Semiconductor Devices Principles: PN Diode, LED, Solar Cell, APD

Posted on Mar 2, 2026 in Physics

Here is a detailed explanation of the principle, construction, and working of the following semiconductor devices:

1. PN Junction Diode

2. LED (Light Emitting Diode)

3. Solar Cell

4. Avalanche Photodiode

1. PN Junction Diode

Principle

A PN junction diode works on the principle of unidirectional current conduction — it allows current to flow in forward bias but blocks it in reverse bias.

Construction

• Made by joining P-type and N-type semiconductors.

• A depletion region forms at the junction due to diffusion of electrons and holes.

• Two terminals: Anode (P) and Cathode (N).

Working

• Forward bias: External voltage reduces the barrier; current flows.

• Reverse bias: Barrier increases; only leakage current flows.

• Breakdown occurs if reverse voltage exceeds a critical value (not desirable).

2. LED (Light Emitting Diode)

Principle

When a forward-biased PN junction diode made from direct bandgap materials (e.g., GaAs, GaP) conducts, electrons recombine with holes, releasing energy in the form of light (electroluminescence).

Construction

• Made from III-V semiconductors (for example, Gallium Arsenide, Gallium Phosphide).

• Transparent epoxy encapsulation or lens to allow light emission.

• Anode and Cathode terminals for electrical connection.

Working

• Forward bias: Electrons move from the N side to the P side and recombine with holes.

• Recombination releases photons (visible or infrared light).

• The emission wavelength (color) depends on the material bandgap.

3. Solar Cell (Photovoltaic Cell)

Principle

A solar cell works on the photovoltaic effect — when sunlight falls on a PN junction, it generates electron-hole pairs; the built-in electric field at the junction separates them, producing electric current.

Construction

• Large-area PN junction exposed to sunlight.

• Anti-reflective coating to reduce reflection and trap light.

• Transparent glass cover for protection and durability.

• Metal contacts for current collection (front and back contacts).

Working

• Photons from sunlight excite electrons → electron-hole pairs are generated.

• Electric field at the junction drives electrons to the N side and holes to the P side.

• This creates a potential difference and current flows through an external load.

4. Avalanche Photodiode (APD)

Principle

An APD operates under reverse bias and is based on avalanche multiplication — a single photon can trigger a cascade of charge carriers, amplifying the photocurrent.

Construction

• A specially doped PN junction designed to withstand high reverse voltage.

• Multilayer structure with a depletion region wide enough to allow impact ionization.

• Often includes a guard ring to prevent edge breakdown.

Working

• Incident light creates electron-hole pairs in the depletion region.

• Under high reverse bias, carriers gain enough energy to impact-ionize atoms, producing more carriers (avalanche).

• The result is a large multiplication of photocurrent, enabling detection of weak light signals.

Summary Table

Here is a detailed explanation of the principle, construction, and working of the following semiconductor devices:

1. PN Junction Diode

2. LED (Light Emitting Diode)

3. Solar Cell

4. Avalanche Photodiode

1. PN Junction Diode

Principle

A PN junction diode works on the principle of unidirectional current conduction — it allows current to flow in forward bias but blocks it in reverse bias.

Construction

• Made by joining P-type and N-type semiconductors.

• A depletion region forms at the junction due to diffusion of electrons and holes.

• Two terminals: Anode (P) and Cathode (N).

Working

• Forward bias: External voltage reduces the barrier; current flows.

• Reverse bias: Barrier increases; only leakage current flows.

• Breakdown occurs if reverse voltage exceeds a critical value (not desirable).

2. LED (Light Emitting Diode)

Principle

When a forward-biased PN junction diode made from direct bandgap materials (e.g., GaAs, GaP) conducts, electrons recombine with holes, releasing energy in the form of light (electroluminescence).

Construction

• Made from III-V semiconductors (for example, Gallium Arsenide, Gallium Phosphide).

• Transparent epoxy encapsulation or lens to allow light emission.

• Anode and Cathode terminals for electrical connection.

Working

• Forward bias: Electrons move from the N side to the P side and recombine with holes.

• Recombination releases photons (visible or infrared light).

• The emission wavelength (color) depends on the material bandgap.

3. Solar Cell (Photovoltaic Cell)

Principle

A solar cell works on the photovoltaic effect — when sunlight falls on a PN junction, it generates electron-hole pairs; the built-in electric field at the junction separates them, producing electric current.

Construction

• Large-area PN junction exposed to sunlight.

• Anti-reflective coating to reduce reflection and trap light.

• Transparent glass cover for protection and durability.

• Metal contacts for current collection (front and back contacts).

Working

• Photons from sunlight excite electrons → electron-hole pairs are generated.

• Electric field at the junction drives electrons to the N side and holes to the P side.

• This creates a potential difference and current flows through an external load.

4. Avalanche Photodiode (APD)

Principle

An APD operates under reverse bias and is based on avalanche multiplication — a single photon can trigger a cascade of charge carriers, amplifying the photocurrent.

Construction

• A specially doped PN junction designed to withstand high reverse voltage.

• Multilayer structure with a depletion region wide enough to allow impact ionization.

• Often includes a guard ring to prevent edge breakdown.

Working

• Incident light creates electron-hole pairs in the depletion region.

• Under high reverse bias, carriers gain enough energy to impact-ionize atoms, producing more carriers (avalanche).

• The result is a large multiplication of photocurrent, enabling detection of weak light signals.

Summary Table