Electrical breakdown

1.07 Electrical breakdown

It is important to point out that when we are describing the conduction properties of materials we are considering fairly normal operating conditions and we are not talking about situations involving extreme voltages. Air for instance is an excellent insulator, however in thunderstorms voltages in the order of a hundred million volts can force a current through the air in the form of a lightning bolt. It would not take such an extreme voltage to break down a small piece of silicon and force it to conduct electricity. There are two stages that occur as a material begins to breakdown due a large applied voltage. These are zener breakdown and avalanche breakdown.

Zener breakdown
In Zener breakdown the electrostatic attraction between the negative electrons and a large positive voltage is so great that it pulls electrons out of their covalent bonds and away from their parent atoms. ie Electrons are transferred from the valence to the conduction band. In this situation the current can still be limited by the limited number of free electrons produced by the applied voltage so it is possible to cause Zener breakdown without damaging the semiconductor.

Avalanche breakdown
Avalanche breakdown occurs when the applied voltage is so large that electrons that are pulled from their covalent bonds are accelerated to great velocities. These electrons collide with the silicon atoms and knock off more electrons. These electrons are then also accelerated and subsequently collide with other atoms. Each collision produces more electrons which leads to more collisions etc. The current in the semiconductor rapidly increases and the material can quickly be destroyed.

Zener and avalanche breakdown

  • It is very clear from the reverse bias characteristics of diode that the reverse current increases very rapidly as soon as the voltage is increased above breakdown voltage. The sudden rise in the current can be explained with the help of two mechanisms namely – Zener breakdown and Avalanche breakdown.
  • In this article, we discuss the mecahnisms of Zener and avalanche breakdown.

We saw in the earlier section that the diode in reverse bias configuration conducts very little amount of current. This current is of the order of nano-amperes and it does not change significantly with the change in reverse bias voltage. Let us first have a look at the reverse bias characteristics of diode and then we shall discuss what is Zener and avalanche breakdown.

Reverse bias characteristics of PN junction diodeIt can be seen from the above graph that the reverse current increases drastically when the diode voltage reaches breakdown voltage. Two type of effects (mechanisms) are responsible for such sudden rise in the value of reverse current.

  1. Zener effect.
  2. Avalanche effect.

Zener and avalanche effects are responsible for such a dramatic increase in the value of current at the breakdown voltage. Having said that, there arises an obvious question – “which effect occurs when”? Such a simple question. And the answer is simple too. The factor determining which effect occurs is the relative concentration of impurities in the semiconductor. Having said that, let us understand the effect of impurity concentration and the types of breakdown mechanisms in detail.

Zener effect : 

If the impurity concentration is very high, then the width of depletion region is very less. Less width of depletion region will cause high intensity of electric field to develop in the depletion region at low voltages. Confused?? Lets take an example to understand things clearly. Let say the width of depletion region is 200 Å (very small). If a reverse bias voltage of just 4 V is applied to the diode, then the electric field intensity in the depletion region will be

4                = 2 x 108      V/m
200 x 10-10

Merely a voltage of 4 V is responsible to generate an electric field intensity of 2 x 108 V/m (very high intensity). This electric field is sufficient to rupture the bonds and separate the valence electrons from their respective nuclei. Large number of electrons gets separated from their atoms, resulting in sudden increase in the value of reverse current. This explanation was given by scientist C. E. Zener. Such diodes are called Zener diodes.

Zener effect predominates in diodes whose breakdown voltage is below 6 V.

schematic symbol of zener diode

Symbol of Zener diode is shown above. Did you notice the ‘z-like’ appearance instead of vertical line on the cathode side?

  • It is very clear from the reverse bias characteristics of diode that the reverse current increases very rapidly as soon as the voltage is increased above breakdown voltage. The sudden rise in the current can be explained with the help of two mechanisms namely – Zener breakdown and Avalanche breakdown.
  • In this article, we discuss the mecahnisms of Zener and avalanche breakdown.

We saw in the earlier section that the diode in reverse bias configuration conducts very little amount of current. This current is of the order of nano-amperes and it does not change significantly with the change in reverse bias voltage. Let us first have a look at the reverse bias characteristics of diode and then we shall discuss what is Zener and avalanche breakdown.

Reverse bias characteristics of PN junction diodeIt can be seen from the above graph that the reverse current increases drastically when the diode voltage reaches breakdown voltage. Two type of effects (mechanisms) are responsible for such sudden rise in the value of reverse current.

  1. Zener effect.
  2. Avalanche effect.

Zener and avalanche effects are responsible for such a dramatic increase in the value of current at the breakdown voltage. Having said that, there arises an obvious question – “which effect occurs when”? Such a simple question. And the answer is simple too. The factor determining which effect occurs is the relative concentration of impurities in the semiconductor. Having said that, let us understand the effect of impurity concentration and the types of breakdown mechanisms in detail.

Zener effect : 

If the impurity concentration is very high, then the width of depletion region is very less. Less width of depletion region will cause high intensity of electric field to develop in the depletion region at low voltages. Confused?? Lets take an example to understand things clearly. Let say the width of depletion region is 200 Å (very small). If a reverse bias voltage of just 4 V is applied to the diode, then the electric field intensity in the depletion region will be

4                = 2 x 108      V/m
200 x 10-10

Merely a voltage of 4 V is responsible to generate an electric field intensity of 2 x 108 V/m (very high intensity). This electric field is sufficient to rupture the bonds and separate the valence electrons from their respective nuclei. Large number of electrons gets separated from their atoms, resulting in sudden increase in the value of reverse current. This explanation was given by scientist C. E. Zener. Such diodes are called Zener diodes.

Zener effect predominates in diodes whose breakdown voltage is below 6 V.

schematic symbol of zener diode

Symbol of Zener diode is shown above. Did you notice the ‘z-like’ appearance instead of vertical line on the cathode side?

Avalanche effect :

Zener effect predominates on diodes whose breakdown voltage is below 6 V. The breakdown voltage can be obtained at a large value by reducing the concentration of impurity atom. We know that very little amount of current flows in the reverse biased diode. This current is due to the flow of minority charge carriers i.e., electrons in the p type semiconductor and holes in the n type semiconductor. Taking this into consideration, let us understand how avalanche breakdown takes place in a diode.

The width of depletion region is large when the impurity concentration is less. When a reverse bias voltage is applied across the terminals of the diode, the electrons from the p type material and holes from the n-type materials accelerates through the depletion region. This results in collision of intrinsic particles (electrons and holes) with the bound electrons in the depletion region.With the increase in reverse bias voltage the acceleration of electrons and holes also increases. Now the intrinsic particles collides with  bound electrons with enough energy to break its covalent bond and create an electron-hole pair. This is shown in the figure below.

Avalanche breakdown mechanism.

The collision of electrons with the atom creates an electron-hole pair. This newly created electron also gets accelerated due to electric field and breaks many more covalent bond to further create more electron-hole pair. This process keeps on repeating and it is called carrier multiplication. The newly created electrons and holes contribute to the rise in reverse current. The process of carrier multiplication occurs very quickly and in very large numbers that there is  apparently an avalanche of charge carriers. Thus the breakdown is called avalanche breakdown.

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