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 MBE layers. It was found that the use of an undoped setback layer of 200-500angstroms to offset Be diffusion in the emitter resulted in significant current gain increases. Maximum current gains of 1150 for a base width of 0.18micron 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 T(r) ~ 150 ps and pulsed collector currents of I(c) > 100 mA which is useful for high current laser drivers.