Semiconductor-diodes operated in the breakdown region show large noise fluctuations. The noise fluctuations associated with the onset of astable burning microplasmas are well known. The noise spectral density for such statistical switching microplasma pulses has been calculated and agrees with measured values. ZEXER-diodes, in which internal field emission is the cause of the breakdown or ZENER-diodes with stable burning microplasmas, the so-called avalanches diodes, show large noise fluctuations too. These spontaneous fluctuations of voltage and current can be explained by the thermal fluctuations of the breakdown-carriers heated up by the high field in the transition region of a p-n junction. ZENER-diodes break down by internal field emission, when the measured noise temperature does not exceed about 18 000 °K. In this case the aequivalent noise temperature grows proportional to the square root of the breakdown current. When the noise temperature of the breakdown is higher than about 18 000°K, the carriers have sufficient energy to produce electron-hole pairs by ionization. The stable parts of breakdown characteristics with noise temperatures above about 18 000°K show avalanche behavior. Often microplasma switching noise is observed. In the stable burning parts of avalanche breakdown characteristics, noise temperatures as high as 105 to 107°K are measured. These high noise temperatures can not be fully explained by thermally heated carriers in the breakdown region. It is believed that fluctuations of the ionization breakdown contribute to these noise temperatures.