A photodiode made on high-resistivity silicon (greater than 100 Ω-cm) is characterized by a relatively wide junction-depletion layer. As a consequence, for most of the spectral response region the optical generation of charge carriers occurs within the depletion layer and the separation and collection of charge is primarily controlled by electric field rather than by diffusion. Short collection times thus obtained are estimated to be in the range from 10 to 100 nsec and have been shown to be 200 nsec or less. By choice of sufficiently high resistivity and moderate reverse biases the device capacitance can be kept smaller than circuit stray capacitances.
Low values of saturation current have been obtained resulting in high diode sensitivities. Best measured noise-equivalent-power values of 2×10-13 W at 0.9 µ are about a factor 2 short of theoretical values.
R. L. WILLIAMS, "Fast High-Sensitivity Silicon Photodiodes," J. Opt. Soc. Am. 52, 1237-1244 (1962)