Transconductance Degradation in Near-THz InP Double-Heterojunction Bipolar Transistors

We examine the relationship between transconductance gm and emitter current density JE for InP/InGaAs/InP abrupt emitter-base (EB) double-heterojunction bipolar transistors operating at high JE. High JE is needed to increase gm for reduced C/gm delays. We observe a significant degradation in measured gm below qIE/kT with increased JE. This degradation primarily results from the Fermi-Dirac statistics governing current injection at high current densities and from quantum-mechanical reflection at the EB junction arising from changes in the electron effective mass and in the conduction band potential. Transconductance is further reduced by gradients in the quasi-Fermi level in the EB space-charge region and by modulation of the heterointerface energy barrier by the applied bias.