Photoelectric detectors are components that can convert light radiation into electricity, widely used in various fields such as military, national economy, industrial control, medical equipment, optical communication, etc. Its working principle is based on the photoelectric effect, that is, when light shines on the surface of a substance, the electrons inside the substance are excited by the light, generating electrical energy. This article will delve into the working principle of photodetectors and their applications in different fields.




The basic working principle of photodetectors

The basic working principle of photodetectors can be divided into three main processes: the generation of photo generated carriers, the diffusion or drift of carriers to form current, and the amplification and conversion of photocurrent into voltage signals in the amplification circuit.

Generation of photo generated charge carriers
When light shines on the surface of a photodetector, if the photon energy of the incident light is greater than or equal to the material's bandgap width (i.e. the minimum energy required for electrons to transition from the valence band to the conduction band), the electrons in the valence band will absorb the photon energy and transition to the conduction band, forming photo generated carriers (including electrons and holes).

The diffusion or drift of charge carriers forms current
Photogenerated carriers will generate a concentration gradient inside the material, causing carriers to diffuse from high concentration regions to low concentration regions. At the same time, under the action of an external electric field, charge carriers will also undergo drift motion. Both of these modes of motion will generate an electric current, known as photocurrent.
Photocurrent is amplified and converted into a voltage signal in an amplification circuit

The generated photocurrent is often very weak and needs to be amplified through an amplification circuit to convert it into a voltage signal for subsequent processing and analysis.




 Classification of photodetectors
According to the working principle and structural characteristics of photodetectors, they can be divided into various types, including phototubes, photomultiplier tubes, photoconductive detectors, photovoltaic detectors, etc. Different types of photodetectors have different characteristics and application scenarios.

Photocell
Photocell is a type of photodetector based on the external photoelectric effect. Its working principle is that when light shines on the cathode of the photocell, electrons are emitted, forming a photocurrent. Phototubes have the characteristics of wide spectral response range and high sensitivity, and are commonly used in instruments such as spectrophotometers.

Photomultiplier tube
Photomultiplier tube is a photodetector that utilizes the photoelectric effect and secondary electron emission effect to amplify signals. Its working principle is that when light shines on the cathode of a photomultiplier tube, it emits electrons, which accelerate and collide with the multiplier electrode under the action of an electric field, producing more electrons, thereby achieving signal amplification. Photomultiplier tubes have the characteristics of high magnification and fast response speed, and are commonly used in fields such as astronomy, spectroscopy, and laser ranging.

Photoconductive detector
A photoconductive detector is a type of photodetector that utilizes the photoconductive effect of semiconductor materials. Its working principle is that when light is irradiated on semiconductor materials, electrons inside the material are excited and transition to the conduction band, forming photo generated carriers, thereby changing the conductivity of the material. Photoconductive detectors have the characteristics of fast response speed and high sensitivity, and are commonly used in fields such as infrared detection and missile guidance.

Photovoltaic detector
Photovoltaic detector is a type of photodetector based on the photovoltaic effect. Its working principle is that when light shines on the PN junction of the photovoltaic detector, a photo generated voltage is generated on both sides of the PN junction. Photovoltaic detectors have the characteristics of simple structure and high reliability, and are commonly used in fields such as solar cells and photoelectric sensors.



 Conclusion
As a component that converts light radiation into electricity, photodetectors have broad application prospects in military, national economy, industrial control, medical equipment, optical communication and other fields. With the continuous development of technology, the performance of photodetectors continues to improve, and their applications in various fields will also become increasingly widespread.