Functional heterostructure-emitter bipolar transistor (HEBT) with graded-confinement and pseudomorphic-base structure

In this paper, a new functional heterostructure-emitter bipolar transistor (HEBT) with a graded Al_xGa_1-xAs confinement layer and a pseudomorphic InGaAs/GaAs base structure is fabricated and demonstrated. Due to the insertion of an InGaAs quantum well (QW) between the emitter-base (E-B) junction, the valence band discontinuity can be enhanced and a high emitter injection efficiency may be achieved. Furthermore, the potential spike of the graded Al_xGa_1-xAs/GaAs heterojunction is expected to be smoothed out which results in a lower offset voltage. The excellent transistor characteristics include a high current gain of 120 and a low offset voltage of 100 mV. In addition, an interesting S-shaped multiple negative differential resistance (MNDR) phenomenon is observed in the inverted operation mode. This may be attributed to an avalanche multiplication and a sequential two-stage barrier lowering effect resulting from the accumulation of holes at the base and electrons at the InGaAs QW, respectively. Consequently, owing to the remarkable transistor performance and MNDR characteristics, the studied HEBT shows good promise for applications in amplifiers and multiple-valued logic circuits.