Evaluation of a Specially Designed GaAS Schottky-Barrier Photodiode Using 6328-Å Radiation Modulated at 4 GHz

Abstract

This paper describes a method of evaluating the high frequency signal and noise performance of photodiodes suitable for mixers in an optical heterodyne receiver. The method uses a single laser beam modulated at a high frequency, 4 GHz, to simulate the signal and local oscillator in a conventional optical receiver thus avoiding the problem of aligning two separate optical beams; the results obtained can be applied to the design of an optical heterodyne receiving system. A new Schottky-barrier gallium arsenide photodiode is described, and measurements of a typical diode of this type are used as an example for comparison with theory. The photodiodes employed are of the same basic gold-dot epitaxial gallium arsenide type that has previously been used for millimeter wave and microwave mixers, but they have been modified to allow for the introduction of an optical beam from beneath the gold-dot contact area. A typical diode has a barrier capacitance of 0.027 pF, a series resistance of 30 Omega, and an effective quantum efficiency of 40%, which could be improved to perhaps 60% by an antireflection coating on the photodiode surface.