Zener diode is basically like an ordinary PN junction diode but normally operated in reverse biased condition. But ordinary PN junction diode connected in reverse biased condition is not used as Zener diode practically. A Zener diode is a specially designed, highly doped PN junction diode.
A Zener diode allows current to flow from its anode to its cathode like a normal semiconductor diode, but it also permits current to flow in the reverse direction when its “Zener voltage” is reached. Zener diodes have a highly doped p-n junction. Normal diodes will also break down with a reverse voltage but the voltage and sharpness of the knee are not as well defined as for a Zener diode. Also normal diodes are not designed to operate in the breakdown region, but Zener diodes can reliably operate in this region.
|Working principle||Zener breakdown|
|Invented||Clarence Melvin Zener|
|Pin configuration||anode and cathode|
OPERATION OF ZENER DIODE
When a PN junction diode is reverse biased, the depletion layer becomes wider. If this reverse biased voltage across the diode is increased continually, the depletion layer becomes more and more wider. At the same time, there will be a constant reverse saturation current due to minority carriers. After certain reverse voltage across the junction, the minority carriers get sufficient kinetic energy due to the strong electric field. Free electrons with sufficient kinetic energy collide with stationary ions of the depletion layer and knock out more free electrons. These newly created electrons also get sufficient kinetic energy due to the same electric field, and they create more free electrons by collision cumulatively. Due to this commutative phenomenon very soon huge electrons will be created in the depletion layer, and the entire diode will become conductive.
This type of breakdown of the depletion layer is known as avalanche breakdown, but this breakdown is not quite sharp. There is another type of breakdown in depletion layer which is sharper compared to avalanche breakdown, and this is called Zener breakdown. When a PN junction is diode is highly doped, the concentration of impurity atoms will be high in the crystal. This higher concentration of impurity atoms causes the higher concentration of ions in the depletion layer hence for same applied reverse biased voltage, the width of the depletion layer becomes more thinner than that in a normally doped diode. Due to this thinner depletion layer, voltage gradient or electric field strength across the depletion layer is quite high. If the reverse voltage is continued to increase, after a certain applied voltage, the electrons of ions come out from parent impurity ions and make the depletion layer conductive. This breakdown is called Zener breakdown. The voltage at which this breakdown occurs is called zener voltage. If the applied reverse voltage across the diode is more than zener voltage, the diode provides a conductive path to the current through it hence, there is no chance of further avalanche breakdown in it. Theoretically, zener breakdown occurs at lower voltage level then avalanche breakdown in a diode which is specially doped for zener break down. The zener voltage of diode can be adjusted during manufacturing with the help of required and proper doping. When a zener diode is connected across a voltage source, and the source voltage is more than zener voltage, the voltage across a zener diode remain fixed irrespective of the source voltage. Although at that condition current through the diode can be of any value depending on the load connected with the diode. That is why zener diode is mainly used for controlling voltage in different circuits.
Zener Diode Circuit
Zener Diode is nothing but a single diode connected in a reverse bias, we have already stated that. A diode connected in reverse bias positive in a circuit is shown below,
The circuit symbol of Zener diode is also shown below. For convenience and understanding, it is used normally
Characteristics of a Zener Diode
Now, discussing about the diode circuits we should look through the graphical representation of the operation of the zener diode. Normally it is called the V-I characteristics of a general p – n junction diode.
The above diagram shows the V-I characteristics of the zener diode. When the diode is connected in forward bias, this diode acts as a normal diode but when the reverse bias voltage is greater than a predetermined voltage zener breakdown voltage takes place. To make the breakdown voltage sharp and distinct, the doping is controlled and the surface imperfections are avoided. In the V-I characteristics above Vz is the zener voltage, we can say. It is also the knee voltage because at this point the current is the current is very rapid.