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 energizes the PN junction. They are different from a typical rectifier diode. A typical diode passes current only in forward bias and blocks the current in reverse bias mode.
When a typical diode is reverse biased, a very small current is flowing due to minatory carriers. This current is leakage current. Similarly, a small current is flowing in photodiode in reverse biased. This small current is a dark current by supposing that the diode in the darkroom.
By exposing the diode to the light, electrons from the valence band excite 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. Dark current is when no light hitting the PN junction, a small current is flowing in reverse bias. The dark current denoted with Iλ. A careful calculation shows that a photodiode works like a resistor controlled by light intensity.
Circuit of photodiode
The photodiodes are always used in reverse bias in a circuit. P-type connects to the negative of the battery terminal. And n-type connects to the positive of the battery terminal. A simple circuit of a photodiode is presented here with a photodiode, 10k resistor, and a 5v DC voltage.
Speed and sensitivity of photodiodes
The response speed and sensitivity can be achieved by increasing the revere bias voltage. Another way to improve the sensitivity can be by introducing the high resistivity intrinsic material. It is placed in between the P-type and N-type materials and called 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. Where the germanium InGaSa photodiodes have high sensitivity 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 has a fast response.
The responsivity of the photodiodes can be calculated using the following formula.
R= η (e/hv)
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 photodiode out there in the market. All of them are working over the same basic principles. Each type has a smaller modification is to improve the quality of the photodiode. The classification of photodiode by functionality and construction.
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.
The most used photodiodes are PIN photodiodes. PIN photodiodes are using intrinsic semiconductor 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 photodiodes are hight gain devices. Avalanche photodiode is using low light but it produces high noise comparatively.
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 photodetector and are used in optical fiber links.
The commonly used photodetectors are;
- a photodiode
- avalanche photodiode
- photodiode array, and charged coupled devices (CCD).
Photodiodes are cheap, compact, and easy to use devices therefore most common photodetectors. The range of detection depends upon the materials.
Uses of photodiodes as a photodetector in the following products.
- Cameras, Compact disk player (CD), Television, and remote control.
- In medical devices in computed tomography and blood gas monitors
- Safety products i.e smoke detector
- Optical communication receivers
- Barcode scanners
- Automotive devices
- Surveying instrument
- Position sensors