We have fabricated a heteroemitter that consists of a microcrystalline silicon (mu c-Si) and a very thin carbon-doped amorphous crystalline silicon (a-SiCx) double layer which is optimized for high emitter efficiency and high-speed transistor action. The emitter layer was deposited using electron cyclotron resonance, plasma-enhanced chemical vapor deposition. Critical factors for the optimization are treatments of the substrate surface before deposition of the emitter layer and the temperature and SiH4/CH4 gas ratio during deposition of the very thin a-SiCx layer. The emitter efficiency of transistors using the heteroemitter is two to three times higher than that of conventional polysilicon emitter bipolar transistors, and the emitter resistance of the transistors is reduced to only three times that of conventional transistors. A self-aligned silicon heterojunction bipolar transistor (SiHBT) using the heteroemitter demonstrated a cutoff frequency, f(T), of 47 GHz,which is almost two times larger than the highest value previously reported for SiHBTs using hydrogenated mu c-Si or mu c-SiCx as an emitter.