Atlanta Fiber System Experiment: Planar Epitaxial Silicon Avalanche Photodiode

A silicon avalanche photodiode (APD) has been developed for optical fiber communications systems. It has been optimized for optical wavelengths of 800 to 850 nm and exhibits a quantum efficiency greater than 90 percent. The APD operates between typical voltages of 100 and 400 V, exhibiting photocurrent gains of approximately 8 and 100, respectively, at those biases. The device has a short response time of ∼ 1 ns and low excess noise characterized by an excess noise factor approximately 5 times the shot noise limit for operation at a photocurrent gain of 100. The APD has a four-layer n+-p-π-p+structure and is fabricated on large-diameter epitaxial wafers using planar technology. Uniform avalanche gain, low dark currents, and good reliability are achieved through the use of (i) a diffused guard ring, (ii) a diffused channel stop, (iii) metal field plates, (iv) the removal of impurities in the surface oxides and the bulk of the APD, (v) passivation with silicon nitride and (vi) a processing sequence that maintains low dislocation density material.