Zener Diode is a reverse-biased heavily-doped PN junction diode which operates in the breakdown region. The reverse breakdown of a PN- junction may occur either due to Zener effect or avalanche effect. Zener effect dominates at reverse voltages less than 5 volt whereas avalanche effect dominates above 5 V. Hence, first one should be called Zener diode. But for simplicity, both types are called Zener Diodes. The breakdown voltage of a Zener diode can be set by controlling the doping level. For Zener diodes, silicon is preferred to Ge because of its high temperature and current capability. This post includes explanation of operation of Zener diode and V-I Characteristics of Zener Diode.
Operation of Zener Diode:
- Zener Diodes are normally used only in the reverse bias direction.
- It means that the anode must be connected to the negative side of the voltage source and the cathode must be connected to the positive side.
- A main difference between Zener diodes and regular silicon diodes is the way they are used in the circuits.
- It is primarily used to regulate the circuit voltage as it has constant Vz.
- A large change in IR will cause only a small change in Vz. It means that a zener diode can be used as an alternate current path. The constant Vz developed across the diode can then be applied to a load.
- Thus the load voltage remains at constant by altering the current flow through the Zener diode.
The V-I Characteristics of a Zener Diode can be divided into two parts
(i) Forward Characteristics
(ii) Reverse Characteristics
The forward characteristics of a Zener diode is shown in figure. It is almost identical to the forward characteristics of a P-N junction diode.
As we increase the reverse voltage, initially a small reverse saturation current Io. Which is in A, will follow. This current flows due to the thermally generated minority carriers. At a certain value of reverse voltage, the reverse current will increase suddenly and sharply . This is an indication that the breakdown has occurred. This breakdown voltage is called as Zener breakdown voltage or Zener voltage and it is denoted by Vz.
The value of Vz can be precisely controlled by controlling the doping levels of P and N regions at the time of manufacturing a Zener diode. After breakdown has occurred. The voltage across Zener diode remains constant equal to Vz. Any increase in the source voltage will result in the increase in reverse Zener current. The Zener current after the reverse breakdown must be controlled by connecting a resistor R as shown in figure. This is essential to avoid any damage to the device due to excessive heating.
Zener Region and its importance
Reverse breakdown of the zener diode operates in a region called zener region, as shown in figure. In this region the voltage across zener diode remains constant but current changes depending on the supply voltage. zener diode is operated in this region when it is being used as a voltage regulator. The complete v-i characteristics of zener diode is as shown in figure
Application of zener Diode are as follows:
Zener diodes have a large number of application. few of them are
(i) Zener diode is used as a voltage regulator.
(ii) Zener diode is used as a peak clipper in wave shaping circuits.
(iii) Zener diode is used as a fixed reference voltage in transistor biasing circuits.
(iv) Zener diode is used for meter protection against damage from accidental application of excessive voltages.
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