Investigation of GaAs homojunction bipolar transistor with delta doping emitter structure

A GaAs homojunction bipolar transistor with a delta doping emitter structure is proposed and demonstrated. The proposed device makes use of a delta doping structure inducing a triangular barrier for minority carrier confinement, resulting in a high emitter injection efficiency. Based on the minority carrier transport in the bulk emitter region with drift-diffusion mechanism, and in the triangular barrier region with tunnelling and thermionic-emission mechanisms, an analytical derivation of current-voltage characteristics, including the effect of bandgap shrinkage, was obtained. The calculated results show that the triangular barrier is the key parameter in determining the electrical properties. In addition, due to the absence of heterojunction, the proposed device exhibits more nearly constant current gain with collector current than for the AlGaAs/GaAs heterojunction bipolar transistor (HBT). The proposed device, grown by molecular beam epitaxy, shows a differential current gain of 13 and an offset voltage of 60mV at a base-to-emitter doping ratio of 10. The offset voltage is attributed mostly to the geometric limits. With a simple chemical treatment of Na₂S·9H₂O, the differential current gain is enhanced to be 16 due to the reduced surface recombination. Theory and experiment indicate the potential application of the proposed device.