Noise Figure, Noise Factor and Noise Temperature are all figures of merit to evaluate the sensitivity of a given system to random and uncorrelated fluctuations adding to the signal of interest. There are two mechanisms that contribute to the total noise of a system: charge carrier velocity fluctuations (thermal noise) and fluctuations in the number of charge carriers (shot noise). Both thermal and shot noise are purely random and have a Gaussian amplitude distribution. Generally, noise power per unit bandwidth is uniform with frequency but excess (or flicker or 1/f) noise does not have a uniform power spectral distribution. Shot noise can become dependent on frequency when the period of the signal approaches the charge carrier transit time within the device.

The Noise Factor is the ratio of the signal-to-noise ratio at the input to the signal-to-noise ratio at the output (SNRin/SNRout).

The Noise Figure (dB) is 10*Log_{10}(Noise Factor).

The Noise Temperature (K) is 290*(Noise Factor -1).