Pseudo-HBT with polysilicon emitter contact and an ultrashallow highly doped bases by photoepitaxy

It is noted that reducing the base width to below 50 nm and increasing the base concentration to above 10/sup 19/ cm/sup -3/ are the key to high-speed pseudo-HBT (heterojunction bipolar transistor) operation at 77 K. To overcome the limitation on the base concentration due to tunneling, an emitter was fabricated with a reduced impurity concentration (4.4*10/sup 18/ cm/sup -3/) and a 55-nm ultrathin heavily doped base using a photoepitaxial growth technique. The device was free from tunneling current despite a base concentration of 1.2*10/sup 19/ cm/sup -3/. Current gain degradation at low temperatures was less than that of conventional devices. Heavily doped polysilicon between the medium-doped emitter layer and metal electrode improves the pseudo-HBT´s current gain by about three times at 300 K and by 1.5 times at 77 K. The transit time due to this polysilicon layer, although large at 300 K, is negligible at 77 K.<>