Zener Breakdown

Normal diodes comprise a P-N junction that conducts in the forward direction, but blocks any reverse voltage applied across it. If this reverse voltage exceeds a certain value, the junction breaks down, usually failing permanently in the process. However, some devices, known as Zener diodes, exhibit a Zener effect; they can tolerate a reverse voltage, and start conducting reliably once this reaches a critical level, without ill effect.

The triggering level at which the Zener effect starts - typically between 4 and 8 V – can be very accurately set, usually to less than 1% tolerance, during the doping process. This, together with the device’s reverse voltage characteristic, means that Zeners are useful for voltage regulation applications. Downstream electronics can be protected from excessive voltage levels, while the voltage across the diode remains almost constant throughout large changes in current at any level between the breakdown current value and the maximum current rating. These two effects mean that the diode stabilises voltage sources with protection from both supply and load variations.

The Zener effect arises because under a high reverse-bias voltage the p-n junction's depletion region widens, leading to a high-strength electric field across the junction. Sufficiently strong electric fields enable tunneling of electrons across the depletion region of a semiconductor, leading to a large number of free charge carriers. This sudden generation of carriers rapidly increases the reverse current and gives rise to the high slope conductance of the Zener diode.