We compare the basic detector structures used in receivers for optical communication system applications. The three types of detectors most commonly considered for the direct detection of digital signals are the photoconductor, the p-i-n detector and the avalanche photodiode (APD). A comparison of these structures from a noise (and hence, sensitivity) and bandwidth perspective indicates that reversed biased detectors such as p-i-n photodiodes and APDs afford the greatest advantages, although the latter structure can be extremely difficult to fabricate.
S. R. Forrest, S. R. Forrest,
"Optical Detectors for Long-Wavelength Lightwave Communication", Proc. SPIE 0722, Components for Fiber Optic Applications, (14 January 1987); doi: 10.1117/12.937664; https://doi.org/10.1117/12.937664