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• A Zener diode is like any semiconductor diode which allows current to flow in the forward direction but also in the reverse direction during a certain voltage called breakdown voltage 
• Symbol of Zener diode 
  
• Operation of Zener diode: A normal semiconductor diode usually is damaged due to overheating when reverse voltage is applied for long duration. But in a Zener diode, a large amount of current can be passed through it and only a small change would be observed in the voltage level. It is usually used in reverse bias condition.

Types of Zener diodes :-

• Based on the breakdown voltage, Zener diodes can be classified into 2 types: Surface Zener and Sub Surface Zener .

• Surface Zeners

1. Breakdown Voltage: 6.8 – 10 V 
2. NPN transistor where Collector and Emitter are at cathode and Base at Anode 
3. Charges accumulated in the diode cause Zener walk out

• Sub surface Zeners 

1. This diode is similar to surface zener, the only difference being that the avalanche is deeper inside in the structure. 
2. Thus there is no Zener walk out and the zener voltage is maintained.