In real-world applications, photodiodes are frequently employed with other optical devices to simplify light signal acquisition, processing, and analysis.

What are photodiodes?

Photodiodes are optoelectronic devices that generate electric current when subjected to light or radiation. They are composed of p-n junction semiconductor material operating in reverse bias mode.

In contrast to LEDs, these devices can convert light into electricity. They are mostly used to measure the brightness or intensity of light. They are also known as light detectors, photosensors, or photodetectors.

A photodiode is commonly represented, as depicted in Fig. 1, in circuits similar to a diode, with the only difference being the arrow, which indicates incident light or photon.

Fig. 1 Symbol of the photodiode (Arrows indicate incident photon or light). Source: Rakesh Kumar, Ph.D.

P-N Junction

Photodiodes usually comprise two types of semiconductor material, p and n-type, with two leads, anode, and cathode, enclosed in an insulating material as shown in Fig. 2. Commonly used semiconductor materials include indium gallium arsenide, germanium, and silicon.

Fig. 2 Illustration of P-N junction photodiode. Source: Rakesh Kumar, Ph.D.

Usually, every silicon atom is surrounded by four silicon atoms. These silicon atoms desire eight electrons in the valence shell (octet rule). Still, they only have four, so they covalently share electrons with the adjacent atom to form a stable structure.

Each silicon atom contributes an electron to the covalent bond and satisfies the octet rule of each atom. Hence, there are no free electrons and holes in silicon in normal conditions, as depicted in Fig. 3.

Fig. 3 Illustration of covalent bonding of silicon atom. Source: Rakesh Kumar, Ph.D.

P-N junctions in diodes are produced through the doping procedure. Doping is the process of adding impurities to the semiconductor material, as shown in Fig. 4, to enhance its conductivity.

The n-type of semiconductor materials are doped with pentavalent impurities with five electrons in their valence shell. One excess electron becomes free because silicon only requires four electrons in the valence for covalent bonding. Because of this, n-type semiconductors have an abundance of free electrons.

Fig. 4 Illustration of doping of semiconductor material with impurities resulting in n-type semiconductor having an excess of electrons and p-type having holes. Source: Rakesh Kumar, Ph.D.

On the other hand, p-type semiconductors are doped by trivalent impurities, such as aluminum, which only has three electrons in the valence shell and lacks one electron to covalently bond with a silicon atom, resulting in a hole. Hence, p-type semiconductor materials have excess holes (positively charged ions).

Depletion Region

The junction between the p and n-type of semiconductor material is called the depletion region, as shown in Fig. 5, which is the active region in the photodiode. The active region of the photodiode must be kept transparent so that the incident light can come in contact with the p-n junction.

In the p-n junction, a few electrons from n-type semiconductors are initially drawn to holes in p-type material, and holes in p-type semiconductor material are attracted toward the electrons in n-type material. Charged ions are produced due to this action in the depletion region.

When a negatively charged electron is added to an acceptor ion (p-type), it increases the negative charge. As a result, the acceptor ion has a negative charge.

On the other hand, donor ions (n-type) have a positive charge because they donate a negatively charged electron and contain many positively charged protons.

The accumulation of negatively and positively charged regions results in an electric field. This potential difference is created due to the electric field acting as a barrier, preventing further diffusion of electrons and holes in the junction.

Fig. 5 Illustration of the depletion region. Source: Rakesh Kumar, Ph.D.

Reverse Bias

In reverse bias mode, the p-type semiconductor material is linked to the negative terminal of the power supply. In contrast, n-type semiconductor material is connected to the positive terminal, as shown in Fig. 6.

This results in a broader depletion region because holes are drawn toward the negative terminal of the power supply. Electrons are drawn toward the positive side. As a result, the barrier becomes larger.

Fig. 6 Illustration of Reverse Bias. Source: Rakesh Kumar, Ph.D.

Under normal circumstances, in the absence of incident light, only a few holes and electrons in the photodiodes are attracted toward the positive and negative sides.

This causes a relatively small reverse bias current known as the "dark current," which is caused by the temperature of the surroundings. The dark current should be as minimal as possible for an ideal photodetector.

Band Theory of Solids

Solid band theory is the basis for how photodiodes operate. Solid band theory is a key component in determining the state of electrons in a solid. The valence and conduction bands are the two key bands in solid band theory.

Valence Band

Atoms comprise a nucleus in the center surrounded by an orbital shell containing electrons. These electrons are kept in the orbital shell with the help of the nucleus.

The outermost shell, called the valence shell, as shown in Fig. 7, can contain one or more electrons. The electron's energy in the valence shell contributes to its valence band energy.

Fig. 7 Illustration of Valence band and Conduction band. Source: Rakesh Kumar, Ph.D.

Conduction Band

The conduction band in the outermost shell, as depicted in Fig. 7, where electrons can easily move if they reach this layer, results in the current flow. This conduction band is typically empty in normal circumstances. When exposed to temperature or radiation, the electron from the valence shell absorbs the incident photon and is excited by this conduction band.

The distance between the conduction and valence bands is known as the band gap or forbidden gap. This bandgap determines the material's conductivity, as shown in Fig. 8.

Fig. 8 Difference between conductor, semiconductor, and insulator. Source: Rakesh Kumar, Ph.D.

● In a conductor such as copper, the conduction band and valence shell overlap, allowing electrons to move freely, resulting in current flow.

● Semiconductor materials have lower band gaps, and electrons reach the conduction band when subjected to temperature or radiation, resulting in the current passage.

● In the case of insulators such as rubber, the nucleus holds the electron with more energy. It has a larger band gap, preventing the electron from reaching the conduction band, and they do not conduct electricity.

All of these basic concepts, such as the formation of a p-n junction by doping, the larger depletion zone brought about by reverser bia, and the band theory of solids, which is in charge of the excitation of an electron to conduct electricity, are involved in the functioning of a photodiode.

Part 2 of the essay discusses the operation, variables that affect the operation, benefits, drawbacks, and applications of photodiodes.

Summarizing the Key Points

● Photodiodes convert light into electricity through p-n junction semiconductor materials, serving as essential light detectors.

● Doping processes introduce impurities to create p-type and n-type semiconductors, influencing electron and hole formation.

● Depletion regions at p-n junctions are vital in photodiodes, facilitating the separation of charges and generating electric fields.

● Understanding reverse bias in photodiodes is crucial to minimizing dark current and optimizing their performance as photodetectors.

● Band theory showcases the behavior of electrons in solids, with valence and conduction bands playing crucial roles.