How Does a PN Junction Work in Forward and Reverse Bias?
A PN junction is a fundamental building block in electronics, created when a piece of p-type semiconductor is joined with an n-type semiconductor. This interface between the P and N regions leads to unique electrical properties that form the basis of devices like diodes, rectifiers, LEDs, and solar cells.
When silicon is doped with a small amount of pentavalent element (such as Antimony), it becomes an N-type semiconductor, which provides free electrons as majority charge carriers. Doping silicon with a trivalent element (such as Boron) results in a P-type semiconductor, which contains holes (absence of electrons) as majority charge carriers.
Simply having P-type or N-type material separately does not show interesting electrical behavior. However, when these two are combined to form a PN junction, a new set of phenomena emerges due to the interaction between electrons and holes across the junction.
PN Junction Formation and Depletion Region
At the junction, a high concentration of electrons on the N-side and holes on the P-side causes some electrons to diffuse into the P-region, filling available holes. Similarly, holes move into the N-region. As this happens, the regions near the interface lose free carriers and become populated by fixed ions. The N-side near the junction develops positive ions, and the P-side develops negative ions.
This process continues until an equilibrium is reached where the movement of charge carriers is balanced by an electric field that arises due to these fixed charges. The region around the junction that is depleted of free carriers is called the depletion layer or depletion region.
This depletion region acts as a barrier to further movement of carriers, creating a built-in “potential barrier” that must be overcome for current to flow.
Key Properties and Potential Barrier
The presence of the depletion region means a PN junction does not conduct current freely in both directions.
The built-in potential difference across the junction (often called the barrier voltage) at room temperature is typically around 0.6 to 0.7 volts for silicon and about 0.3 to 0.35 volts for germanium.
This potential barrier opposes further diffusion of electrons and holes, making the junction act as a one-way valve for current.
Current Flow in PN Junction: Biasing
Applying an external voltage across the PN junction affects the width of the depletion region and current flow:
- Forward Bias:
Connecting the P-side to the positive terminal and the N-side to the negative terminal of a battery reduces the potential barrier. The depletion region narrows, and current flows easily.
- Reverse Bias:
If the P-side is connected to the negative terminal and N-side to the positive, the barrier increases. The depletion region widens, and only a very small leakage current passes through.
| Condition | Depletion Region | Current Flow |
|---|---|---|
| Forward Bias | Narrows | High (easy) |
| Reverse Bias | Widens | Very low (leakage only) |
Step-by-Step Approach: Formation to Equilibrium
- Doping creates P-type (holes) and N-type (electrons) regions in a single crystal.
- When joined, electrons from N-type diffuse to fill holes in P-type, forming negative ions on the P-side and positive ions on the N-side near the junction.
- This transfer establishes a “depletion layer,” removing free carriers and forming a zone of fixed charged ions only.
- An internal electric field (the potential barrier) builds up, halting further net diffusion; the system reaches equilibrium.
Key Formulas for PN Junction
| Formula | Meaning |
|---|---|
| Eo = VT ln((ND NA)/ni²) | Open-circuit (zero bias) junction voltage |
| Dp×NA = Dn×ND | Charge neutrality in depletion region |
Applications of PN Junction
| Device | Role of PN Junction |
|---|---|
| Diode | Allows current in one direction only (More on Diodes) |
| Rectifier | Converts AC to DC (Rectifier Working) |
| Transistor | Combination of two PN junctions (Learn About Transistors) |
| LED/Solar Cell | Emission/detection of light using the junction (Solar Cell Concept) |
Problem-Solving Steps with PN Junctions
- Identify if the diode (PN junction) is forward or reverse biased in the circuit.
- Apply the correct potential barrier value based on the material (Si or Ge).
- Use equilibrium relationships or current-voltage equations if required.
- For current calculation, analyze direction and magnitude according to the biasing.
Example: Potential Barrier for Silicon PN Junction
| Parameter | Value (Approx.) |
|---|---|
| Barrier Potential (Silicon) | 0.6 – 0.7 V |
| Barrier Potential (Germanium) | 0.3 – 0.35 V |
| Thermal Voltage (Room Temp.) | ≈ 26 mV |
Next Steps and Further Learning
- Practice diode and transistor circuit analysis using Semiconductor Diode Problems.
- Deepen your understanding of electronics via Semiconductor Device modules.
