Three main noise mechanisms are attracted most of the attention: shot, flicker, thermal noise.
The figure helps explain a lot. Thermal noise is generated by the thermal agitation of the charge carriers inside an electrical conductor at equilibrium, which happens regardless of any applied voltage (Wikipedia). In other words, a low-temperature environment could reduce the thermal noise in the circuit. So some sensitive electronic equipment such as radio telescope receivers are cooled to cryogenic temperatures to reduce thermal noise in their circuits. Thermal noise in an ideal resistor is approximately white, meaning that the power spectral density is nearly constant throughout the frequency spectrum. When limited to a finite bandwidth, thermal noise has a nearly Gaussian amplitude distribution. The valuable kT/C noise consideration is also caused by the switching resistor instead of the sampling capacitor because there is no ideal switch without on-resistance.
Short noise or Poisson noise is a type of electronic noise which can be modeled by a Poisson process (Review
6 common probability distributions). For CMOS imager design, short noise needs to be concerned, because it is associated with the particle nature of light.
Photon noise simulation. The number of photons is increased from left to right and from top to bottom.
Flicker noise, 1/f noise, or pink noise, is a type of electronic noise with a 1/f power spectral density. Flicker noise is often characterized by the corner frequency fc between the region dominated by the low-frequency flicker noise and the higher-frequency flat band noise. MOSFETs have a higher fc than JFETs or bipolar transistors, which is usually below 2kHz for the later.
