The photoelectric detection technology has a very wide range of applications in many fields, from simple infrared sensors to optical fiber communication equipment, as well as the image sensor in our daily mobile phones. Whether it is a single receiver tube or a large-scale photoelectric cell array, the basic principle is light absorption in the photoelectric effect. 
As for the photoelectric effect, this is the knowledge point of middle school textbooks. To put it simply, the electron outside the nucleus of the atom absorbs the energy of the photon and breaks away from the nucleus of the atomic nucleus and runs out. This energy is related to the frequency, that is, E = hv, and the minimum energy value is called the band gap. 
1. The concept and parameters of photoelectric detection
1. Responsiveness
The electrons generated by the photoelectric effect move under the action of the electric field to generate a photocurrent, and the current is proportional to the power of the incident light Ip = RPin. R is the responsivity of the detector, the unit is A / W.
2. Quantum efficiency
The ratio of the number of photons that can be converted into photocurrent to the total number of incident photons is quantum efficiency. Can be expressed as: 
namely: 
3. Response bandwidth
Bandwidth is the response time. The fundamental reason is the time it takes photogenerated carriers to pass through the absorption zone. But the width of the absorption zone is positively related to the responsivity, so the two are mutually constrained. Another parameter related to this is the parasitic capacitance.
4. Dark current
The so-called dark current is the current when there is no light. At this time, although no external photons excite the electrons, carriers generated by thermal effects or stray light inside the semiconductor will also form a current, but this current is generally very small, mostly below nA.
The above are some typical parameters of photodetectors, but different semiconductors have different absorption of photons. In addition to our most common ones such as Si, Ge, etc., other new materials are also emerging to adapt to different technical solutions in. And the exploration of new materials can be said to be the hottest frontier field in the field of sensing and detection