Linear theory of the quasi-unipolar photodiode

Quasi-unipolar (QU) operation, in which unequal numbers of electrons and holes participate in the photocurrent, is proposed as a strategy for optimizing III-V photodetector bandwidth. Analytic expressions are derived for the transport of photogenerated charge, based on a linearization of the first two moments of the Boltzmann transport equation. Microscopic charge and current densities are shown to imply an equivalent circuit model for the QU detector, for which the familiar heterojunction p-i-n and unitraveling-carrier detectors are limiting cases. Simulations demonstrate that a maximum 3-dB bandwidth may be achieved via a QU rather than a purely bipolar or unipolar operation and without penalty to junction capacitance or quantum efficiency.