Current Gain-Early Voltage Product in SiGe Base HBT´s with Thin θ-Si:H Emitters

SiGe heterojunction bipolar transistors (HBTs) with very thin n⁺ hydrogenated amorphous Si (α-Si:H) emitters and various doping and Ge distribution profiles in the bases are reported. An analytical model defining the leverage of the structures in terms of current gain and high Early voltage is presented and verified experimentally. Devices having a base doping concentration of 1??10¹⁹cm^-3, a base Ge gradient of 0 to 20% and an emitter size of 2??4 μm² exhibited a current gain of 100, a current gain-Early voltage product larger than 11000, and a unity gain cutoff frequency fT of 14.5GHz.