- Explore advanced devices in Photodiodes and Solar Energy and Photovoltaic Cells.
Understanding PN junctions lays the foundation for electronic devices. By visualizing carrier diffusion, the nature of the depletion layer, and how current varies with biasing, students develop strong concepts vital for further physics and electronics topics.
Regularly solving practical problems using these concepts helps in mastering both board and competitive examinations.
FAQs on PN Junction: Definition, Working, and Uses
1. What is a pn junction?
A pn junction is the boundary created when a p-type semiconductor (with excess holes) meets an n-type semiconductor (with excess electrons). This junction forms the basic structure of many electronic devices.
Key features include:
- Creates a depletion region due to recombination of charge carriers.
- Establishes a potential barrier that affects carrier movement.
- Allows current to flow mainly in one direction when voltage is applied (rectification).
2. What is the difference between p-type and n-type in a pn junction?
P-type material contains holes as majority carriers, while n-type material contains electrons as majority carriers.
Details:
- P-type (Positive): Created by doping with acceptor impurities (like Boron) resulting in more holes.
- N-type (Negative): Created by doping with donor impurities (like Phosphorus) resulting in more free electrons.
3. Explain the formation of the depletion region in a pn junction.
The depletion region forms where the p-type and n-type materials meet, due to diffusion of electrons and holes.
How it forms:
- Electrons from n-region move to p-region and recombine with holes.
- This leaves behind charged ions and creates an area depleted of free carriers.
- The region develops a built-in electric field called the potential barrier.
4. What does forward biasing and reverse biasing mean in a pn junction?
Forward biasing means connecting the p-side to positive and n-side to negative of a battery, enabling current flow; reverse biasing means the opposite, blocking current.
Forward Bias:
- Reduces depletion layer width.
- Allows significant current flow.
- Increases depletion layer width.
- Very little current flows (leakage current).
5. What is the potential barrier in a pn junction?
The potential barrier is the electric potential difference formed across the depletion region, which prevents further diffusion of charge carriers.
Key points:
- It typically ranges around 0.7 V for silicon and 0.3 V for germanium.
- Must be overcome by external voltage for current to flow in forward bias.
6. What is the VI characteristic of a pn junction diode?
The VI characteristic of a pn junction diode shows the relationship between current (I) and voltage (V) across the diode.
Important aspects:
- Forward bias: Current rises rapidly after threshold voltage (0.7 V for Si).
- Reverse bias: Very small current flows (reverse saturation current) until breakdown voltage.
7. What are the main applications of a pn junction diode?
PN junction diodes are widely used in electronics for several key functions:
- Rectifiers: Converting AC to DC in power supplies.
- Switching circuits: Logic gates and electronic switches.
- Solar cells: Converting light to electrical energy.
- LEDs and photodiodes: Devices for emission or detection of light.
8. What is the diode equation for a pn junction?
The diode current (I) in a pn junction is given by:
I = I0 (eqV/nkT - 1)
Where:
- I0 = reverse saturation current
- V = applied voltage
- n = ideality factor
- k = Boltzmann constant
- T = absolute temperature
- q = electron charge
9. Why does the current increase sharply in forward bias for a pn junction diode?
In forward bias, the applied voltage reduces the potential barrier, allowing charge carriers to cross the junction freely, resulting in a sharp exponential increase in current.
Highlights:
- Barrier becomes thin and easily crossed by electrons and holes.
- Current increases rapidly past threshold (typically 0.7 V for silicon).
10. What factors affect the width of the depletion region in a pn junction?
The width of the depletion region depends on the doping level and applied bias voltage.
Details:
- Higher doping → narrower depletion layer.
- Forward bias → reduces depletion width.
- Reverse bias → increases depletion width.
11. What happens if the reverse bias voltage exceeds breakdown in a pn junction?
When the reverse bias voltage exceeds a certain critical value (breakdown voltage), a large current flows suddenly and can damage the diode.
Types of breakdown:
- Zener breakdown (in heavily doped diodes, at low voltages)
- Avalanche breakdown (at higher voltages in lightly doped diodes)
12. List three conditions necessary for the formation of a pn junction.
Three key conditions for pn junction formation are:
- Use of a single crystal semiconductor (usually silicon or germanium).
- Doping one side with acceptor impurities (p-type) and the other with donor impurities (n-type).
- Physical contact or fusion between p-type and n-type regions to allow recombination and depletion region formation.
