What is a Photodiode?

Photodiodes are electronic components that convert light into an electrical signal. They contain a PN junction that is formed by p-type and n-type semiconductor regions. A p-type semiconductor is a type of semiconductor material where a trivalent impurity like Boron, Aluminium, etc. is added to an intrinsic or pure semiconductor. On the other hand, n-type semiconductor is an intrinsic semiconductor material that is doped with pentavalent impurities like phosphorus, arsenic, or antimony (Sb). They are widely used in various applications, such as optical communication, light detection and measurement, and control systems.

A symbolic representation of a typical photodiode is shown in figure 1.

Figure 1: Symbol of a Photodiode

There are various types of photodiodes, each designed for specific applications. Some of the commonly used photodiodes are:

• PN Photodiode
• PIN Photodiode 
• Avalanche Photodiode 
• Schottky Photodiode

PN Photodiode

Figure 2: PN Photodiode

A PN Photodiode is a simple p-n junction photodiode that operates in reverse bias. It is commonly used in optical communication and light detection applications.

Operation of a PN Photodiode

Figure 3: Working of PN Photodiode

When a photon of sufficient energy (or light) incident on the p-n junction, electron-hole pairs are generated in the depletion region. This mechanism of electron-hole pair generation by an external light source is known as the photoelectric effect. The electric field across the depletion region separates the electrons and holes, causing them to move in opposite directions. Thus, holes move toward the anode as it is negatively charged, and electrons move toward the cathode as it is positively charged. The movement of electrons and holes results in the generation of a photocurrent. Now when an external load is connected to the device, a current flow is observed which is proportional to the intensity of the incident light. The total current through the photodiode is the sum of the dark current (current generated in the absence of light) and the photocurrent. 

PIN Photodiode

Figure 4: PIN Photodiode

A PIN Photodiode is a p-i-n photodiode that has an intrinsic (i) layer made of pure silicon sandwiched between the p and n layers. It has a larger depletion region and hence, exhibits lower capacitance and higher speed. The PIN photodiode is a highly sensitive and fast responding device. It is commonly used in high-speed communication systems and optical fiber communication. 

Operation of a PIN Photodiode

Figure 5: Working of PIN Photodiode

The cross-sectional view of p-i-n photodiode is shown in figure 5. The PIN photodiode is biased in reverse bias mode, which creates a depletion region between the p and n regions. When light strikes the intrinsic layer of the pin photodiode, it generates electron-hole pairs. The electric field present within the depletion region created by the p-type and n-type layers of the photodiode causes the electrons to drift towards the n-type layer, while the holes drift towards the p-type layer. This results in a flow of current through the photodiode, which is proportional to the amount of light that is incident on the device. The intrinsic layer of the photodiode is specifically designed to be relatively thick, which allows it to absorb a greater amount of light and thus produce a higher signal output. Additionally, the p-type and n-type layers of the photodiode are typically heavily doped, which increases their conductivity and allows faster response times.

Avalanche Photodiode

Figure 6: Avalanche Photodiode

An Avalanche Photodiode is a type of p-n junction photodiode that operates in reverse bias beyond its breakdown voltage. It exhibits internal multiplication of photocarriers, resulting in a higher gain and sensitivity. It is commonly used in low-light-level detection and optical communication applications.

Operation of an Avalanche Photodiode

Figure 7: Working of Avalanche Photodiode

The APD is similar in structure to a pin photodiode, with a p-type layer, an intrinsic layer, and an n-type layer. The intrinsic layer of an APD is designed to have a high electric field, which allows for the creation of an avalanche of electron-hole pairs when light is incident on the device. This avalanche effect produces a large number of secondary electron-hole pairs that in turn produce a larger electrical signal than would be produced in a pin photodiode.

The avalanche effect is achieved by applying a reverse bias voltage across the APD creating a high electric field in the intrinsic layer. When a photon is absorbed in the intrinsic layer, it creates an electron-hole pair. If the electric field is strong enough, this electron can gain enough energy to create additional electron-hole pairs through collisions with other atoms in the material. This process repeats, resulting in an avalanche of electron-hole pairs that produce a much larger electrical signal.

The gain of an APD, or the degree of signal amplification achieved by the avalanche process, is determined by the applied bias voltage and the physical properties of the device, such as the thickness of the intrinsic layer and the doping concentration of the p-type and n-type layers. APDs can achieve very high gain, up to several hundred or thousand times that of a pin photodiode, making them useful for applications that require high sensitivity and low noise.

Schottky Photodiode

Figure 8: Schottky Photodiode

A Schottky Photodiode is a metal-semiconductor junction photodiode that exhibits low capacitance and high speed. It is commonly used in high-frequency communication and microwave applications. 

Operation of a Schottky Photodiode

Figure 9: Working of Schottky Photodiode

The Schottky photodiode consists of a metal layer (usually gold, platinum, or tungsten) that is in contact with a semiconductor material, such as silicon or gallium arsenide. When light is incident on the metal-semiconductor interface, it creates an electric field that separates the charge carriers in the semiconductor. This results in the generation of a photocurrent that is proportional to the intensity of the incident light.

The working principle of the Schottky photodiode is based on the Schottky barrier, which is a potential barrier formed at the metal-semiconductor interface due to the difference in the work functions of the metal and the semiconductor. The height of the barrier determines the sensitivity and speed of the photodiode. A higher barrier height results in faster response times, but lower sensitivity, while a lower barrier height results in higher sensitivity, but slower response times.

