Analog Electronics
Zener Diode
Dr. R . K Saxena Professor, Electrical & Electronics Engineering Global Institute of Technology, Jaipur (Raj.) Unit 1- Diode Circuits Zener Diode
To see the video explanation, click here :
https://www.youtube.com/watch?v=rtnmn_MNgsg
Dr R K Saxena, Prof-EED, Global Institute of Technology, Jaipur 2 Zener and Avalanche Breakdown
The Avalanche
Breakdown and Zener Breakdown are two different mechanisms by which a PN junction breaks.
The Zener and Avalanche breakdown both occur in diode under reverse bias
Dr R K Saxena, Prof-EED, Global Institute of Technology, Jaipur 3 Zener Breakdown
Width of depletion region depends on doping of P and N-type semiconductor material. If the
material is heavily doped, the width of the depletion region becomes very thin. The phenomenon of the Zener breakdown occurs in the very thin depletion region. The thin depletion region has more numbers of free electrons. The reverse bias applies across the PN junction develops the electric field intensity across the depletion region. The strength of the electric field intensity becomes very high. 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 .The high reverse current starts flowing in the junction because of which depletion region entirely vanishes. This process is known as the Zener breakdown.
Dr R K Saxena, Prof-EED, Global Institute of Technology, Jaipur 4 Avalanche Breakdown
Avalanche breakdown occurs in lightly dopped material because of the reverse saturation current. The P and N-type materials of the PN junction are not perfect, and they have some impurities in it, i.e., the p-type material has some electrons, and the N-type material has some hole in it. The reverse bias increases the electrical field across the depletion region. When the high electric field exists across the depletion, the velocity of minority charge carrier crossing the depletion region increases. These carriers collide with the atoms of the crystal. Because of the collision, the charge carrier takes out the electrons from the atom. The collision increases the electron-hole pair. As the electron-hole induces in the high electric field, they are quickly separated and collide with the other atoms of the crystals. The process is continuous, and the electric field becomes so much higher then the reverse current starts flowing in the PN junction. After the breakdown, junction can’t regain its original position as diode is completely burnt off.
Dr R K Saxena, Prof-EED, Global Institute of Technology, Jaipur 5 Zener Diode
1. Zener diode allows current to flow in the forward direction in the
same manner as an ideal diode, but 2. also permits current to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage. 3. The zener diode differs from rectifier diodes because it is designed for operation in the reverse-breakdown region. The breakdown voltage of a zener diode is set by carefully controlling the level of doping during manufacture. 4. The basic function of zener diode is to maintain a specific voltage across it’s terminals within given limits of line or load change. 5. Typically it is used for providing a stable reference voltage for use in power supplies and other equipment.
Dr R K Saxena, Prof-EED, Global Institute of Technology, Jaipur 6 Zener diodes are designed to operate in reverse breakdown.
Two types of reverse breakdown in a zener diode are avalanche and zener. The
avalanche break down occurs in both rectifier and zener diodes at a sufficiently
high reverse voltage.
Zener breakdown occurs in a zener diode at low reverse voltages.
A zener diode is heavily doped to reduced the breakdown voltage. This causes
a very thin depletion region.
The zener diodes breakdown characteristics are determined by the doping
process.
Zeners are commercially available with voltage breakdowns of 1.8 V to 200 V.
Dr R K Saxena, Prof-EED, Global Institute of Technology, Jaipur 7 I-V Characteristic
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.
Zener effect is responsible for such a dramatic increase in the value of current at the breakdown voltage.
Dr R K Saxena, Prof-EED, Global Institute of Technology, Jaipur 8 Thank you
To see the video explanation, click here :
https://www.youtube.com/watch?v=rtnmn_MNgsg
Dr R K Saxena, Prof-EED, Global Institute of Technology, Jaipur 9