Photodetectors are critical for a wide range of commercial and scientific applications, and their unique material features have resulted in superior optoelectronic performance in recent years and significant potential in various fields. Part 1 of this article discusses the basic principles involved in working with a photodiode.

How does a photodiode work?

When light or radiation passes through the photodiode's depletion region as shown in Fig. 1, and when the incident photon's energy is greater than the bandgap of the semiconductor material, it can knock off the electron from the atom in the depletion region and produce empty holes.

The energy from the incident photon is absorbed by the electron in the valence shell and is excited to enter the conduction band; these electrons are now free to move and conduct electric current.

As a result of incident light, an electron-hole pair is created. Because of the direction of the electric field in the depletion region, holes are drawn toward the negative terminal, and electrons are drawn to the positive terminal. A current flow known as photocurrent is produced by this electron movement.

When the intensity of light increases, more electrons in the valence shell absorb photons, resulting in more electron-hole pairs and increased current. As a result, light intensity is measured by exposing the photodiode's depletion zone to light or radiation.

On the other hand, no current is generated when light travels across the p and n regions because the electron-hole pair created will recombine. Hence, the incident light or radiation is only applied to the depletion region.

Fig. 1 Working of photodiode. Source: Rakesh Kumar, Ph.D.

Factors Affecting the Efficiency of Photodiodes

Various elements can impact the effectiveness of photodiodes.

The wavelength of the incident photon

The wavelength of the incident photon influences the quantity of photocurrent produced. The incident photon wavelength for semiconductor materials such as silicon must fall within 400 nm to 1100 nm.

Meanwhile, semiconductor materials more frequently employed in optical communications, such as indium gallium arsenide, operate in higher wavelengths.

The energy of the incident photon

The incident photon's energy must be greater than the semiconductor material's band gap energy for electrons to absorb the energy and be stimulated to higher energy states. Most regularly used semiconductor materials, such as silicon, have a band energy of approximately 1.12 eV.

Responsivity

Responsivity is the term used to describe the photodiode's capacity to generate photocurrent with the given incident photon efficiently. Responsivity is an important parameter to check in the datasheet when selecting a photodiode.

Response time

Equivalent capacitance and resistance are two important factors affecting a photodiode's response time—the amount of time it takes to react to an optical stimulus. Capacitance is, in turn, influenced by the diode's area and reverse bias condition.

As the photodiode's area increases, junction capacitance increases and the response time slows down. Hence, the smaller the photodiode, the faster the response time.

Reverse biasing

One of the key factors influencing the amount of photocurrent generated is reverse biasing, which determines the width of the depletion region. A broader depletion region means more photons are adsorbed, and more electron-hole pairs are created.

This phenomenon increases the flow of electric current. Reverse biasing also lowers the junction capacitance, which fastens photodiodes' response time.

Noise equivalent factor

Noise equivalent power (NEP) is a significant factor affecting photodiodes' efficiency. The noise equivalent power is the least optical input that may be detected by a photodetector, resulting in a photocurrent equal to the system's inherent noise.

Advantages

There are numerous benefits to photodiodes.

● They are appropriate for various optical applications due to their high sensitivity, quick response to light, and relatively low noise.

● They are available in a compact, lightweight form ideal for space-constrained applications.

● They can detect a broad spectrum of wavelengths and be employed in various applications.

● Because of their linear response, photodiodes are well-suited for accurate light measurement.

● This photodiode has a comparatively higher durability.

● They are cost-effective.

Disadvantages

● One significant drawback is their susceptibility to temperature changes, as rising temperatures cause an increase in the flow of dark current, which raises noise levels.

● The p-n junction's transparent active region, which allows light to interact with it, is comparatively smaller.

● Many applications require current amplification since the output current is often quite low in the range of picoampere.

Applications

Because of their many benefits, photodiodes can be applied in a wide range of sensing domains, including medical (optical imaging, computerized tomography, pulse oximetry, etc.), scientific research, optical communication, spectroscopy and remote applications in electronic devices (television, air conditioners, smoke detectors,), space applications, etc.

For better understanding, photodetectors' applications in smoke detectors are shown diagrammatically in Fig. 2. In fire accidents; smoke particles can divert the light beam from an LED, which is detected by a photodetector positioned at an angle to the LED. The photodetector triggers the fire alarm when it detects a sufficient intensity of light.

Hence, photodiodes are employed in numerous applications.

OSI Optoelectronics PIN-RD15

One such photodiode to consider is the OSI optoelectronics manufacturer's PIN RD15, as shown in Fig. 3. It is a high-speed photodiode with a large active area.

Fig. 3 Image of PIN RD15 Source: OSI Optoelectronics

Features

● Active area 14.9 mm²

● Responsivity: 0.5 A/W

● Dark current: 1.0 nA

● Capacitance: 14 pF

● Package: T0-8

● Peak wavelength: 900 nm

● It also operates at maximum reverse voltage, which results in a wider depletion region, allowing more photons to be absorbed by the electron, resulting in an electron-hole pair, increased photocurrent, and improved responsivity of the photodiode.

● Additionally, the photodiode is fully depleted, reducing the junction capacitance and resulting in faster response time.

● Operating in maximum reverse bias mode also minimizes junction capacitance, resulting in a faster response time.

● It has an extremely low dark current that lowers noise and increases photodiode efficiency.

Applications

They can be used for

● High-energy X-ray detection

● Gamma rays detection

● Detection of high-energy particles such as electrons, alpha rays, and heavy ions.

Summarizing the Key Points

● Photodiodes convert incident photons into electric current, which is crucial for various applications.

● Factors like wavelength and energy of incident photons significantly impact the efficiency of photodiodes.

● Photodiodes offer high sensitivity, quick response to light, and low noise, making them ideal for various optical applications.

● Despite susceptibility to temperature changes and low output current, photodiodes remain cost-effective and versatile for numerous sensing domains.