Some electrons detach from covalent bonds, producing electron hole pairs, also known as photo generated charge carriers. The number of photogenerated charge carriers is limited, and the majority of charge carriers before light are much larger than the number of photogenerated charge carriers. Therefore, the influence of photogenerated carriers on the number of carriers is minimal, but the number of minority carriers has a significant impact. This is why photodiodes operate at reverse voltage instead of forward voltage. Therefore, under reverse voltage, a small number of carriers increased by photogenerated carriers participate in drift motion. In the P region, photogenerated electrons diffuse to the PN junction. If the thickness of the P region is smaller than the electron diffusion length, most of the photo generated electrons will reach the PN junction through the P region. The same applies to Zone N. 1、 Photocurrent photodiodes operate in a reverse state, and their reverse current increases with the increase of light intensity. When there is no light, the reverse current is very small (usually less than 0.1 μ A), referred to as dark current. At this point, the reverse resistance is several tens of m Ω. When there is light, a reverse current is formed, called photocurrent. At this point, the reverse resistance of the photodiode drops to a few k Ω. Dark current: In photoconductive mode, when there is no light, the current passing through the photodiode is called dark current. The dark current in photodiodes includes the radiation current and saturation current of semiconductor junctions. It must be measured in advance. Especially in precision optical power measurement, it is necessary to carefully consider and correct the errors caused by dark current. 2、 Response time response rate is the ratio of photocurrent in photoconductive mode to emergency light, in A/W. The response characteristic can also be expressed as the quantum efficiency of a photodiode, which is the ratio of the number of charge carriers generated by light to the number of emergency photons. Noise Equivalent Power (NEP) refers to the minimum optical power required to generate photocurrent, which is equivalent to the RMS of the noise power at 1 Hz. It is approximately equivalent to the minimum detectable input power of a photodiode. The specific application of VI photodiode is phototube, which is essentially a large area PN junction. When light strikes a PN junction surface, such as the P-region surface, if the photon energy is greater than the bandgap bandwidth of the semiconductor material, each photon in the P-region will generate a free electron hole pair. 3、 The electromotive force hole pairs rapidly diffuse inward and form an electromotive force related to light intensity under the junction electric field. At this point, if we use it as a power source and connect it to an external circuit, as long as there is light, it will continue to supply power, which is called a photocell. In other words, a photocell is a PN junction optoelectronic device without bias voltage. It can directly convert light energy into electrical energy. According to the purpose of photovoltaic cells, they can be divided into solar photovoltaic cells and measurement photovoltaic cells. The difference between photodiodes and photocells: Photocells are used for energy conversion. Advantages: Improved conversion rate. Photodiode: used for detection. Advantages: High sensitivity, fast response, and high quantum efficiency. 3、 Solar cells are a type of semiconductor device. When sunlight shines on a semiconductor, some parts are reflected and the rest are absorbed or penetrated through the semiconductor. Some absorbed light becomes heat, while other photons collide with the valence electrons that make up the semiconductor, resulting in electron hole pairs. In this way, light energy is converted into electrical energy. Therefore, after exposure to sunlight, a DC voltage is generated at both ends of the solar cell, which directly converts solar energy into DC current. If we weld the metal leads to the P and N layers and connect the load, the current will flow through the external circuit. In this way, if we connect the photocells in series and parallel, we can generate a certain voltage and current, thereby outputting power. 4、 Photovoltaic power generation lighting system is a power generation system that uses solar cells to convert solar energy into electrical energy. It utilizes the photovoltaic effect. The main components are solar cells, batteries, controllers, and inverters. High reliability, long service life, pollution-free, independent power generation, and grid connected operation of photodiodes. Due to the significant influence of external environmental factors such as light and temperature on the photovoltaic mode of photodiodes, the operating point changes rapidly. There are independent power generation systems and grid connected power generation systems. Independent photovoltaic power generation system: Independent photovoltaic power generation system is a power generation method that is not connected to the power grid. It requires batteries to store energy at night.