Break Down Mechanism
If the reverse-bias applied to a p-n junction is increased, a point will reach when the junction breaks down and reverse current rises sharply to a value limited only by the external resistance connected in series. This specific value of the reverse bias voltage is called breakdown voltage (vz). After breakdown, a very small further increase the reverse current. The breakdown voltage depends upon the with of depletion layer. The width of depletion layer depends upon the doping level. With the help of this post we have tried to explain the concept of Zener Breakdown and Avalanche Breakdown Mechanism as Basic Electronics Notes in detail.
The following two processes cause junction breakdown due to the increase in reverse bias voltage.
(i) Zener Breakdown
(ii) Avalanche Breakdown
The Zener Breakdown is observed in the Zener diodes having Vz less than 5V or between 5 to 8 volts. When a reverse voltage is applied to a Zener diode, it causes a very intense electric field to appear across a narrow depletion region. Such an intense electric field is strong enough to pull some of the valence electrons into the conduction band by breaking their covalent bonds .these electrons then become free electrons which are available for conduction. A large number of such free electrons will constitute a large reverse current through the Zener diode and breakdown is said to have occurred due to the Zener effect.
Characteristics of Zener Breakdown is shown below:
A current limiting resistance should be connected in series with the Zener diode to protect it against the damage due to excessive heating. In Zener breakdown, the breakdown voltage depends on the temperature of P-N junction.The breakdown voltage decreases with increase in the junction temperature.
The avalanche breakdown is observed in the Zener Diodes having Vz having than 8 V. In the reverse biased condition, the conduction will take place only due to the minority carriers. As we increase the reverse voltage applied to the Zener diode, these minority carriers tend to accelerated. Therefore, the kinetic energy associated with them increases. While travelling, these accelerated minority carriers will collide with the stationary atoms and impart some of the kinetic energy to the valence electrons present in the covalent bonds.
Characteristics of Avalanche Breakdown is shown below:
Due to this additionally acquired energy, these valence electrons will break their covalent bonds and jump into the conduction bond to become free conduction. Now these newly generated free electrons will get accelerated. They will knock out some more valence electrons by means of collision. This phenomenon is called as carrier multiplication.
Comparison of Zener Breakdown and Avalanche Breakdown
|S.No.||Zener breakdown||Avalanche breakdown|
This is observed in Zener diodes having Vz 5 to 8 volts.
This is observed in Zener diodes having Vz greater than 8 volts.
The valence electrons are pulled into conduction due to very intense electric field appearing across the narrow depletion region.
The valence electrons are pushed into conduction band due to the energy imparted by colliding accelerated minority carries.
V-I characteristics with Zener the break down is very sharp.
The V-I characteristics with the avalanche breakdown increases gradually. It is not as sharp as that with the Zener breakdown.
The breakdown voltage decreases with increase in temperature.
The breakdown voltage increases with increase in temperature.
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