Short Notes Photodiode - Electronic Devices - Electronics and Communication

Table of Contents
1. Introduction
2. What is a Photodiode?
3. Types of Photodiode
4. Key Features and Performance Parameters
5. Construction
6. Working Principle
7. Modes of Operation
8. Photodiode vs Phototransistor
9. Photodiode Circuit
10. Photodiode Efficiency
11. PIN Photodiode versus PN Photodiode
12. Advantages
13. Disadvantages
14. Applications
15. V-I Characteristics of a Photodiode
View more

Introduction

Photodiode is a PN-junction diode designed to convert light energy into electrical current. It is also called a photo-detector, light detector, or photo-sensor. Photodiodes are normally used in reverse-bias so that the P side is connected toward the negative potential and the N side toward the positive potential of the supply. They are very sensitive to light: when photons strike the device, electron-hole pairs are generated and a measurable photocurrent flows. A solar cell can be regarded as a large-area photodiode that converts solar radiation into electrical energy; solar cells require bright illumination to produce useful power.

What is a Photodiode?

A photodiode converts incident light into current (or into a voltage, depending on the external circuit). It typically includes an optical window or anti-reflection coating and may include surface-shaping features or lenses to guide light onto the photosensitive area. Larger active areas collect more light but increase junction capacitance and reduce speed, so there is a trade-off between sensitivity and response time. Many photodiodes use a PIN structure (an intrinsic layer between P and N) rather than a simple PN junction to increase light collection and reduce capacitance.

Physically a photodiode looks similar to a regular diode. Conventionally the shorter lead is the cathode and the longer lead the anode. Under forward bias conventional current flows from anode to cathode; photocurrent produced under illumination is collected in the reverse direction when the device is operated as a photodetector.

Types of Photodiode

Photodiodes differ by structure and materials. They all rely on the same basic photoelectric process but their detailed performance (speed, sensitivity, spectral range, gain) varies. Common types are:

• PN photodiode
• PIN photodiode
• Schottky photodiode
• Avalanche photodiode (APD)

PN Photodiode

The earliest photodiodes use a simple PN junction. Photodetection occurs in the depletion region. PN photodiodes are compact but have lower sensitivity and higher capacitance than PIN types; they are still used where cost or simplicity is important.

PIN Photodiode

The PIN photodiode inserts an intrinsic (undoped) layer between the P and N regions. The wider depletion region collects more photons and reduces junction capacitance, improving both sensitivity and speed. For this reason PIN diodes are the most commonly used photodiodes in optical communication and measurement.

Avalanche Photodiode

Avalanche photodiodes (APD) operate at high reverse bias so that impact ionisation multiplies carriers produced by a single absorbed photon, providing internal gain. APDs give higher sensitivity in low-light but also generate higher noise and require careful biasing and temperature control.

Schottky Photodiode

Schottky photodiodes use a metal-semiconductor junction (Schottky barrier) rather than a PN junction. The small junction capacitance enables very fast operation, so these devices are used in high-bandwidth optical receivers and communications equipment.

Choice of type depends on the application; key selection parameters include noise, spectral response (wavelength), allowable reverse bias, gain, speed (transit time), and active area.

Key Features and Performance Parameters

• Good linearity of output current with incident optical power (in the linear region)
• Low noise (especially for Si devices compared with Ge)
• Wide spectral response (depends on material)
• Mechanically rugged, lightweight and compact
• Long operational life
• Trade-offs between sensitivity (active area), speed (capacitance and transit time), and noise

Materials and Spectral Ranges

• Silicon (Si): approximately 190-1100 nm
• Germanium (Ge): approximately 400-1700 nm
• Indium gallium arsenide (InGaAs): approximately 800-2600 nm
• Lead(II) sulfide (PbS): approximately 1000-3500 nm
• Mercury cadmium telluride (MCT): approximately 400-14000 nm

Because of the wider bandgap and lower intrinsic carrier generation, Si-based photodiodes generally produce lower dark noise than Ge devices for near-visible and near-IR applications.

Construction

Typical photodiode construction uses P-type and N-type semiconductor layers. In many silicon devices an N-type epitaxial layer is grown on an N+ substrate and a P+ diffusion is formed on top to create the junction. Metal contacts form the anode and cathode. The front surface is divided into an active area (where light is admitted) and non-active regions protected by dielectric such as silicon dioxide (SiO2). An anti-reflection coating is often applied to the active surface to reduce reflection losses and maximise photon absorption.

Working Principle

When a photon with energy equal to or greater than the semiconductor bandgap is absorbed it produces an electron-hole pair (internal photoelectric effect). If absorption occurs within the depletion region, the built-in electric field separates the carriers: electrons drift toward the cathode and holes toward the anode, producing a photocurrent. The total current through the diode is the sum of the photocurrent and the dark (bias) current; minimising dark current improves detector sensitivity.

