Forward and Reverse Bias in Diodes: Explained with Examples
Understanding the Difference Between Forward Bias and Reverse Bias is crucial for students learning about semiconductors and diode operations. Comparing these two modes helps in analyzing how diodes behave in circuits, which is essential for advanced mathematics and electronics-based examinations like JEE.
Mathematical Perspective on Forward Bias in Diodes
Forward bias occurs when the positive terminal of a voltage supply is connected to the p-type region and the negative terminal to the n-type region of a diode.
This configuration reduces the width of the depletion region and lowers the potential barrier at the PN junction, allowing substantial current to flow through the device.
$I = I_0 (e^{\frac{qV}{kT}} - 1)$
The forward current increases exponentially with the applied voltage, as described mathematically by the diode equation. For related comparisons, see the Difference Between AC and DC Generators.
Essence of Reverse Bias in Semiconductor Devices
Reverse bias occurs when the positive terminal of the external voltage is connected to the n-type region and the negative terminal to the p-type region of the diode.
In this arrangement, the potential barrier increases and the depletion region widens, resulting in a very small reverse saturation current across the junction.
$I \approx -I_0$
The current in reverse bias is almost independent of the applied voltage and is extremely low. For further studies, review the Difference Between Electric Potential.
Comparative View of Forward Bias and Reverse Bias in Diodes
| Forward Bias | Reverse Bias |
|---|---|
| Positive terminal to p-region, negative to n-region | Positive terminal to n-region, negative to p-region |
| Decreases potential barrier | Increases potential barrier |
| Depletion layer becomes thinner | Depletion layer becomes wider |
| Current flows easily through the diode | Only a small leakage current flows |
| Resistance of diode is low | Resistance of diode is high |
| Acts as a conductor | Acts as an insulator |
| Voltage drop mainly across external circuit | Voltage drop mainly across the depletion region |
| Useful for rectification in AC circuits | Used for voltage regulation and signal demodulation |
| Current increases rapidly with voltage | Current remains nearly constant with voltage |
| Power is mainly dissipated as heat in external resistance | Power dissipation is negligible |
| Application in light-emitting diodes | Application in photodiodes and Zener diodes |
| Barrier potential is reduced (0.3V for Ge, 0.7V for Si) | Barrier potential is increased |
| Conventional current direction: p to n | Conventional current direction: n to p |
| Majority charge carriers dominate current | Minority charge carriers dominate current |
| Used in signal detection circuits | Used in voltage multiplier circuits |
| Breakdown does not occur under normal voltage | Breakdown may occur at high reverse voltage |
| Emits energy in some special diodes | Absorption of energy in some devices |
| Threshold voltage must be exceeded to conduct | No conduction unless breakdown happens |
| Input voltage must be more than barrier potential | Any reverse voltage increases the barrier |
| Mainly for current-driven applications | Mainly for voltage-driven applications |
Main Mathematical Differences
- Forward bias reduces the barrier; reverse bias raises it
- Current is significant in forward, negligible in reverse
- Depletion layer narrows in forward, widens in reverse
- Forward bias gives low resistance; reverse bias yields high resistance
- Majority carriers control forward current; minority in reverse
- Forward bias is for conduction; reverse bias is for blocking
Simple Numerical Examples
Consider a silicon diode with a threshold voltage of 0.7 V. In forward bias, applying 2 V results in the diode conducting with a significant current. In reverse bias, applying -5 V allows only a tiny leakage current until breakdown occurs.
A diode used in a rectifier is forward biased during positive half-cycle and reverse biased during negative half-cycle, illustrating both behaviors in operation. For more circuit-related examples, refer to Difference Between AM and FM.
Mathematical Applications and Usage
- Used in rectifiers for AC to DC conversion
- Protecting circuits from voltage spikes via reverse bias
- Employed in LED and photodiode operation
- Signal demodulation and voltage regulation tasks
- Designing logic gates and oscillators in circuits
Summary in One Line
In simple words, forward bias enables the diode to conduct by lowering the barrier, whereas reverse bias blocks current by increasing the barrier at the junction.
FAQs on What Is the Difference Between Forward Bias and Reverse Bias?
1. What is the difference between forward bias and reverse bias in a diode?
Forward bias allows current to flow through a diode, while reverse bias blocks current.
• In forward bias, the positive terminal of the battery connects to the p-side and the negative to the n-side.
• In reverse bias, the positive terminal connects to the n-side and the negative to the p-side.
• Forward bias reduces the potential barrier and permits electron flow.
• Reverse bias increases the potential barrier and prevents current, except for a very small leakage current.
2. Define forward bias and reverse bias with examples.
Forward bias means a diode is connected so it conducts electricity, while reverse bias means it stops current.
Examples:
• Forward bias: Connecting a p-n junction diode with the p-side to the positive terminal and the n-side to the negative results in current flow.
• Reverse bias: Connecting the p-side to the negative terminal and the n-side to the positive prevents current through the diode.
3. How does a diode behave under forward bias and reverse bias conditions?
Diodes allow current to flow in forward bias, but not in reverse bias.
• Under forward bias, the diode conducts and current passes easily.
• Under reverse bias, the diode blocks current except for a tiny leakage (reverse saturation current).
4. What are the main characteristics of a diode in forward bias?
Forward bias in a diode reduces resistance and allows current to flow.
Key points:
• Barrier potential decreases
• High current flows after threshold voltage is reached
• Acts like a closed switch
• Widely used in rectifiers and circuits requiring controlled current direction
5. What happens to the potential barrier of a p-n junction diode in forward bias?
In forward bias, the potential barrier across the p-n junction decreases.
• The applied voltage opposes the barrier potential
• Diffusion of majority charge carriers increases, enabling significant current
6. Why does current not flow in reverse bias?
In reverse bias, the diode blocks current as the width of the depletion region increases.
• Applied voltage supports the junction barrier
• Only minor leakage current due to minority carriers occurs
• No significant current flows until breakdown voltage is reached
7. List differences between forward bias and reverse bias of a p-n junction diode.
The main differences are:
• Forward bias: External voltage reduces barrier, current flows.
• Reverse bias: External voltage increases barrier, current does not flow.
• Forward bias: Depletion region narrows.
• Reverse bias: Depletion region widens.
• Forward bias: Used for rectification.
• Reverse bias: Used in detectors or voltage regulation.
8. What is the effect of reverse bias on the depletion layer of a diode?
Reverse bias increases the width of the depletion layer in a p-n junction diode.
• The applied voltage pulls majority carriers away from the junction
• Higher barrier restricts current except for leakage due to minority carriers
9. What are the uses of forward bias and reverse bias in electronic circuits?
Forward bias and reverse bias are fundamental to diode operation in various circuits.
• Forward bias is used in rectifiers, LED circuits, and as clippers
• Reverse bias is used in voltage regulation (Zener diodes), detectors, and switching devices
10. How does current-voltage (I-V) characteristic of a diode differ for forward and reverse bias?
I-V characteristics show how diode current varies with applied voltage.
• Forward bias: Current remains almost zero until threshold voltage, then rises rapidly
• Reverse bias: Current is minimal (leakage) until breakdown voltage is reached, then it spikes sharply
• This behavior demonstrates the one-way conduction property of diodes
11. Why does the depletion region widen in reverse bias?
In reverse bias, the electric field across the p-n junction causes majority charges to move away from the junction.
• This widens the depletion region
• Increases the barrier potential
• Prevents flow of majority carrier current through the diode
