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When a diode is reverse-biased no current flows and it behaves like an open switch. If the current is increased, there is a critical point that is reached, where the PN depletion zone breaks down and current flows. When using a ‘regular’ diode, this is not a desirable effect. However, this normally undesirable effect is used as a feature in a special type, called a Zener diode.
A Zener diode is a type of diode that has been designed to allow current to flow in the reverse-biased configuration, in a reliable manner. This only happens when a voltage, known as the Zener voltage, has been reached. The Zener diode is designed to have a voltage drop equal to the Zener voltage, even with a changing current. In this way a Zener diode can be used as an accurate voltage reference.
There are actually two mechanisms that occur in Zener diodes that we are interested in: the Zener effect and the avalanche effect. In silicon diodes up to around 5.6V, it’s the Zener effect that we are interested in. Above 5.6V we are interested in the avalanche effect.
Zener breakdown
During the Zener breakdown electrons move across the barrier from the p-type material to the n-type material. Electrons are pulled into conduction due to a high-electric field in a narrow depletion region. The temperature increases, which caused the breakdown voltage to decrease.
avalanche breakdown
During the avalanche breakdown electrons are pushed into conduction due to energy from accelerated electrons. They gain their velocity due to their collision with other atoms. The temperature increase, which causes the breakdown voltage to decrease.
series resistor
There are some scenarios where you will need to place a resistor in series with a Zener diode to prevent it from being destroyed. The reason for this is that the power rating of the Zener diode will be exceeded! An example is shown in figure 1. In this example, I am using a Zener diode rated at 1W.