Low-temperature characterization of high-current-gain graded-emitter AlGaAs/GaAs narrow-base heterojunction bipolar transistor

Narrow-based heterojunction bipolar transistors (NBHBTs) in the AlGaAs/GaAs material system, with a nominal base thickness of 50 AA, exhibit maximum small-signal common-emitter current gains of 1400 at 300 K and 3000 at 80 K. The performance of the device is attributed to the superlattice graded-emitter contact and a novel planar base access fabrication process. Low-temperature measurements indicate that the maximum current gain increases exponentially with decreasing temperature until it saturates around 200 K, suggesting a tunneling-limited current transport mechanism.>