Shot Noise

- the basics of shot noise - a form of noise that is detected and encountered in RF and other electronic circuits.

Shot noise is a form of noise that arises because of the discrete nature of the charges carried by charge carriers, electrons or holes.

It is particularly manifest in areas where current levels are low, and hence the statistical nature of the current flow together with the discrete charge levels is more obvious.

Shot noise is particularly noticeable in semiconductor devices, such as tunnel junctions, Schottky barrier diodes and p-n junctions.

First observations of shot noise

The first understanding of shot noise was developed by Walter Schottky, the German physicist who played a major role in the development of the theory of electron and ion emission. He invented the tetrode thermionic valve or vacuum tube and also number of effects that bear his name. Interestingly he also developed the ribbon microphone.

While working on vacuum tubes he noted that even when all external sources of noise had been eliminated two types of noise remained. One he determined was a a result of the temperature and this is now referred to as Johnson-Nyquist or thermal noise. The other was shot noise.

Shot noise basics

Shot noise arises because current consists of a vast number of discrete charges, and is not a totally analogue phenomenon.

The continuous flow of these discrete pulses gives rise to almost white noise. There is a cut-off frequency which is governed by the time it takes for the electron or other charge carrier to travel through the conductor.

Unlike thermal noise, this noise is dependent upon the current flowing and has no relationship to the temperature at which the system is operating.

Shot noise is more apparent in devices such as PN tunnelling junctions. The electrons are transmitted randomly and independently of each other.

For metallic resistors, shot noise is virtually non-existent because the inelastic electron-phonon scattering smoothes the current fluctuations that result from the discrete nature of the electrons, leaving only thermal noise.

However recent developments have recognised that there is still some very small level of shot noise.

By Ian Poole