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 close to 10 GHz. Due to the simplicity of fabrication, such a device could represent an effective way to add a bipolar stage to a GaAs MESFET IC without recurring to AlGaAs/GaAs heterostructures.