The structure of a photodiode is similar to that of a semiconductor diode, and its core is a PN junction with photosensitive characteristics, which has a single conductivity. Therefore, a reverse voltage must be applied during operation. When there is no light, there will be a small dark current, at which point the photodiode will cut off. When exposed to light, the saturation reverse leakage current increases, forming a photocurrent that changes with the intensity of the illuminated light. When light shines on the PN junction, electron hole pairs can be generated in the PN junction, thereby increasing the density of minority carriers. These carriers drift under reverse voltage, increasing reverse current. Therefore, the intensity of light can be used to change the current in the circuit. Common products include 2CU, 2DU, and other series. The volt ampere characteristics of photodiodes. The volt ampere characteristic of a photodiode refers to the relationship between the photocurrent generated on the photodiode and the voltage applied at both ends. Volt ampere characteristics of photodiodes: 1 When a forward voltage is applied, unidirectional conductivity is displayed. When a reverse voltage is applied, light will generate a photoelectric current, which is much greater than the reverse saturation current. 3. Reverse bias can reduce the transmission time of charge carriers and the capacitance between diodes. The optical characteristics of photodiodes. The relationship between luminous flux and photocurrent when the voltage applied between the cathode and anode of the photocell is constant. The slope of the light characteristic curve is called the sensitivity of the photocell. The optical characteristics of photodiodes: firstly, the light is not linear, and secondly, the saturation is not linear. Spectral characteristics of photodiodes. The relationship between the photocurrent of a photodiode and the wavelength of the incident light is called the spectral characteristic. The energy of photons is related to the wavelength of light: the longer the wavelength, the smaller the energy of photons; On the contrary, the shorter the wavelength, the greater the energy of the photon. Spectral characteristics of photodiodes. Due to the relatively small load resistance, photodiodes have a good linear relationship between photocurrent and incident light power. 2. The response wavelength of the photodiode is the same as that of the GaAs laser tube and the light-emitting diode, and is composed of a combination of optocoupler devices. 3. The photodiode has a small junction capacitance, high frequency response, and a bandwidth of up to 100kHz.