Introduction

Photodiodes and Light Emitting Diodes (LEDs) are optoelectronic semiconductor devices that form the foundation of modern optical communication systems, imaging technology, and display applications. These devices convert between electrical signals and light through the principles of quantum mechanics and semiconductor physics. Together, they enable technologies ranging from fiber-optic communications carrying terabits of data per second to simple indicator lights in consumer electronics.

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Part 1: Photodiodes

Introduction to Photodiodes

Photodiodes are semiconductor devices that detect light and convert it into electrical current. Unlike photoresistors (LDRs) that change resistance based on light, photodiodes generate an output signal proportional to incident light intensity through the photovoltaic effect.

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Operating Principle and Physics

Fundamental mechanism:

1. Photons with sufficient energy strike the semiconductor material

2. Each photon excites an electron from the valence band to the conduction band

3. This creates an electron–hole pair

4. The built-in electric field of the reverse-biased p-n junction separates the charges

5. Electrons flow to the n-side, holes to the p-side

6. External circuit measures this flow as photocurrent

Energy requirement:

$$E_{\text{photon}} = h \times f = \frac{h c}{\lambda} \ge E_g$$

Where:

• h = Planck's constant (6.626 × 10⁻³⁴ J·s)

• f = light frequency (Hz)

• c = speed of light (3 × 10⁸ m/s)

• λ = wavelength (meters)

• E_g = bandgap energy of semiconductor (eV)

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Photodiode Configuration and Operating Modes

Photodiode Mode (Reverse Bias)

• Most common configuration

• Reverse bias voltage applied

• Wide depletion region

• Fast response time

• Low capacitance

• Requires transimpedance amplifier

Photovoltaic Mode (Zero Bias)

• No external bias voltage

• Device acts as small solar cell

• Generates open-circuit voltage

• Slower response time

• Used in light-powered applications

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Types of Photodiodes

1. PIN Photodiode

Structure: p-type layer, intrinsic (undoped) region, n-type layer

Advantages:

• High quantum efficiency

• Low capacitance

• Fast response (nanoseconds)

• High-frequency operation

Disadvantages:

• Requires reverse bias

• Higher dark current

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2. Avalanche Photodiode (APD)

Operating Principle:

• Operates at very high reverse bias voltage (200–500 V)

• Strong electric field causes impact ionization

• One photon generates multiple charge carriers

Multiplication relationship:

$$I_{\text{photo}} = M \times q \times \Phi$$

Where:

• M = multiplication factor (10–1000)

• q = electron charge

• Φ = photon flux

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Key Photodiode Parameters

Dark Current (Id)

$$I_d(T_2) = I_d(T_1) \times 2^{\frac{T_2 - T_1}{10}}$$

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Quantum Efficiency (QE)

$$QE($$

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Responsivity (R)

$$R = \frac{I_{\text{photo}}}{P_{\text{optical}}} ; \text{A/W}$$

Relation to quantum efficiency:

$$R = \frac{QE \times \lambda \times e}{h \times c}$$

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Bandwidth and Response Time

$$\tau = (R_d + R_L) \times C_j$$

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Photodiode Noise Sources

Shot Noise

$$i_{\text{shot}} = \sqrt{2 \times e \times I \times BW}$$

Thermal Noise

$$i_{\text{thermal}} = \sqrt{\frac{4 \times k_B \times T \times BW}{R_L}}$$

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Part 2: Light Emitting Diodes (LEDs)

Introduction to LEDs

Light Emitting Diodes are semiconductor devices that emit light when forward biased. They convert electrical energy directly into light through electroluminescence, making them highly efficient light sources used in displays, lighting, and indicator applications.

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Operating Principle

$$E_{\text{photon}} = h \times f = \frac{h c}{\lambda} \approx E_g$$

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Bandgap Energies and Corresponding Colors

Material	Bandgap (eV)	Wavelength (nm)	Color
GaAs	1.42	875	Infrared
GaAsP	1.9–2.26	550–650	Red–Yellow
GaN	3.44	360	Ultraviolet