Photodiode Basics: Types, Working, Modes, Uses

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Imagine a world where machines could actually see like humans. No more robots awkwardly crashing into stuff or tripping over random obstacles. Instead, they’d be moving through their surroundings with the same smoothness and precision as we do. That’s where photodiodes come in—their electronic eyes.

Just like our eyes, photodiodes detect light, but they take it to a whole new level. These things can see in the dark, in blinding sunlight, and even catch wavelengths that we can’t. Plus, they translate that light into electrical signals that machines can read, giving them legit vision.

Introduction to Photodiodes

A semiconductor device that transforms light into electrical current is called a photodiode. It works on the basis of the photoelectric effect, in which photons impacting the diode produce pairs of electrons and holes in the semiconductor material. Because light can produce a quantifiable current when it strikes a photodiode, it is a crucial part of many optical applications.

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Operating Principle

A photodiode is a device composed of a PIN (p-n junction) structure that, through the process known as the inner photoelectric effect, produces electron-hole pairs when light strikes it. 

An electric field inside the diode moves these particles, sending holes to the anode and electrons to the cathode, creating a photocurrent. When light is absorbed close to the diode’s electric field, this occurs. 

The photocurrent, which comes from light, and the dark current, which runs even in the absence of light, make up the total current. It is crucial to lower the dark current in order to increase the photodiode’s sensitivity.

Modes of Operation

Photovoltaic Mode:

Photocurrent travels via a short circuit from the anode to the cathode in photovoltaic mode (zero bias). A voltage builds up that forward biased the diode in the event of an open circuit or resistance in the circuit. Solar cells are built on this principle, with the device tuned to provide the most power at the lowest possible voltage and forward current. In essence, a conventional solar cell is a large photodiode. 

Photoconductive Mode:

The photodiode is reverse biased in photoconductive mode, which means the cathode is more positive than the anode. The depletion layer expands as a result, decreasing capacitance and enabling faster electron collection, which quickens the response time. The photocurrent largely stays constant, although the opposing bias also results in dark current. 

This mode is faster, but it may also produce more electrical noise due to avalanche effects or dark current. Because good PIN diodes have very low leakage currents (less than 1 nA), the resistance of the circuit usually causes the majority of the noise.

Materials

A photodiode’s material selection is essential to determining its characteristics since only photons with enough energy to activate electrons across the material’s bandgap can result in appreciable photocurrents.

The materials shown in the table below are frequently used to make photodiodes.

Silicon-based photodiodes produce less noise than germanium-based photodiodes due to their larger bandgap.

New materials for photodiode manufacture have emerged, including binary materials like graphene and MoS2.

Features of Photodiode 

Critical performance parameters of a photodiode consists of:

1. Critical Performance Parameters:

• Spectral responsivity
• Dark current
• Response time
• Noise-equivalent power

2. Spectral Responsivity:

• Ratio of generated photocurrent to incident light power (A/W in photoconductive mode).
• Can also be expressed as quantum efficiency (photogenerated carriers per incident photon).

3. Dark Current:

• Current through the photodiode in absence of light (includes background radiation and semiconductor saturation current).
• Needs calibration for accurate optical power measurements and adds noise in optical communication systems.

4. Response Time:

• Time for the detector to respond to optical input.
• Affected by transit-time spread (movement of electron-hole pairs) and RC time constant (resistance and capacitance of the circuit).
• Determines bandwidth for data transmission in optical systems.

Silicon photodiode response vs incident light wavelength

Types of Photodiode

1. PN photodiode: This photodiode can be useful in a number of circumstances. This is used in all minimal light conditions since it does not require reverse bias.
2. PIN photodiode: PIN photodiodes have a larger surface area for both collecting and converting light photons, which allows them to capture photons of light with a bigger effect than PN photodiodes.
3. Avalanche photodiode: This type of photodiode can multiply both electrons and holes since it operates at a much higher reverse bias.
4. Schottky photodiode: This type of photodiode combines several Schottky diodes to provide both long- and short-wavelength identification.

Photodiode vs Photomultipliers

Advantages of Photodiodes

1. Fast Response Time: Photodiodes are perfect for high-speed applications because of their fast response to light, particularly the PIN and Schottky varieties.
2. High Sensitivity: These detectors are capable of detecting low light levels, particularly when they are built with reverse bias or avalanche setups.
3. Compact Size: Photodiodes are lightweight, compact, and simple to incorporate into a variety of systems.
4. Low Cost: In comparison to other light-detecting technologies such as photomultipliers, they are comparatively affordable.
5. Durability: Photodiodes can function in a variety of environmental situations and have a lengthy lifespan.
6. Modest electricity Consumption: They usually use very little electricity and demand modest operating voltages.
7. Broad Spectrum Response: They can be made to detect light in a range of wavelengths, from infrared to ultraviolet.

Disadvantages of Photodiodes

1. Low Internal Gain: Standard photodiodes, with the exception of avalanche varieties, do not magnify the signal, which results in lesser sensitivity in low-light situations. This is in contrast to devices like photomultipliers.

2. Dark Current: Even in the absence of light, photodiodes produce a tiny current known as “dark current,” which can introduce noise and lower the light detection accuracy.

3. Temperature Sensitivity: Changes in temperature can have an impact on photodiode performance, with an increase in dark current at higher temperatures.

4. Restricted Dynamic Range: Photodiodes’ inability to precisely detect extremely high or low light intensities limits their applicability in particular situations.

5. Noise: Photodiodes have the potential to produce additional electrical noise in reverse-biased modes. This noise may impede the quality of the signal, especially in sensitive applications such as communication systems.

Applications of Photodiodes

1. Optical Communication Systems: In fiber optic communication, photodiodes are utilized to transform light signals into electrical impulses that are then used to transfer data.
2. Consumer electronics: Included in gadgets such as optical sensors in smartphones, remote controllers, and cameras.
3. Medical Devices: Used to identify the body’s absorption of light in devices like heart-rate monitors and pulse oximeters.
4. Environmental Monitoring: Photodiodes are used to measure light intensity in solar energy systems, UV index detectors, and light meters.
5. Industrial Automation: Optical sensors used in automated systems for item detection, location sensing, and counting are examples of industrial automation.
6. Safety Systems: Used in smoke detectors and other safety devices to perceive changes in light conditions.

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This article was published in August 2020 and recently updated in September 2024.