3.Explain about Zener diodes

Zener diode is very much used in voltage regulator to maintain a constant voltage across a varying load.  In order to understand Zener diode operation let us first recall the VI characteristic of a PN diode. The VI characteristic of PN diode as you can see here is in two regions. One is the forward biased and the other one is reverse biased.

        In the forward bias region the current rises sharply after crossing the threshold voltage which is around 0.7 volt for silicon and 0.3 volt for germanium. Let us concentrate in the reverse bias condition. When the reverse bias voltage that is applied across the PN diode is increased current does not increase significantly. Whatever little current flows that is called the reverse saturation current  IS and this is due to the minority carriers present in the diode. Even though we increase the reverse bias voltage the current does not rise significantly. If we go on increasing the reverse bias potential then if it is sufficiently large after a point we see that there is sharp increase in the reverse current IR. That means here the voltage which is to be reached before the breakdown condition that voltage is known as breakdown voltage and the breakdown in a PN diode occurs generally due to two phenomena. One is called the avalanche breakdown. That is because when the applied reverse bias potential across the diode is very large then the electrons get sufficient kinetic energy to break open the covalent bond and in the process they release. other electrons and these electrons also get the energy to knock out other electrons from near by covalent bonds.

This process is actually a cumulative process. There will be an avalanche of carriers present and that is why there is a large rise in the current and this process is the avalanche breakdown. But this avalanche breakdown phenomenon generally happens for lightly doped PN diodes. If we increase the doping concentration in P and N type, then we can get the breakdown occurring at even a lower potential. If the doping is high then we could get the breakdown occurring as low as at even -5 volt. That phenomenon for heavily doped diode is known as Zener breakdown.

         There is a fundamental difference between avalanche and Zener breakdown. What is Zener breakdown? Because of the reverse bias potential being high there will be strong electric field present across the junction. Due to this high field intensity, the rupturing of the covalent bonds will  occur and whole electron pairs will be generated. Here tearing of the covalent bonds occur because of the high electric field across the junction, because of the application of a high reverse bias potential. This breakdown is called Zener breakdown. This Zener breakdown will have a sharp rise of the reverse current in the diode. That means this current will suddenly very sharply rise after crossing this Zener breakdown voltage, VZ. The major reason for the Zener breakdown is because of this rupturing of the covalent bonds.

            
This breakdown potential VZ can be brought to lower levels by increasing the doping levels in the p-type and n-type materials. These Zener diodes exploit this breakdown phenomenon which is effectively used for some special applications. For example a common application is the voltage regulator.