A Photodiode is a semiconductor device that operates in reverse bias. The photodiode has a small opening on the top for light entering the light and energize the PN junction. They are different from a typical rectifier diode. A typical diode passes current only in forward bias and blocks current in reverse bias mode. There are types of it like Avalanche, Schottky, PN, and PIN photodiodes.
When a typical diode is reverse biased, a very small current flows due to minatory carriers. This current is the leakage current. Similarly, a small current is flowing in the photodiode in reverse bias. This small current is a dark current by assuming that the diode is in the darkroom.
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By exposing the diode to the light, electrons from the valence band are excited and jump to the conduction band. And thermally, electron-hole pairs are generated. As the current carriers increase, the reverse current also increases. It causes the photodiodes to operate in reverse bias. The reverse current of the diode strongly depends upon the light hitting the PN Junction through the transparent opening. A dark current is when no light hits the PN junction, and a small current flows in reverse bias. The dark current is denoted with Iλ. A careful calculation shows that a photodiode works like a resistor controlled by light intensity.
Circuit of PIN Photodiode
The photodiodes are always used in reverse bias in a circuit. P-type connects to the negative of the battery terminal. And the n-type connects to the positive of the battery. A simple circuit of a photodiode is presented here with a photodiode, a 10k resistor, and a 5V DC voltage.
Speed and sensitivity of photodiodes
The response speed and sensitivity can be achieved by increasing the reverse bias voltage. Another way to improve the sensitivity can be by introducing a high-resistivity intrinsic material. It is placed between the P-type and N-type materials and called a PIN photodiode. PIN photodiode has a response of a few hundred MHz. Silicon photodiodes are commonly used and have high sensitivity at 0.8-0.9µm. The germanium InGaSa photodiodes have a high sensitivity of around 1.1-1.7µm.
The response time also depends upon the capacitance of the PN Junction. To improve the capacitance, PIN photodiodes are introduced that decrease the capacitance. Another factor that affects the response time is the photo-sensitive area. The smaller active area has smaller capacitance and a faster response.
The responsivity of the photodiodes can be calculated using the following formula.
R= η (e/hv)
Where,
η – is the quantum efficiency,
e – is the charge of an electron,
hv – is the energy of the photon
Types of photodiode
There are many types of photodiodes out there in the market. All of them are working on the same basic principles. Each type has a smaller modification to improve the quality of the photodiode. The classification of photodiodes by functionality and construction.
PN photodiode
PN photodiode is the first developed type of photodiode. It is a simple PN Junction diode with a transparent window over the junction. Its sensitivity is not that great not used in advanced applications.
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PIN Photodiode
The most commonly used photodiodes are PIN photodiodes. PIN photodiodes use intrinsic semiconductors between the P-type and N-type of the PN Junction. It reduces the junction capacitance and improves the light absorption by the junction.
Avalanche photodiode
Avalanche photodiodes are high-gain devices. Avalanche photodiode uses low light, but it produces high noise comparatively.
Schottky photodiode
Schottky photodiodes use the small junction area to reduce the capacitance of the junction. It improves the response rate of the photodiode. These diodes are high-frequency photodetectors and are used in optical fiber links.
Photodetector
The commonly used photodetectors are;
- photo-resistors
- a photodiode
- avalanche photodiode.
- Photodiode array, and charge-coupled devices (CCD).
Photodiodes are cheap, compact, and easy-to-use devices; therefore the most common photodetectors. The range of detection depends on the materials.
Photodiode application
Uses of photodiodes as a photodetector in the following products.
- Cameras, Compact disc player (CD), Television, and remote control.
- In medical devices, computed tomography and blood gas monitors
- Safety products, i.e, smoke detector
- Optical communication receivers
- Barcode scanners
- Automotive devices
- Surveying instrument
- Position sensors