A general model for the avalanche photodiode is presented. It is shown that the diode consists of four regions: 1) guard ring, 2) uniform avalanche region, 3) high-field absorption region and 4) zero-field absorption region. Expressions are given for the ac quantum efficiency, the dc quantum efficiency, and the transit time cutoff frequency. Material requirements are discussed. Based on an entire detector system, an expression is derived for the signal-to-noise ratio. An example is given with the result that a noise-equivalent power (NEP) of 10-12W/Hz1/2is obtained with an optimum avalanche gain of approximately 23.