Under large enough reverse bias, a semiconductor diode experiences either Zener or avalanche breakdown. The avalanche breakdown process is inherently random, so an ordinary avalanche Zener diode is a great source of noise and depending on one s application, this can be a bane or a blessing. Manufacturers of commercial noise diodes control the factors that determine the avalanche breakdown process in order to enhance the amount of noise. The noise from these diodes are surprisingly broadband and flat over several decades. For example it is not uncommon to find a diode that will generate noise over a range such as 100 Hz 100 MHz, or 10 MHz 10 GHz, with noise levels flat within 1 dB over this frequency range.
Manufacturers often combine the diodes with filters to limit the noise to a desired frequency range.
References Micronetics . Manufacturer of noise diodes and noise measurement equipment. NoiseCom is another manufacturer of noise diodes and measurement equipment. Loral Microwave manufactures noise diodes.