Flicker Noise

Flicker noise is a type of electronic noise with a 1/f, or pink power density spectrum. It is therefore often referred to as 1/f noise or pink noise, though these terms have wider definitions. It occurs in almost all electronic devices, and can show up with a variety of other effects, such as impurities in a conductive channel, generation and recombination noise in a transistor due to base current, and so on. 1/f noise in current or voltage is always related to a direct current because it is a resistance fluctuation, which is transformed to voltage or current fluctuations via Ohm's law. In mechanics, it was found in the earth’s rate of rotation, undersea currents and the hourglass flow of sand fluctuations.

In electronic devices, it shows up as a low-frequency phenomenon, as the higher frequencies are overshadowed by white noise from other sources. In oscillators, however, the low-frequency noise can be mixed up to frequencies close to the carrier which results in oscillator phase noise.

Flicker noise is often characterized by the corner frequency f_c between the region dominated by the low-frequency flicker noise and the higher frequency "flat-band" noise. MOSFETs have a higher f_c (can be in the GHz range) than JFETs or bipolar transistors which is usually below 2 kHz for the latter.

The flicker noise voltage power in MOSFET can be expressed by K/(Cox•WLf), where K is the process-dependent constant, Cox is the oxide capacitance in MOSFET devices, W and L are channel width and length respectively.

Flicker noise is found in carbon composition resistors, where it is referred to as excess noise, since it increases the overall noise level above the thermal noise level, which is present in all resistors. In contrast, wire-wound resistors have the least amount of flicker noise. Since flicker noise is related to the level of DC, if the current is kept low, thermal noise will be the predominant effect in the resistor, and the type of resistor used may not affect noise levels, depending the frequency window.