The Schottky photodiode has several advantages over other types of photodiodes. It has a fast response time, high sensitivity, and low noise, making it suitable for use in applications that require high-speed and high-precision optical detection. It also has a large active area, which allows it to detect light over a wide range of wavelengths.

Modes of Photodiode

• Operation of a photodiode in Photovoltaic mode

Figure 10: Photodiode in Photovoltaic mode

A photodiode can operate in photovoltaic mode where photovoltaic means the direct conversion of light into electric power using semiconducting materials. The photovoltaic mode is also known as zero bias and biasing is the setting of initial operating conditions like current and voltage, of a photodiode. In photovoltaic mode, the short circuit current produced by the diode is measured. Ideally, it is presented with 0 impedance (the short circuit). Thus, when the photodiode is irradiated by a flash of light, voltage is produced which is directly proportional to the photocurrent. However, photodiodes in photovoltaic mode are rarely used as they are inefficient when attached to a high load resistance. Thus, photodiodes are mostly operated in photoconductive mode. 

• Operation of a photodiode in Photoconductive mode

Figure 11: Photodiode in Photoconductive mode

In photoconductive mode, the diode is reverse biased i.e., the cathode is attached to the positive side of a battery with respect to the anode. 

Here the incident photon in the active region generates electron-hole pair. And in this biased region, there is a built-in potential, which leads to the movement of electrons towards the n side and holes towards the p side and therefore the carriers are separated by the electric field, resulting in a reverse photocurrent.

The response time is decreased by applying a reverse bias, which widens the depletion layer (intrinsic layer) and reduces the junction's capacitance. This expansion also creates a larger region with an electric field, which causes the rapid collection of electrons. Also, after the separation of electrons and holes, they do not recombine further because of the highly generated electric field. So, the photocurrent and illuminance are linearly proportional.

Materials for construction of photodiode

Only photons with sufficient energy excite electrons across the material's bandgap to produce photocurrents. Therefore, suitable materials are only used to produce photodiodes, some of them are listed below:

Features of a photodiode 

Parameters that govern a photodiode are:

Spectral responsivity: The spectral responsivity of a photodiode is a measure of its sensitivity to incident light of different wavelengths. It is the ratio of the generated photocurrent to incident light power and is expressed in A/W.

where R(λ) is the spectral responsivity at a given wavelength λ, I(λ) is the photocurrent generated by the photodiode in response to light of wavelength λ, and P(λ) is the incident optical power at wavelength λ.

The spectral responsivity of a photodiode can vary depending on the operating conditions, such as temperature and bias voltage.

Dark current: The dark current of a photodiode is the current that flows through the device even in the absence of light. It is generated by background radiation (which includes the cosmic rays generated by the sun) and the saturation current of the semiconductor junction. Here, saturation current is a combination of the light generated by the thermal generation of electron-hole pairs within the depletion region of the diode and the diffusion current due to minority carriers in the n and p regions diffusing across the depletion region. It is also a source of noise when a photodiode is used in an optical communication system. To maximize the sensitivity of the device, the dark current must be minimized by using a protective guard ring of conductive material at the junction region.

The equation for dark current in a photodiode can be expressed as:

Where I_d is the dark current, Is is the reverse saturation current, q is the charge of an electron, V_d is the voltage across the diode, k is the Boltzmann constant, and T is the temperature in Kelvin

Response time: Response time is the measure of time it takes for the device to respond to an optical input. It is determined by several factors, including the carrier lifetime, transit time, and diffusion length within the device. These factors depend on the material properties of the photodiode and the operating conditions, such as temperature and bias voltage. The resistance (R) and capacitance (C) of the photodiode and the external circuitry give rise to response time known as RC time constant.

When used in an optical communication system, the response time determines the bandwidth available for signal modulation and thus data transmission.

Noise-equivalent power: Noise-equivalent power is the minimum input optical power to generate photocurrent. It depends on several factors, including the dark current, the responsivity, and the noise characteristics of the device and can be calculated using the following equation:

where q is the elementary charge, Id is the dark current, k is the Boltzmann constant, T is the temperature in Kelvin, B is the electrical bandwidth, Δf is the noise equivalent bandwidth, and R is the responsivity of the photodiode.

The first term in the equation represents the noise due to the dark current, while the second term represents the noise due to thermal fluctuations. The NEP is usually expressed in units of watts per square root of hertz (W/√Hz).

Applications

Photodiodes are also used in control systems to detect the presence of light and control the operation of devices. They are used in electronic devices like compact disc players, and smoke detectors. In household remote controls, the photodiode is used as the receiver of data encoded on an infrared beam. It can be used as an optocoupler, allowing the transmission of signals between circuits without a direct metallic connection between them, making it isolated from high voltage differences. 

They are also used in medical equipment, such as pulse oximeters, to measure blood oxygen saturation levels.

Photodiodes are used as receivers in optical communication systems to convert optical signals into electrical signals. P–n photodiodes are used in photodetectors, such as photoconductors, charge-coupled devices (CCD), and photomultiplier tubes. They are used to generate an output that is dependent upon the illumination.

They are also used in light meters and photometers to measure the intensity of light. Photodiodes are used in solar cell that generates solar power using a large-area photodiode.