1. Zenerdiode:
When an ordinary diode is reverse biased, the diode remains non conducting. As the reverse bias voltage is increased, the minority electrons passing through the depletion region acquire enough kinetic energy to knock out electrons from the semiconductor atoms. In this process, the covalent bonds are broken and the electron – hole pairs are produced. Hence the number of free electrons and holes goes on increasing. This cumulative phenomenon is called avalanche break down. Thus an ordinary junction diode of lower rating will get destroyed on applying reverse biase above its break down voltage.
If a diode is heavily doped, depletion layer will be thin and consequently the breakdown of the junction will occur at a lower reverse voltage. On the other hand a lightly doped diode has a higher breakdown voltage. When an ordinary crystal diode is properly doped so that it has a sharp breakdown voltage, it is called zener diode. The process by which covalent bonds in the depletion region are directly broken by the strong electric field is called zener breakdown.
A properly doped crystal diode which has a sharp breakdown voltage is known as a zener diode and the corresponding voltage is called zener voltage Vz.
a. A zener diode is like an ordinary diode expect that it is properly doped so as to have a sharp break down voltage.
b. A zener diode is always reverse connected or reverse biased.
c. A zener diode has a sharp break down voltage called zener voltage Vz.
d. When forward biased, its characteristics are just same as that of an ordinary diode.
e. The zener diode is not immediately burnt just because it has entered the breakdown region. The current through the diode is limited by its resistance.
Zener diode as a voltage regulator
A zener diode can be used as a voltage regulator to provide a constant voltage from a source whose voltage may vary over sufficient range. The circuit diagram is as shown in the figure. The zener diode of zener voltage Vz is reverse connected as the load RL across which a constant output is desired. The series resistance R acts as current limiter which keeps the current through a zero diode with in the permissible limit so as to maintain a constant voltage across the load.
Suppose that the input voltage Vin exceeds the zener voltage Vz the diode turns into the breakdown region. Here after the output voltage remains constant at Vz,across the load resistance.
The current equation is I = Iz + IL
 |
