Analog Electronics

Breakdown of a diode

When the reverse voltage applied across diode becomes greater than the breakdown voltage, then the diode breaks down and very high current starts flowing in the circuit. There are generally two types of breakdowns in a diode:

  1. Zener breakdown
  2. Avalanche breakdown

And based on the above classifications of breakdown of diode, we have the two special types of diode as

  1. Zener Diode
  2. Avalanche Diode

The difference between the Zener Diode and avalanche Diode is the doping level. The doping level of Zener diode is more than avalanche diode or we can say diodes which have higher doping level undergo Zener breakdown when reverse bias voltage is increased while diodes with lesser doping level undergo Avalanche breakdown.

Zener diode

As we have already mentioned doping level of Zener diode is very high and hence width of depletion region is less. As we know

                                                E = VB / d

                                VB is the barrier voltage

                                E is the electric field

                                d is the depletion width

As doping is high, hence width (d) is less and as barrier voltage varies with doping as stated by the formula:

From the formula we can get that the voltage varies proportional to log of doping and hence the barrier voltage is almost constant.

So from the above discuss we find that Electric field in the depletion region would be large as VB is almost constant and d has decreased. Due to this large electric field, electrons from the outer shell of the atom in the depletion region are expelled out and hence carriers are generated within the depletion region. The high electric field in the depletion region pulls out large number of electrons from the large number of atoms. This leads to large current flow and this type of breakdown is called Zener breakdown.

Avalanche diode

The diode which have lesser doping undergo avalanche breakdown when high reverse voltage is applied. The lesser doping means the depletion width is large and so electric field within depletion region is not so high. Hence the electric field would not be able to pull out electrons from the outer shell of atoms and breakdown doesn’t occur in depletion region. But as the depletion region is large and hence when the minority charge carriers move through the depletion region, they get accelerated by the electric field and that even for larger time (as distance through which acceleration is provided is large). Hence minority charge carriers acquire high velocity and so high kinetic energy. When these charge carriers strike with atoms in the n-type and p-type regions, the high kinetic energy gets converted to thermal energy and hence due to this energy electrons from the outermost shell are pulled out and large current starts flowing. This type of breakdown is called avalanche breakdown.

                But due to the high thermal energy, the temperature rises and diode gets burned. Due to this reason the simple diodes (where avalanche breakdown occurs) is not used in the applications and instead Zener diode is used in the application circuits of breakdown diodes such as regulating power supply.

Differences between Zener breakdown and Avalanche breakdown:

           Zener breakdown                                                  Avalanche breakdown

  1. The Zener breakdown occurs in HIGH        1.  The avalanche breakdown occurs in LOW  doping diodes.                                                    doping diodes.
  2. The breakdown occurs within the                 2.  The breakdown occurs outside the depletion depletion region.                                                  region.
  3. The breakdown voltage is lesser than            3.  The breakdown voltage is more than zener that of avalanche breakdown.                            breakdown voltage.

As Zener breakdown voltage is less than that of avalanche breakdown voltage, hence Zener breakdown is said to occur before the avalanche breakdown.

Hence we can say if we increase the doping of a diode, the chances of zener breakdown increases and hence breakdown voltage decreases

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