Introduction

In 1925 J.B. Johnson, studying the current fluctuations of electronic emission in a thermionic tube with a simple technique, found, apart from the shot noise whose spectral density was independent of frequency and was in agreement with the Schottky formula (1.5.10) (Schottky, 1918), also a noise whose spectral density increased with decreasing frequency f (Johnson, 1925). Schottky (1926) suggested that this last noise arises from slow random changes of the thermocathode's surface, and proposed for this kind of noise the name ‘flicker effect’, or ‘flicker noise’. The same type of current noise spectrum was found also in carbon microphones (Christensen & Pearson, 1936), and later, in the 1940s and 1950s, in various semiconductors and semiconductor devices. It has become evident that the flicker noise is a very often encountered, if not universal, phenomenon in conductors.

Up to the present, measurements of the current noise spectra have been performed on a vast number of semiconductors, semiconductor devices, semimetals, metals in normal state, superconductors and superconductor devices, tunnel junctions, strongly disordered conductors etc. One observes, in practically all cases, an increase of the spectral density of current noise with decreasing frequency f approximately proportional to 1/f, down to the very lowest frequencies at which the measurements of the spectral density have been performed. Therefore this current noise is usually called 1/f noise (or of 1/f type). The term proposed by Schottky (flicker noise) and the term ‘excess noise’ are now more rarely used.