High-gain, high-frequency AlGaAs/GaAs graded band-gap base bipolar transistors with a Be diffusion setback layer in the base

AlGaAs/GaAs heterojunction bipolar transistors (HBT's) with graded band-gap bases were fabricated from computer-controlled molecular beam epitaxy layers. It was found that the use of an undoped setback layer of 200-500 Å to offset Be diffusion in the emitter resulted in significant current gain increases. Maximum current gains of 1150 for a base width of 0.18 μm were obtained which are the highest yet reported for graded base HBT's. Zn diffusion was used to contact the base and provides a low base contact resistance. Microwave s-parameter measurements yielded f_T=5 GHz and f _max=2.5 GHz. Large signal pulse measurements resulted in rise times of τ_r∼150 ps and pulsed collector currents of I _c>100 mA which is useful for high current laser drivers.