A study of low-bias photocurrent gradient of avalanche photodiodes

Presents a study of the photo-response of avalanche photodiodes (APDs) under low reverse bias at unity gain. Some wafers exhibit a linear increase of photocurrent as a function of bias voltage; the "slope" of this linear current-voltage (I-V) curve has been examined under different illumination conditions, fabrication procedures and growth conditions. The observed characteristics are explained in terms of the extension of the depletion region edge toward the surface. An analytical model, based on minority drift-diffusion and current continuity equations, provides good fits to the experimental results for small (/spl sim/0.1 /spl mu/m) "effective diffusion lengths.".