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Forward biasing a PN junction | Class 12 (India) | Physics | Khan Academy
Khan Academy India - English
Overview
This video explains how a PN junction behaves when a voltage is applied in a forward bias configuration. It builds upon the concept of diffusion and drift currents present at equilibrium. By connecting the positive terminal of a battery to the P-type material and the negative terminal to the N-type material, the applied voltage reduces the potential barrier at the junction. This reduction allows majority charge carriers (holes and electrons) to diffuse more easily across the junction, significantly increasing the diffusion current. The video illustrates that this effect is non-linear and that as the applied voltage approaches the built-in potential (around 0.7V for silicon), the junction conducts heavily, allowing a large current to flow.
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Chapters
- At equilibrium, a PN junction has two opposing currents: diffusion and drift.
- Diffusion current is due to majority carriers moving across the junction.
- Drift current is due to minority carriers being pushed by the electric field.
- At equilibrium, these currents are equal and opposite, resulting in zero net current.
Understanding the state of a PN junction before applying external voltage is crucial for comprehending how external factors alter its behavior.
The video mentions the barrier height for silicon is about 0.7V at room temperature, which prevents significant diffusion at equilibrium.
- Forward biasing involves connecting the positive terminal of a battery to the P-type side and the negative terminal to the N-type side.
- This connection is made using metallic leads and wires, with an ammeter to measure current.
- Initially, with zero applied voltage, the current is zero.
This setup establishes the conditions for observing how an external voltage influences the internal dynamics of the PN junction.
Connecting the positive terminal of a variable voltage source to the P-type material and the negative terminal to the N-type material.
- Applying a small voltage (e.g., 0.2-0.3V) pushes majority carriers towards the junction.
- This reduces the potential barrier height, making diffusion easier.
- The diffusion current increases, becoming larger than the drift current.
- A net current flows from P to N within the semiconductor, and through the external circuit.
This demonstrates the initial response of the PN junction to external voltage, showing how even small voltages can initiate current flow.
When 0.2V is applied, the barrier height decreases by 0.2V, allowing more holes and electrons to diffuse across the junction.
- As the applied voltage increases, the potential barrier is further reduced.
- The diffusion current increases non-linearly; it can more than double when the voltage doubles.
- The depletion region width also decreases as the barrier height reduces.
- The drift current is assumed to remain relatively constant due to its small initial value.
Understanding the non-linear relationship between voltage and current is key to predicting the behavior of diodes in circuits.
Doubling the applied voltage from 0.2V to 0.4V causes the diffusion current to increase by more than double, potentially tripling.
- When the applied voltage approaches the built-in potential (around 0.7V for silicon), the barrier height is almost eliminated.
- The depletion region nearly vanishes.
- Majority carriers can diffuse freely, causing the diffusion current to increase dramatically.
- The PN junction conducts heavily, acting like a closed switch.
This explains why a forward-biased diode allows significant current flow once the applied voltage overcomes the internal barrier, a fundamental characteristic of diodes.
At 0.7V for silicon, the barrier is gone, the depletion region vanishes, and the diffusion current 'skyrockets'.
Key takeaways
- Forward biasing a PN junction occurs when the positive terminal connects to P-type and the negative to N-type material.
- The applied voltage in forward bias effectively lowers the potential barrier at the PN junction.
- Lowering the barrier allows majority charge carriers to diffuse more easily, increasing the diffusion current.
- The relationship between applied voltage and diffusion current in forward bias is non-linear.
- As the applied voltage approaches the built-in potential, the PN junction begins to conduct heavily.
- Forward biasing allows a PN junction (diode) to act as a conductor, enabling significant current flow.
Key terms
PN junctionEquilibriumDiffusion currentDrift currentMajority charge carriersMinority charge carriersPotential barrierDepletion regionForward biasingBuilt-in potential
Test your understanding
- What are the two types of current present in a PN junction at equilibrium, and how do they relate?
- How does connecting a battery in forward bias affect the potential barrier of a PN junction?
- Why does the diffusion current increase significantly when a PN junction is forward-biased?
- What is the non-linear effect observed between applied voltage and diffusion current in forward bias?
- What happens to the PN junction's conductivity when the applied forward bias voltage is close to the built-in potential?