It is high conductivity PN junction diode having doping density about 1000 times higher than ordinary PN junction diode. It reduces the reverse break down voltage so that diode break down for any reverse voltage. It produce a negative resistance region, because -5 of its extremely thin depletion layer (10 mm) electrons are able to tunnel through the potential barrier at relatively low forward bias voltage. The tunnelling is much faster than normal crossing by the electron across the depletion layer. This enable a diode to switch ON and OFF at a speed (comparable with speed of light) much faster than ordinary diode. Due to this, these are used in high speed computers memories, where very fast switching speeds are involved. It can be used in an amplifier, oscillator and switching elements where ordinary diode can not be used. The graph between current and voltage is as shown where I peak current pe corresponding to peak voltage Vpe. The voltage & current corresponding to minimum are called as valley voltage V and V valley current Iv.