Modes of Operation

Photodiodes are used in three principal modes:

• Photovoltaic (zero-bias) mode: the diode delivers a small voltage and current under illumination without external bias. This mode is simple but has limited dynamic range and nonlinear voltage output.
• Photoconductive (reverse-bias) mode: the diode is reverse biased. Reverse bias widens the depletion region, reduces junction capacitance and transit time, and gives faster response. This is the most common operating mode for high-speed detection but increases electronic noise (dark current).
• Avalanche mode: the diode is biased near avalanche breakdown so each photogenerated carrier can trigger an avalanche multiplication process, providing internal gain. Avalanche mode increases sensitivity but also increases noise and requires precise high-voltage biasing.

Why operate in reverse bias?

Reverse bias increases the depletion width and reduces junction capacitance. Reduced capacitance and wider collection region lower the carrier transit time, giving faster device response and improved bandwidth. In reverse bias the photocurrent is approximately proportional to incident optical power, giving a linear detector response over a wide range.

Photodiode vs Phototransistor

Both devices convert light into electrical signals, but they differ in operating principle and performance:

• Phototransistor: provides internal gain because absorbed light modulates the base current of a transistor, producing a larger collector current. This gives higher sensitivity but slower response due to charge storage and transistor dynamics.
• Photodiode: typically faster and more linear than a phototransistor. PIN photodiodes used with reverse bias achieve high speed and are preferred when fast, linear detection is needed.

Photodiode Circuit

A simple photodiode circuit uses a reverse-biased photodiode and a load resistor. The photocurrent generated by illumination flows through the resistor and produces a measurable voltage. The photocurrent is approximately proportional to incident light intensity over the linear operating range.

Typical connection for reverse-bias operation: the photodiode cathode is connected to the positive supply through a bias resistor or supply, and the anode is connected toward ground (or to the negative rail). Under illumination current flows from the cathode toward the anode (in the external circuit it appears as a current leaving the cathode toward the supply). Because the raw photocurrent is often small, external amplification (transimpedance amplifier) is commonly used to convert photocurrent to a usable voltage.

Photodiode Efficiency

Quantum Efficiency

Quantum efficiency (η) is the fraction of absorbed photons that produce charge carriers contributing to the photocurrent. If P is the optical power incident on the detector and hν is the photon energy, the rate of incident photons is P/(hν). If a fraction η of those photons produce charge carriers that contribute to current, the photocurrent I_ph is given by the relation:

\[I_{ph} = \eta\,e\,\frac{P}{h\nu}\]

Where η is the quantum efficiency (0 ≤ η ≤ 1), e is the electron charge, and hν is the photon energy.

High quantum efficiency (in some devices above 95% at certain wavelengths) requires minimising reflection (anti-reflection coatings) and maximising internal carrier collection.

Responsivity

Responsivity (S or R) is the ratio of photocurrent to incident optical power in the linear region:

\[R = \frac{I_{ph}}{P} = \frac{\eta\,e}{h\nu}\]

Responsivity depends on wavelength through \(h\nu\) (or via \(\nu = c/\lambda\)) and on quantum efficiency. Example: if \(\eta = 0.90\) at wavelength \(\lambda = 800\ \text{nm}\), the responsivity is approximately 0.58 A/W.

Devices with internal multiplication (APD or photomultiplier) may show effective responsivity greater than 1 A/W due to internal gain; when reporting quantum efficiency the internal multiplication is usually not included in η.

PIN Photodiode versus PN Photodiode

• A PN photodiode can be used without reverse bias in some low-light, low-noise applications but typically has smaller depletion width and larger capacitance, reducing speed.
• A PIN photodiode operated with reverse bias gives a wider depletion region, lower capacitance and faster response, but reverse bias increases dark current which can reduce signal-to-noise ratio if not controlled.
• For high dynamic range and high bandwidth (BW) applications, reverse-biased PIN diodes are preferred because of the smaller junction capacitance and shorter carrier storage time.

Advantages

• Low series resistance
• High speed of operation (especially PIN and Schottky types)
• Long operational life
• High spectral response and good linearity
• No very high voltages required for normal operation (except APD)
• Compact and mechanically rugged
• High quantum efficiency and low dark current (especially silicon devices)
• Good frequency/temporal response

Disadvantages

• Temperature sensitivity (performance varies with temperature)
• Photocurrent magnitude can be very small and often requires amplification
• Active area is limited (large area increases capacitance and reduces speed)
• Simple PN photodiodes have slower response and lower sensitivity than specialised detectors
• APDs have higher noise and require high bias voltages and temperature compensation
• Offset (dark) current exists and can limit sensitivity in low-light situations

Applications

• Optical communications (fiber-optic receivers)
• Light-level sensing and precision light measurements in scientific instruments
• Consumer electronics: remote controls, CD/DVD players, smoke detectors, and sensor modules
• Medical instruments: CT detectors, blood-gas monitors, and analytical instrumentation
• Industrial instrumentation and position, colour and intensity sensing
• High-speed photometry and optical timing applications

V-I Characteristics of a Photodiode

A photodiode is commonly operated in reverse bias. The V-I characteristic shows that the photocurrent is nearly independent of the magnitude of the reverse bias (in the linear region) and depends primarily on the optical power incident on the device. In dark conditions (zero illumination) the current equals the small dark current (typically nanoampere range for silicon devices). As optical power increases, the photocurrent increases approximately linearly until limitations such as series resistance, saturation or device heating occur. The maximum photocurrent is limited by device power dissipation and geometry.