Zener Diode

A Zener diode is an exclusive type of diode that enables current to flow in the forward direction like a regular diode but regulates voltage in the reverse direction by operating in the breakdown region without damage. This ability to maintain a stable voltage makes Zener diodes essential for voltage regulation, surge protection, and providing reference voltage in circuits.

1.0Introduction And Symbol of  Zener Diode

• Special purpose diode invented by C. Zener. Heavily doped P-N junction. Works like a normal diode in forward bias. Mainly operated in reverse bias breakdown region.
• It is a semiconductor device with high doping levels, specifically designed to function under reverse bias, particularly within the breakdown region.

2.0Working Of Zener Diode

A Zener diode operates primarily in the reverse breakdown region.

1. Forward Bias: It behaves like a regular diode, allowing current to flow with a small voltage drop (0.7V for silicon).
2. Reverse Bias: In reverse bias, it allows minimal current until the reverse voltage reaches the Zener breakdown voltage, at which point it starts conducting.
3. Zener Breakdown: Once in breakdown, it maintains a stable voltage, regardless of current fluctuations.
4. Voltage Regulation: Zener diodes are used to regulate voltage, clamping it at the Zener voltage to protect circuits and ensure stability.

3.0Zener Diode As A Voltage Regulator

• Regulation is achieved because, in the breakdown region, the Zener voltage stays constant despite variations in the current passing through the Zener diode.      
• When the input voltage rises, the current through both the series resistor(RS) and the Zener diode also rises. Conversely, if the input voltage decreases, the current through(RS)  and the Zener diode drops. As a result, any fluctuations in the input voltage lead to corresponding changes in the voltage drop across RS, while the voltage across the load remains unchanged. This is how the Zener diode functions as a voltage regulator.
• Load voltage is constant and load current is also constant $\frac{V_z}{R_L}$
• Input current is, $I_S=\frac{V_s-V_z}{R_S}$
• Zener Current, $I_Z=I_S-I_L$
• Power, $P_Z=V_Z{I}_Z$

4.0Breakdown In Zener Diode

• Increase in reverse bias voltage increases the electric field in the depletion region. The electric field may become large enough causing electron-hole pairs to be created. This creates a huge reverse bias current.
• Voltage at which this phenomenon happens is called Breakdown voltage and breakdown voltage is a function of doping concentration in p-region and n-region.
• In reverse biasing, breakdown can occur in two ways:

1. Avalanche Breakdown: It occurs in p-n junctions having low doping. Breakdown voltage is very high. Depletion layer width is also more. Charge carriers crossing the depletion region gain enough kinetic energy and make collisions with other atoms, and hence it starts the avalanche effect. Damage to the diode is permanent.
2. Zener Breakdown: Occurs in a heavily doped p-n junction. Breakdown voltage is very small. The depletion layer is very thin. Breaking of covalent bonds is mainly due to the electric field. Hence damage is not permanent.

5.0Characteristics Of Zener Diode

1. Reverse Breakdown: Operates in the reverse breakdown region, conducting once the reverse voltage exceeds the Zener voltage.
2. Zener Voltage: The voltage at which the diode conducts in reverse, remaining constant even as current increases.
3. Voltage Regulation: Maintains stable voltage across the load despite input voltage fluctuations.
4. Forward Bias: Behaves like a regular diode, with a small voltage drop (~0.7V).
5. Current-Voltage Characteristics: Current remains low until Zener voltage is reached, then increases sharply, while voltage stays constant.

6.0Uses Of Zener Diode

Zener diodes have several important uses, including:

1. Voltage Regulation
2. Overvoltage Protection
3. Voltage Clipping
4. Surge Protection
5. Shunt Regulation