PN Junction Diode – Detailed Notes & Video
A PN junction diode is a fundamental semiconductor device that allows current to flow in one direction while blocking it in the opposite direction. It is widely used in rectifiers, voltage regulators, and signal processing circuits.
1. Structure of PN Junction Diode
A PN junction diode consists of two types of semiconductor materials:
(A) P-Type Semiconductor:
Doped with trivalent elements like Boron (B), Gallium (Ga), or Indium (In).
Contains holes (positive charge carriers) as the majority carriers and electrons as the minority carriers.
(B) N-Type Semiconductor:
Doped with pentavalent elements like Phosphorus (P), Arsenic (As), or Antimony (Sb).
Contains electrons (negative charge carriers) as the majority carriers and holes as the minority carriers.
(C) Depletion Region:
When the P-type and N-type materials are joined, electrons from the N-region diffuse into the P-region and recombine with holes, forming a depletion region near the junction. This region:
Contains immobile ions (positive ions in the N-region and negative ions in the P-region).
Acts as a potential barrier that prevents further charge carrier movement.
Has an inbuilt potential (typically 0.7V for silicon diodes and 0.3V for germanium diodes).
2. Biasing of PN Junction Diode
The behavior of a PN junction diode depends on how voltage is applied across it.
(A) Forward Bias Condition:
Positive terminal of the battery is connected to the P-region, and the negative terminal is connected to the N-region.
This reduces the depletion width, and the potential barrier decreases.
Once the applied voltage exceeds the barrier potential (0.7V for Si, 0.3V for Ge), the diode conducts current exponentially.
The current follows the diode equation:
(B) Reverse Bias Condition:
Positive terminal of the battery is connected to the N-region, and negative terminal to the P-region.
This widens the depletion region and increases the barrier potential.
Only a small leakage current flows due to minority carriers.
If the reverse voltage exceeds a critical value (breakdown voltage), a large current flows, which can damage the diode (except in Zener diodes).
3. Volt-Ampere (V-I) Characteristics
The V-I characteristics of a PN junction diode describe how current changes with applied voltage.
(A) Forward Bias Region (First Quadrant)
For voltages less than the threshold (0.7V for Si, 0.3V for Ge), current is negligible.
Once the applied voltage crosses the threshold, current increases exponentially.
This is because more charge carriers gain enough energy to cross the junction.
(B) Reverse Bias Region (Third Quadrant)
A small leakage current (IsI_s) flows due to the thermally generated minority carriers.
The current remains nearly constant until breakdown voltage is reached.
At breakdown, the diode suddenly allows a large current to flow, potentially damaging the diode unless it is a Zener diode (designed to operate in breakdown mode).
4. Breakdown Mechanisms in Reverse Bias
If a high reverse voltage is applied, the diode may enter breakdown mode:
(A) Avalanche Breakdown (High Voltage)
Occurs in lightly doped diodes with a wide depletion region.
High reverse voltage accelerates free electrons, which collide with atoms, generating more free carriers (avalanche effect).
Leads to a sudden increase in current without damage if a current-limiting resistor is used.
(B) Zener Breakdown (Low Voltage)
Occurs in heavily doped diodes with a narrow depletion region.
The strong electric field causes direct electron tunneling across the junction.
Used in Zener diodes for voltage regulation.
6. Applications of PN Junction Diode
Rectifiers – Converts AC to DC.
Clipping Circuits – Limits voltage levels in signal processing.
Clamping Circuits – Shifts voltage levels.
Voltage Regulation – Zener diodes maintain constant voltage.
Signal Demodulation – Used in radios and TVs.
V-I characteristics graph of a PN junction diode:
Graph Explanation:
Forward Bias Region (Right Side, First Quadrant)
Below threshold voltage (0.7V for Si, 0.3V for Ge), very little current flows.
Beyond threshold, current increases exponentially.
Reverse Bias Region (Left Side, Third Quadrant)
Small reverse leakage current flows.
At breakdown voltage, a sudden large current flows (dangerous unless using a Zener diode).
Here is the V-I characteristics graph of a PN junction diode! It clearly shows the forward bias (exponential rise in current) and reverse bias (small leakage current and breakdown region).
