Design considerations for a-Si:H/SiGe/Si heterojunction bipolar transistors

A SiGe/Si heterojunction bipolar transistor (HBT) with a thin n⁺ hydrogenated amorphous silicon (a-Si:H) emitter is discussed, in particular with reference to the dc current gain, by means of numerical simulations. The role of the fundamental geometric design parameters on the device performance is analyzed for a set of devices withstanding the same maximum emitter-collector voltage. It is shown that the emitter thickness has a minor effect on the device current gain, which is instead strongly influenced by the base thickness. However, due to the poor carrier mobility typical of a-Si:H, the total current handled by the device strongly depends on the emitter thickness and resistance. A dc current gain close to 10000 can be predicted for optimized a-Si:H/SiGe HBTs with a thin emitter and a narrow highly doped base.