Fundamentals of Noise in Optoelectronics

Electromagnetic radiation can be described as a stream of individual photons. In solid-state detectors (e.g., photocathodes or semiconductor photosensors), each photon of sufficient energy creates one or several mobile charge carriers which can be subsequently detected with sensitive electronic circuits, possibly after charge packet multiplication employing the avalanche effect. Two types of noise limit the resolution with which individual photons can be detected: (1) The number of detectable photons or photoelectrons shows a statistical variation, which is often well-described as a Poisson distribution. (2) Photogeneration of mobile charge carriers competes with thermal generation, and thermal noise is compromising the generation of photocharges (“dark current”) as well as the performance of electronic charge detection circuits (“Johnson noise” and “random telegraph signal noise”). It is shown that the laws of physics and the performance of today’s semiconductor fabrication processes allow the detection of individual photons and photocharges in image sensors at room temperature and at video rate.