Explain the formation of depletion layer in external bias pn-junction. How is it different from the depletion layer formed in forward bias. Give V-I characteristics of Zener diode. With the help of circuit diagram explain the working of Zener diode as a voltage regulator.

Formation of Depletion Layer in External Bias PN-Junction

Step 1: Understanding the PN-Junction

A PN-junction is formed when a p-type semiconductor is joined with an n-type semiconductor. The p-type semiconductor has an abundance of holes (positive charge carriers), while the n-type has an abundance of electrons (negative charge carriers).

Step 2: Initial Contact

When the two types of semiconductors are brought into contact, electrons from the n-type region diffuse into the p-type region and recombine with holes. Similarly, holes from the p-type diffuse into the n-type region and recombine with electrons.

Step 3: Formation of Depletion Region

As the charge carriers recombine near the junction, they leave behind charged ions (donor ions in the n-type region and acceptor ions in the p-type region). This region void of free charge carriers is called the depletion region. The depletion region acts as a barrier that prevents further movement of charge carriers.

Step 4: External Bias

When an external voltage is applied across the PN-junction, it can be either in forward bias or reverse bias.

• Forward Bias: The positive terminal of the battery is connected to the p-type material and the negative terminal to the n-type material. This reduces the width of the depletion region as the external voltage opposes the built-in potential barrier, allowing charge carriers to cross the junction easily.
• Reverse Bias: The positive terminal of the battery is connected to the n-type material and the negative terminal to the p-type material. This increases the width of the depletion region as the external voltage adds to the built-in potential barrier, preventing the flow of charge carriers across the junction.

Differences in Depletion Layer Formation

Bias Type	Forward Bias	Reverse Bias
External Voltage	Opposes the built-in potential barrier	Adds to the built-in potential barrier
Depletion Width	Decreases	Increases
Charge Flow	Increases (allows current flow)	Decreases (inhibits current flow)

V-I Characteristics of Zener Diode

A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" when a certain set reverse voltage, known as the Zener voltage, is reached.

Zener Diode V-I Characteristics

The V-I characteristics of a Zener diode can be described as follows:

• Forward Bias: Similar to a regular diode, the Zener diode has a forward voltage drop (about 0.7V for silicon diodes) after which it conducts.
• Reverse Bias (Below Zener Voltage): The Zener diode behaves like a regular diode in reverse bias. It has a very small leakage current.
• Reverse Bias (At Zener Voltage): When the reverse voltage reaches the Zener voltage, the diode starts to conduct in reverse, and the current increases sharply while the voltage remains relatively constant.

Working of Zener Diode as a Voltage Regulator

Circuit Diagram

  +V_in ----R_s----|>|----+V_out
                    |
                   ---
                    Z
                   ---
                    |
                   GND

• V_in is the input voltage.
• R_s is the series resistor.
• Z is the Zener diode.
• V_out is the regulated output voltage.

Steps for Voltage Regulation

1. Choosing the Zener Diode: A Zener diode with a Zener voltage close to the desired output voltage is chosen.
2. Series Resistor (R_s): A resistor is placed in series with the Zener diode to limit the current through the diode.
3. Operation Below Zener Voltage: When the input voltage (V_in) is below the Zener voltage, the Zener diode does not conduct, and there is no output voltage (V_out).
4. Operation Above Zener Voltage: When V_in exceeds the Zener voltage, the Zener diode starts to conduct in reverse bias. The voltage across the Zener diode stabilizes at the Zener voltage, providing a constant V_out.
5. Load Regulation: The Zener diode adjusts the current flowing through it to maintain a constant voltage across varying load conditions.

Example

Suppose we have a Zener diode with a Zener voltage of 5V, and we want to regulate an input voltage that varies between 9V and 12V.

• The series resistor (R_s) is calculated to ensure that the current through the Zener diode is within its operating limits when V_in is at its maximum.
• When V_in is 9V or higher, the Zener diode conducts in reverse bias, and V_out is regulated to 5V.
• If the load connected to V_out draws more current, the Zener diode allows more current to flow through it to maintain the voltage at 5V.

By following these steps and understanding the characteristics and operation of a Zener diode, one can effectively use it as a voltage regulator in electronic circuits.