Thermal noise is caused by statistical fluctuations of the charge carriers in all electrically conductive materials. These irregularities are caused by the temperature movement of the charge carriers and generate a noise voltage at conductors and resistors
. Since thermal noise contains all frequencies and has a continuous power spectrum, the spectral noise density is the same regardless of the frequency range. This is also known as white noise
However, the noise energy depends on the electrical resistance and increases with increasing resistance values. Thermal noise or white noise was demonstrated by the American physicist Johnson and is also known as Johnson noise.