Reverse Biasing of Zener Diode
Fig 1.13 Symbolic Representation of Zener Diode.
- A conventional diode does not permit large current to flow when in reverse biased. When a p-n junction is reversing biased, the current through the junction is very small. However if the magnitude of reverse bias reaches a critical value, avalanche breakdown may take place. Thus a rapid avalanche breakdown occurs and the diode conducts a large current in the reverse biased mode and may get damaged permanently.
- However, diodes may be specially built to operate in the breakdown region. By varying the degree of doping, diodes the specific breakdown voltages (ranging from about one to several hundred volts) can be fabricated. If the junction is well-designed, the breakdown will be very sharp and the current after the breakdown will be independent of voltages; such diodes designed for a specific breakdown voltage are shown as Zener diodes.
- They are useful in voltage regular circuits. As the load current or supply voltage changes, the current through a Zener diode will accommodate itself to these changes to maintain a constant load voltage. The upper limit on the diode current is determined by the power dissipation rating of the diode.
- Working Of Zener Diode (Biasing of Zener Diode):
- Forward Biasing of Zener Diode:
- When the anode of the zener diode is connected to the positive terminal if DC Source and the cathode is connected to the Negative terminal, the zener diode is said to be forward biased.
- The forward biased zener diode is behaves identical to the forward biased diodeThe fig shows the forward biased connection of zener diode.
- Zener diode generally not used in forward biased condition.
- Reverse Biasing of Zener Diode:
- When the cathode is connected to the positive terminal of the dc source and the anode is connected to the negative terminal of the dc sourse, the zener diode is said to be reverse biased.
- Zener diode in the reverse biased condition is used as a voltage regulator.
- The breakdown voltage depends upon the following.
1) Width of the depletion region.
2) Doping level.
The characteristics curve has three regions viz. forward, leakage and breakdown. In the diode forward region, it starts conducting at 0.7 V as any other silicon diode. The region between zero and breakdown is the leakage region and only small reverse current flows in this region. The breakdown region is very sharp. When the voltage reaches – 15V, the characteristics becomes almost vertical and the voltage becomes constant at- 15V.
The minus sign in the specification of the breakdown voltage does not have any significance. It only indicates that the Zener diode is reversing biased. It is preferable to say that the Zener diode has a breakdown voltage of (say) 15 V.
The following mechanism is responsible for breakdown under increasing reverse voltage,
1) Zener Breakdown: The Zener breakdown occurs in the junction which being heavily doped and has a very narrow depletion layers, very strong electric field of the order of 10^8 V/M is developed at breakdown voltage. This electric field is strong enough to break the covalent bonds thereby generating electron hole pairs. Further a very small increase in reverse voltage produces a very large number of current carriers.
2) Avalanche breakdown: The avalanche breakdown occurs in junction which are lightly doped, have wide depletion layer where the electric field is not strong enough to generate zener breakdown. The avalanche breakdown occurs when the accelerated free electron acquire
sufficient energy to ionize atoms by bombardment. The additional free electrons created in this manner are accelerated by the reverse field causing more and more ionization.
The zener diode uses a p-n junction is reverse bias to make use of the zener effect, which is a breakdown phenomenon which holds the voltage close to a constant value called the zener voltage. It is useful in zener regulator to provide a more constant voltage, for improvement of regulated power supplies, and for limiter application. Characteristics of zener diode is quite similar to that of simple PN junction diode in forward bias and have a Sharpe breakdown in reverse bias condition.
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