A zener diode is a special type of diode that is designed to conduct large currents in reverse breakdown mode, mainly for voltage regulation purposes.  A zener diode behaves like an ordinary diode, i.e., it conducts current in only one direction and blocks current in the other direction.  Just like a regular diode, a zener diode conducts when it is forward-biased, or when its anode is more positive than its cathode by a certain voltage. It is said to be in reverse bias if its cathode is more positive than its anode, blocking the flow of current in that state. 

If an excessive reverse-bias voltage is applied across an ordinary diode, it goes into a phenomenon known as 'avalanche breakdown'.  Under this state, the diode starts conducting large amounts of current even if it is in reverse bias.  This phenomenon can cause an ordinary diode to get permanently damaged.  A zener diode, on the other hand, is designed to operate in reverse-bias mode and can handle large currents when it is conducting under reverse bias.

A zener diode is fabricated to exhibit a specified reverse bias voltage breakdown that is much lower than that of an ordinary diode. This reverse breakdown voltage of a zener diode is also known as its 'zener knee voltage' or simply its 'zener voltage'.

Figure 1.  Photo of a zener diode (left) and the circuit symbol for a zener diode (right)

A zener diode has a heavily doped p-n junction that allows electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material.  The zener voltage of a zener diode may be set through a precisely controlled doping process, which can achieve tolerances of as high as 0.05%.  Most zener diodes, however, have tolerances of 5% to 10%. 

Since a zener diode is meant to operate in reverse bias, its usual application would have it connected to the circuit in such a way that its cathode is more positive than its anode. Under this connection, the zener diode will not conduct unless the voltage at the cathode exceeds the anode voltage by more than its zener voltage.  Thus, a zener diode starts conducting as soon as it is reverse-biased by a voltage equal to its zener voltage.  Once it conducts, the zener diode tends to pull down the voltage applied across it.  As such, the voltage seen across a conducting zener diode is very close to its zener voltage.

Since a conducting zener diode maintains the voltage across it at a value around its zener voltage, its main purpose is to serve as a voltage regulator.  A very simple circuit that demonstrates this is shown in Figure 2. This is a shunt voltage regulator circuit, since the zener diode is connected in shunt (parallel) with the load.  In this circuit, Vout will be maintained by the zener diode at the zener voltage level even if Vin changes, as long as Vin exceeds the zener voltage.

If Vin increases, the current flowing through the zener diode ZD1 increases as well, causing the voltage across the resistor R to increase while allowing the voltage across the zener diode to remain at the zener voltage level.  Of course, if Vin falls below the zener voltage of ZD1, ZD1 stops conducting, and Vout starts falling with Vin. Note that this circuit is also a very inefficient way to regulate a voltage, since regulation is achieved by shunting current to ground, which is like simply throwing the excess energy away.

Figure 2.  A simple Shunt Voltage Regulator using a zener diode

See Also:   Diode;  Zener Shunt Voltage Stabilizers;  Zener Series Voltage Stabilizers;  More Articles