Surface recombination in GaAlAs/GaAs heterostructure bipolar transistors

Experimental evidence shows that surface recombination in npn GaAlAs/GaAs heterostructure bipolar transistors changes from an exp(qV/2kT) dependence at low currents to an exp(qV/kT) dependence at high currents. Using a one-dimensional analysis, Henry, Logan, and Meritt [J. Appl. Phys. 49, 3530 (1978)] have derived a 2kT dependence for surface recombination in the presence of Fermi-level pinning and flat quasi-Fermi levels between the surface and the bulk. We find that the kT dependence, which dominates at high bias, arises when the recombination rate is limited by availability of minority carriers at high surface recombination velocities. The quasi-Fermi levels do not remain flat at high surface recombination velocities. Two-dimensional effects are also shown to be strong and influential in limiting the availability of carriers.