PN Diode
A PN junction diode is a fundamental semiconductor device formed by joining P-type and N-type semiconductor materials. It acts as a unidirectional switch for electrical current, allowing current to flow more easily in one direction than the other.
Structure:
- P-type region: Contains a high concentration of holes (positive charge carriers) created by doping intrinsic silicon with trivalent elements like Boron (B).
- N-type region: Contains a high concentration of electrons (negative charge carriers) created by doping with pentavalent elements like Phosphorus (P).
- Junction: The interface where P-type and N-type regions meet. It forms a depletion region due to recombination of electrons and holes.
- Contacts: Metal contacts are placed on both sides for external circuit connection.
Working Principle:
Zero Bias:
- No external voltage applied.
- Electrons and holes diffuse across the junction and recombine, forming a depletion region with built-in electric field.
- This field prevents further diffusion and establishes equilibrium.
Forward Bias:
- P-side connected to the positive terminal, N-side to negative.
- External voltage reduces the potential barrier of the depletion region.
- Charge carriers gain energy to cross the junction, resulting in current flow from P to N.
- The diode conducts electricity in this mode.
Reverse Bias:
- P-side connected to negative terminal, N-side to positive.
- External voltage increases the barrier, widening the depletion region.
- Very small leakage current flows due to minority carriers.
- The diode does not conduct under normal reverse bias.
Key Characteristics:
- Threshold/turn-on voltage: Typically ~0.7V for silicon, ~0.3V for germanium.
- Breakdown voltage: Voltage at which reverse current increases drastically due to avalanche or Zener effect.
- Current-voltage (I-V) characteristics: Non-linear; exponential in forward bias, nearly zero in reverse bias (until breakdown).
- Rectification: Converts AC to DC by allowing current in one direction only.
Applications:
- Rectifiers in power supplies.
- Signal demodulation.
- Clipping and clamping circuits.
- Voltage protection using Zener diodes (special PN junction type).
- LEDs and photodiodes are advanced forms of PN junctions.