What is thermal noise? Thermal noise in the resistance of the signal source is the fundamental limit on achievable signal sensitivity is unavoidable, and generated by the random thermal motion of charge carriers (usually electrons), inside an electrical conductor, which happens regardless of any applied voltage. Thermal noise is approximately white, meaning that its power spectral density is nearly equal throughout the frequency spectrum. Noise caused by thermal interaction between free electrons and vibrating ions in a conductor.

The root mean square (RMS) voltage due to thermal noise, generated in a resistance R (ohms) over bandwidth Δf (hertz), is given by

Shot Noise Shot noise is a type of electronic noise which originates from the discrete nature of electric charge. Shot noise is due to the random arrivals of electron packets at the potential barrier of forward biased P/N junctions. The Shot noise is current arising from the random generation and flow of mobile charge carriers. The input power, ideally assumed to be constant, which means the number of photons per unit of time, on average is constant. Shot noise is deviation of the actual number of electrons from the average number

random changes (fluctuations) in voltage and current relative to their mean value in the circuits of amplifiers, radios, and other electronic devices; caused by the shot effect in vacuum electronic and semiconductor instruments. For example, shot noise appears as acoustic noise in the loudspeaker of a radio, “snow” on a television screen, and “grass” on a radar screen. Shot noise is the basic component of the internal noises of electronic devices, which cause the distortion of weak signals and limit the sensitivity of amplifiers.

The noise amplitude is represented by the rms value:

Dark Current Noise

dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons are entering the device. It is referred to as reverse bias leakage current in non-optical devices and is present in all diodes. Physically, dark current is due to the random generation of electrons and holes within the depletion region of the device that are then swept by the high electric field. The charge generation rate is related to specific crystallographic defects within the depletion region. Dark-current spectroscopy can be used to determine the defects present by monitoring the peaks in the dark current histogram's evolution with temperature.

Dark current is one of the main sources for noise in image sensors such as charge-coupled devices. The pattern of different dark currents can result in a fixed-pattern noise; dark frame subtraction can remove an estimate of the mean fixed pattern, but there still remains a temporal noise, because the dark current itself has a shot noise.