A PN diode works by allowing current to flow easily in one direction (forward bias) and severely restricting current flow in the opposite direction (reverse bias) due to the formation and manipulation of a depletion region at the junction between p-type and n-type semiconductor materials.
Understanding the PN Junction
A PN diode is formed by joining p-type and n-type semiconductor materials (typically silicon).
- P-type Semiconductor: Doped with impurities that create an abundance of "holes" (positive charge carriers).
- N-type Semiconductor: Doped with impurities that create an abundance of free electrons (negative charge carriers).
When these two materials are joined, electrons from the n-side diffuse into the p-side, and holes from the p-side diffuse into the n-side.
The Depletion Region
This diffusion process leads to the formation of a depletion region at the junction.
- Formation: As electrons diffuse from the n-side to the p-side, they recombine with holes, and vice versa. This recombination eliminates free charge carriers near the junction.
- Charge Build-up: The n-side of the depletion region becomes positively charged due to the loss of electrons, and the p-side becomes negatively charged due to the gain of electrons (loss of holes).
- Electric Field: This charge build-up creates an electric field that opposes further diffusion of charge carriers. Equilibrium is reached when the electric field is strong enough to prevent further diffusion.
Forward Bias
Applying a positive voltage to the p-side (relative to the n-side) is called forward bias.
- Reducing the Depletion Region: The positive voltage repels the holes in the p-side and the electrons in the n-side, pushing them towards the junction and narrowing the depletion region.
- Current Flow: As the forward voltage increases, the depletion region becomes narrow enough that the electric field is overcome. Electrons can then easily flow from the n-side to the p-side, and holes can flow from the p-side to the n-side, resulting in a significant current flow. The reference mentions, "The flow of electrons from the n-side towards the p-side of the junction takes place when there is an increase in the voltage. Similarly, the flow of holes from the p-side towards the n-side of the junction takes place along with the increase in the voltage".
- Threshold Voltage: A certain voltage (typically around 0.7V for silicon diodes) must be reached before significant current flows. This is the threshold voltage or forward voltage.
Reverse Bias
Applying a negative voltage to the p-side (relative to the n-side) is called reverse bias.
- Widening the Depletion Region: The negative voltage attracts holes in the p-side and electrons in the n-side, pulling them away from the junction and widening the depletion region.
- Minimal Current Flow: The widened depletion region acts as an insulator, preventing the flow of charge carriers. Only a very small leakage current (due to minority carriers) flows.
- Breakdown Voltage: If the reverse voltage becomes too large, the diode can experience reverse breakdown. At this point, a large current can flow in the reverse direction, potentially damaging the diode.
Summary Table
| Bias Type | Voltage Polarity (P-side relative to N-side) | Depletion Region Width | Current Flow |
|---|---|---|---|
| Forward Bias | Positive | Narrowed | Significant |
| Reverse Bias | Negative | Widened | Minimal (Leakage) |
In essence, the PN diode's functionality stems from the manipulation of the depletion region, allowing current to flow readily in the forward direction and blocking it effectively in the reverse direction, making it a fundamental building block in electronics.
