PN Junction Diode

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PN Junction Diode 

1. Basic Concept

A PN junction diode is formed by joining p-type and n-type semiconductor materials into a single crystal.
Its key property is unidirectional conduction: it conducts current mainly in one direction and blocks it in the other.

2. Doping and Charge Carriers

P-type Semiconductor

  • Doped with trivalent impurities (Boron, Gallium)

  • Majority carriers: holes

  • Minority carriers: electrons

N-type Semiconductor

  • Doped with pentavalent impurities (Phosphorus, Arsenic)

  • Majority carriers: electrons

  • Minority carriers: holes

3. Formation of the PN Junction

When P-type and N-type materials are joined:

3.1 Diffusion Process

  • Electrons diffuse from N → P

  • Holes diffuse from P → N

  • They recombine near the junction

3.2 Depletion Region

  • Mobile charge carriers disappear near the junction

  • Leaves behind fixed ions

  • This region is called the depletion layer

3.3 Electric Field Formation

  • Positive ions on N-side

  • Negative ions on P-side

  • Creates an internal electric field opposing diffusion

4. Barrier Potential (Built-in Voltage)

The electric field creates a potential barrier that prevents further carrier diffusion.

Typical values:

  • Silicon: 0.7 V

  • Germanium: 0.3 V

  • GaAs: ≈ 1.2 V

This voltage must be overcome for conduction.

5. Equilibrium Condition (No External Bias)

  • Diffusion current = Drift current

  • Net current = 0

  • Depletion region is stable

6. Biasing of PN Junction

6.1 Forward Bias Operation

Connection:

  • P-side → Positive terminal

  • N-side → Negative terminal

Effects:

  • Barrier potential decreases

  • Depletion region narrows

  • Majority carriers cross the junction

Result:

  • Large current flows

  • Diode acts as a closed switch

Forward current equation:

I=Is(eV/ηVT−1)I = I_s (e^{V/\eta V_T} - 1)

Where:

  • IsI_s = reverse saturation current

  • VTV_T = thermal voltage (~26 mV at room temp)

  • η\eta = emission coefficient (1–2)

6.2 Reverse Bias Operation

Connection:

  • P-side → Negative terminal

  • N-side → Positive terminal

Effects:

  • Barrier potential increases

  • Depletion region widens

Result:

  • Very small reverse saturation current

  • Diode behaves like an open switch

7. Reverse Saturation Current

  • Caused by minority carriers

  • Very small (µA in Ge, nA in Si)

  • Strongly dependent on temperature

8. Breakdown Mechanism

When reverse voltage exceeds a critical value:

8.1 Zener Breakdown

  • Occurs at low voltage

  • Heavily doped junction

  • Strong electric field causes tunneling

8.2 Avalanche Breakdown

  • Occurs at high voltage

  • Lightly doped junction

  • Impact ionization causes carrier multiplication

9. V–I Characteristics

Forward Region

  • No current until cut-in voltage

  • Rapid exponential rise after knee voltage

Reverse Region

  • Small constant current

  • Sudden increase at breakdown

10. Temperature Effects

  • Barrier voltage decreases with temperature

  • Reverse saturation current increases

  • Diode becomes more conductive at higher temperature

11. Practical Parameters

ParameterMeaning
Cut-in voltageMinimum forward voltage
Peak inverse voltage (PIV)Max reverse voltage
Maximum forward currentSafe current limit
Power ratingMax dissipated power

12. Applications

  • Rectifiers

  • Clippers & clampers

  • Signal demodulation

  • Voltage regulation (Zener diode)

  • Protection circuits

13. One-Line Summary 

A PN junction diode is a semiconductor device that conducts in forward bias by reducing the depletion region and blocks current in reverse bias due to an increased potential barrier.

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