Controlling transient enhanced diffusion effects in high-frequency Si_0.7Ge_0.3, heterojunction bipolar transistors with implanted emitters

A method is presented by which the implantation damage induced transient enhanced diffusion of boron and phosphorus is used to advantage to fabricate high-frequency Si_0.7Ge_0.3 heterojunction bipolar transistors with implanted phosphorus emitters. A device with 300 A basewidth, 7 kΩ/□ intrinsic base sheet resistance, and f_T=44 GHz is demonstrated. The very low diffusivity of both boron and phosphorus in high Ge concentration SiGe during 700°C thermal annealing results in a self-alignment of the emitter-base junction to the Si/SiGe interface, giving high current gain with good reproducibility. Methods of counteracting the boron outdiffusion in the Si collector are considered. Rather than reducing this outdiffusion, results indicate that it can be better compensated for by introducing a narrow phosphorus doped peak, epitaxially grown on the collector side of the SiGe. The n doping of this peak, as is also the case for that of the emitter, barely penetrates the SiGe and the p-doped base is well preserved.