Introduction and application of photodiode and phototransistor

Photodiodes and phototransistors are photoelectric conversion semiconductor devices. Compared with photoresistors, they have the advantages of high sensitivity, good high-frequency performance, good reliability, small size, and easy use.

1. Photodiode
1. Structural features and symbols
Compared with ordinary diodes, although photodiodes are both unidirectional nonlinear semiconductor devices, they have their own special features in structure.

The symbol of the photodiode in the circuit is shown in Figure Z0129. When using a photodiode, it must be connected to the circuit in reverse, that is, the positive pole is connected to the negative pole of the power supply, and the negative pole is connected to the positive pole of the power supply.
2. Principle of photoelectric conversion
According to the reverse characteristics of the PN junction, within a certain reverse voltage range, the reverse current is very small and in a saturated state. At this time, if there is no light irradiating the PN junction, the number of electron-hole pairs generated by intrinsic excitation is limited, and the reverse saturation current remains unchanged, which is called dark current in the photodiode. When light irradiates the PN junction, a large number of additional electron-hole pairs (called photogenerated carriers) will be generated in the junction, causing the current flowing through the PN junction to increase dramatically with the increase in light intensity. The reverse current at this time is called photocurrent. Light of different wavelengths (blue light, red light, infrared light) is absorbed in different areas of the photodiode to form photocurrent. The light absorbed by the surface P-type diffusion layer is mainly blue light with a shorter wavelength. In this area, the photogenerated carriers (electrons) generated by light, once drifting to the depletion layer interface,

Under the action of the junction electric field, it will be pulled to the N region, forming a partial photocurrent; the red light with a longer wavelength will pass through the P-type layer and stimulate electron-hole pairs in the depletion layer. These newly generated electron and hole carriers will also reach the N region and P region respectively under the action of the junction electric field, forming a photocurrent. The infrared light with a longer wavelength will pass through the P-type layer and the depletion layer and be directly absorbed by the N region. Once the photogenerated carriers (holes) generated by light in the N region drift to the depletion region interface, they will be pulled to the P region under the action of the junction electric field, forming a photocurrent. Therefore, when light is irradiated, the photocurrent flowing through the PN junction should be the sum of the three parts of the photocurrent.

2. Phototransistor
The structure of phototransistor is similar to that of ordinary transistor. The difference is that phototransistor must have a PN junction that is sensitive to light as the photosensitive surface, and generally uses the collector junction as the light receiving junction. Therefore, photodiode is essentially a common diode with a photodiode connected between the base and the collector. Its structure and symbol are shown in Figure Z0130.

3. Two working states of photodiode

A photodiode, also known as a photodiode, is a photoelectric conversion device. Its basic principle is that when light shines on a PN junction, it absorbs light energy and converts it into electrical energy. It has two working states:

(1) When a reverse voltage is applied to a photodiode, the reverse current in the tube changes with the change of light intensity. The greater the light intensity, the greater the reverse current. Most of them work in this state.

(2) No voltage is applied to the photodiode. The photodiode is used as a miniature photocell by using the principle that the PN junction generates a forward voltage when exposed to light. This working state is generally used as a photodetector.

Photodiodes are divided into PN junction type, PIN junction type, avalanche type and Schottky junction type. Among them, the most commonly used is the PN junction type, which is cheap.

Optical signal amplification and switching circuit