Zener Breakdown Vs Avalanche breakdown

Zener Breakdown Vs Avalanche breakdown

The diode breakdown will happen through one of the following two mechanism.

(1) Zener breakdown
(2) Avalanche breakdown

Before entering into the comparison of the zener breakdown vs avalanche breakdown let us refresh about the diode biasing.

Forward Biased:

  • When the P-type material connected to positive terminal of voltage source with respect to the N-type material the p-n junction said to be in forward biased. 
  • The external applied voltage opposes the contact potential thus closes the depletion layer.
  • Consequently the holes and electrons can cross the junction and a current flows through the device.
  • When the applied voltage exceed the pinch off voltage ( for germanium 0.2V and for silicon 0.6V) the current flow will raise rapidly.  
  • This is the minimum voltage required to narrow the depletion layer. 

Reverse Biased:

  • In a P-N junction, when the P-type material is connected to the negative terminal with respective to the N type material then it is said to be in reverse biased. 
  • The external supply voltage is aides contact potential.
  • So it opposes the movement of holes and electrons due to opening up the depletion layer.
  • Eventually no current flows through the device.
  • But in practical, a small amount of current flows in the reverse direction.
  • This because certain electrons in the covalent bond lattice acquire sufficient energy from the existing heat to leave the lattice, generating mobile electrons and holes at normal room temperature .
  • This process is called electron-hole generation by thermal excitation. 
  • The electrons in the p-type material and holes in the n-type material caused by thermal excitation, are called minority carriers.
  • These minority carriers will be attracted by the applied voltage. Thus a small current of flows. 
  • For germanium it will be in the range of  few microamperes (μA) for Silicon it will be less than one microampere(μA).

Zener breakdown: 

  • In zener breakdown, using heavily doped P and N regions, direct rupture (breaking) of covalent bond takes place.
  • Because of the strong electric field at the junction of PN diode.
  • The newly created electron-hole pairs increase the reverse current in a reverse biased PN diode.
  • The increase in current takes place at an almost constant value of reverse bias typically below 6V for heavily doped diodes.
  • As the P and N regions are heavily doped, the depletion region width becomes very small.
  • So for the applied reverse bias of 6V or less, the field across the depletion region becomes very high.
  • For lightly doped diodes, zener break down voltage becomes high and breakdown is thus predominantly by avalanche multiplication.

Avalanche Breakdown:

  • For normally doped P-N diode, depletion region is thick.
  • In such a diode for reverse bias in excess of 6V, the breakdown is through process of avalanche multiplication (or secondary emission).
  • Thus in a reverse biased diode, a thermally generated carrier falls down the junction barrier and acquire energy from the applied voltage.
  • This carrier collides with a crystal ion, with a sufficient kinetic energy to disrupt a covalent bond. This is known as impact ionization.
  • Thus in addition to the original carriers may also acquire enough energy from the applied field, collide with another crystal ion and create still another electron-hole pair. This is a cumulative process and is known as avalanche multiplication.
  • The net result is a large reverse current at an almost constant voltage drop will appear and avalanche break down will take place.

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