Optoelectronic devices exploit the interaction between photons and charge carriers in semiconductors to either:

• Generate electrical signals from light (photodetection), or
• Produce light from electrical energy (emission).

Three key devices form the foundation of this field:

• Photodiodes: Reverse-biased PN junctions where incident photons generate electron–hole pairs, producing a photocurrent.
• LEDs: Forward-biased PN junctions where injected carriers recombine radiatively, emitting photons.
• Solar Cells: Large-area photodiodes operated in photovoltaic mode to convert sunlight into electrical power.

Specialized variants include:

• Avalanche photodiodes
• CCD/CMOS image sensors
• Laser diodes

These extend optoelectronic principles to high-sensitivity detection, imaging, and coherent light generation.

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Photon–Carrier Interactions

The fundamental processes in optoelectronic devices involve:

1. Photon Absorption → A photon with energy ( h\nu \geq E_g ) excites an electron from the valence band to the conduction band.
2. Carrier Generation & Separation → In the depletion region, the built-in electric field sweeps carriers toward opposite terminals.
3. Radiative Recombination → In direct bandgap semiconductors, electron–hole recombination releases a photon.

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Photodiode Operation

For a reverse-biased photodiode, the total current is:

$$I = I_{\text{dark}} - I_{\text{ph}}$$

Where:

$$I_{\text{dark}} = I_S \left( e^{\frac{qV}{kT}} - 1 \right)$$

and the photocurrent is:

$$I_{\text{ph}} = \frac{q \, \eta \, P_{\text{opt}}}{h\nu}$$

• ( \eta ) → Quantum efficiency
• ( P_{\text{opt}} ) → Incident optical power
• ( h\nu ) → Photon energy

For reverse bias ( V < 0 ), ( I_{\text{ph}} ) dominates.

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LED Operation

For a forward-biased LED, the injected carrier density determines the photon emission rate.
The internal quantum efficiency is:

$$\eta_{\text{int}} = \frac{R_{\text{rad}}}{R_{\text{rad}} + R_{\text{nonrad}}}$$

Where:

• ( R_{\text{rad}} ) → Radiative recombination rate
• ( R_{\text{nonrad}} ) → Non-radiative recombination rate

The emitted optical power is given by:

$$P_{\text{opt}} = \eta_{\text{ext}} \cdot \frac{h\nu I}{q}$$

Where ( \eta_{\text{ext}} ) is the external quantum efficiency, accounting for optical extraction losses.

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Solar Cell Operation

A solar cell behaves as a large-area photodiode operated at zero or forward bias.
The illuminated I–V relationship is:

$$I = I_S \left( e^{\frac{qV}{kT}} - 1 \right) - I_{\text{ph}}$$

Key Performance Parameters

• Short-circuit current:

$$I_{\text{sc}} = I \Big|_{V=0}$$

• Open-circuit voltage:

$$V_{\text{oc}} = V \Big|_{I=0}$$

• Fill Factor:

$$FF = \frac{V_{\text{mp}} I_{\text{mp}}}{V_{\text{oc}} I_{\text{sc}}}$$

• Efficiency:

$$\eta = \frac{V_{\text{oc}} I_{\text{sc}} FF}{P_{\text{in}}}$$

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Specialized Devices

• Avalanche Photodiode (APD): Reverse-biased beyond breakdown; carriers undergo impact ionization, giving high multiplication gain.
• CCD / CMOS Image Sensors: Use arrays of photodiodes for image capture.
  • CCD: Uses charge transfer between pixels.
  • CMOS: Uses per-pixel amplification.
• Laser Diode: Uses double heterostructures to confine carriers and photons, achieving stimulated emission and coherent light output.

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Summary of Key Equations

Photocurrent:

$$I_{\text{ph}} = \frac{q \, \eta \, P_{\text{opt}}}{h\nu}$$

Internal Quantum Efficiency:

$$\eta_{\text{int}} = \frac{R_{\text{rad}}}{R_{\text{rad}} + R_{\text{nonrad}}}$$

Solar Cell Efficiency:

$$\eta = \frac{V_{\text{oc}} I_{\text{sc}} FF}{P_{\text{in}}}$$