How Photodiodes Work and Their Applications

A photodiode is a type of semiconductor device that converts light into electric current. It is also known as a photodetector, a light detector or a photo sensor. Photodiodes are designed to operate in reverse bias conditions, meaning that the voltage applied across them is opposite to the direction of the current flow. Photodiodes are widely used in scientific and industrial instruments to measure light intensity, detect optical signals and generate electric power from solar energy.

What is a Photodiode?

A photodiode is defined as a PN junction diode that generates current when exposed to light. This junction is formed by combining P-type and N-type semiconductor materials. P-type material has extra positive charge carriers (holes), while N-type material has extra negative charge carriers (electrons). When these materials meet, electrons from the N-type region move into the P-type region, recombining with holes and creating a depletion region. This region acts as a barrier to further charge carrier diffusion.

A photodiode has two terminals, an anode and a cathode, which are connected to the P-type and N-type regions, respectively. The anode is usually marked with a tab or a dot on the device package. The symbol of a photodiode is shown below, with two arrows pointing toward the junction to indicate that it is sensitive to light.

How Does a Photodiode Work?

Connected in reverse bias, a photodiode carries a small reverse current from its cathode to its anode. This current, known as dark current, results from the thermal generation of minority charge carriers in the semiconductor. Dark current barely depends on the applied reverse voltage but grows with temperature and doping level.

When the light of sufficient energy strikes the photodiode, it creates electron-hole pairs in the semiconductor material. This process is also known as the inner photoelectric effect. If the absorption of light occurs in or near the depletion region, these charge carriers are swept by the electric field across the junction, creating a photocurrent that adds to the dark current. Thus holes move toward the anode and electrons toward the cathode, and the reverse current increases with light intensity.

The photocurrent is proportional to the light intensity for a specific wavelength and temperature. If the light intensity is too high, the photocurrent hits a maximum value called the saturation current, beyond which it no longer increases. This saturation current depends on the device’s geometry and material properties.

The photodiode can operate in two modes: photovoltaic mode and photoconductive mode.

Photovoltaic Mode

In photovoltaic mode, no external reverse voltage is applied to the photodiode, making it act like a solar cell that generates power from light. The photocurrent flows through a short circuit or load impedance connected to the terminals. If the circuit is open or has high impedance, a voltage builds up across the device, forward-biasing it. This voltage, called the open-circuit voltage, depends on light intensity and wavelength.

The photovoltaic mode exploits the photovoltaic effect, which is used to produce solar energy from sunlight. However, this mode has some disadvantages, such as low response speed, high series resistance, and low sensitivity.

Photoconductive Mode

In photoconductive mode, an external reverse voltage is applied to the photodiode, and it acts like a variable resistor that changes its resistance with light intensity. The photocurrent flows through an external circuit that provides a bias voltage and measures the output current or voltage.

The photoconductive mode has some advantages over the photovoltaic mode, such as high response speed, low series resistance, high sensitivity and wide dynamic range. However, this mode also has some drawbacks, such as higher noise levels, higher power consumption and lower linearity.

Characteristics of Photodiode

The characteristics of a photodiode describe its performance under different conditions of light intensity, wavelength, temperature, bias voltage, etc. Some of these characteristics are:

• Responsivity: the ratio of output photocurrent to input light power at a given wavelength, expressed in amperes per watt (A/W) or milliamperes per milliwatt (mA/mW). Device material, geometry, doping level and junction depth all shape it.
• Quantum efficiency: the ratio of electron-hole pairs generated by light to the photons striking the device at a given wavelength, expressed as a percentage. Absorption coefficient, reflection losses and recombination losses set it.
• Spectral response: how responsivity and quantum efficiency vary with wavelength, showing which wavelengths the device can detect. Band gap energy, absorption coefficient and reflection losses shape it.
• Dark current: the reverse current that flows with no light present, expressed in nanoamperes (nA) or microamperes (µA). Temperature, doping level and leakage currents set it.
• Dark resistance: the ratio of maximum reverse voltage to the device’s dark current, expressed in ohms (Ω) or megaohms (MΩ). Temperature, doping level and leakage currents set it.
• Noise: unwanted fluctuations in the output signal from sources such as thermal noise, shot noise and flicker noise. It limits the signal-to-noise ratio (SNR) and resolution of the device.
• Linearity: how closely the output signal tracks the input over a wide range, which governs the accuracy and precision of the device.
• Response time: how long the output takes to reach a set percentage (usually 90%) of its final value after a step change in the input. It bounds the speed and bandwidth of the device.

Applications of Photodiode

Photodiodes have many applications in various fields, such as:

• Optical communication: Photodiodes are used to receive optical signals transmitted through fiber optic cables or free space. They convert optical signals into electrical signals that can be processed by electronic circuits.
• Optical measurement: Photodiodes are used to measure light intensity, wavelength, color, spectra, etc., for various purposes such as scientific research, quality control, environmental monitoring, etc.
• Optical imaging: Photodiodes are used to form images by detecting light reflected or emitted from objects or scenes. Cameras, scanners, night-vision devices and medical imaging equipment all rely on them.
• Optical switching: Photodiodes are used to control optical switches that can turn on or off optical signals or change their direction or wavelength. Optical modulators, multiplexers/demultiplexers, routers and logic gates use them.
• Solar power generation: Photodiodes are used to convert solar energy into electrical energy by absorbing photons from sunlight. Solar cells, panels, chargers and lamps use them.

Some examples of specific applications of photodiodes are:

• Alarm circuit using photodiode: A photodiode can be used to detect an intrusion by breaking a beam of light that falls on it from a light source. When there is no obstruction in front of the photodiode, a reverse current flow through it due to the incident light. When an obstruction occurs, the reverse current drops to the dark current level. The circuit is designed to trigger an alarm when the reverse current falls below a certain threshold. This arrangement can secure doorways, windows and other entry points.
• Counter circuit using photodiode: A photodiode can be used to count objects that pass through a conveyor belt by breaking a beam of light that falls on it from a light source. When there is no object in front of the photodiode, a reverse current flows through it due to the incident light. When an object passes through the light beam, it blocks the light from reaching the photodiode. The circuit is designed to increment a counter when the reverse current falls below a certain threshold. This arrangement can be used to count items such as bottles, coins, cards, etc.

Conclusion

A photodiode converts light into electric current through the inner photoelectric effect, which creates electron-hole pairs when photons strike the PN junction. Reverse bias and two operating modes, photovoltaic and photoconductive, shape how that current is used, while responsivity, quantum efficiency, spectral response, dark current, noise, linearity and response time measure how well it performs.

Optical communication, measurement, imaging, switching and solar power generation all rest on photodiodes, and beam-interruption alarm and counter circuits show the principle at its simplest. Wherever light must become a signal, a photodiode is usually the first component on the list.