Why Is The Time Response Of A Photodiode Shorter Than That Of A Photoresistor?

Photodiodes and photoresists are two common photosensitive components that can be used to detect changes in the intensity of light, but they have significant differences in their working principles, structures, and performance, which lead to differences in their time response.
Working principle
Photodiode
Photodiode is a semiconductor device that operates based on the photoelectric effect of PN junction. When photons are irradiated onto the PN junction, they are absorbed and generate electron hole pairs. These electrons and holes separate under the built-in electric field of the PN junction, resulting in the generation of current. This process is very rapid because the absorption of photons and the generation of electron hole pairs are almost instantaneous.
Photoresistor
A photoresistor is a type of resistive element typically made of semiconductor materials. Its working principle is based on the photoconductive effect of photosensitive materials. When light is irradiated onto a photoresistor, the energy of photons is absorbed by the material, causing electrons to be excited from the valence band to the conduction band, increasing the conductivity of the material. This process involves the excitation and migration of electrons, which is usually slower than the separation of electron hole pairs in photodiodes.
Structural differences
Photodiode
The structure of a photodiode is relatively simple, mainly composed of a PN junction or PIN structure. The characteristics of PN junction or PIN structure are strong built-in electric field and fast separation speed of electron hole pairs.
Photoresistor
The structure of a photoresistor is usually more complex than that of a photodiode, consisting of multiple layers of semiconductor materials, including a photosensitive layer and an electrode layer. The thickness of the photosensitive layer and the conductivity of the material will affect the response time of the photoresistor.

Performance differences
Response time
The response time of photodiodes is very short, usually in the nanosecond range. This is because the absorption of photons and the generation of electron hole pairs are rapid physical processes.
The response time of photoresists is usually in the millisecond range, because the excitation and migration of electrons require a certain amount of time, and the structure of photoresists is complex, with longer migration paths for electrons in the material.
Sensitivity
The sensitivity of photodiodes is usually higher than that of photoresists because they can directly convert optical signals into electrical signals without any intermediate conversion process.
The sensitivity of photoresists is relatively low because they require the excitation and migration of electrons to change conductivity, which is a process with low efficiency.
Stability
Photodiodes have good stability because they are based on the physical properties of semiconductor materials and are not affected by environmental factors such as temperature.
The stability of photoresists is poor because their conductivity is greatly affected by environmental factors such as temperature and humidity.
Application scenarios
Photodiodes are commonly used in applications that require fast optical signal detection, such as high-speed optical communication and photoelectric sensors, due to their fast response and high sensitivity.
Due to its low response speed and high temperature sensitivity, photoresistors are commonly used in applications that do not require high response speed, such as light intensity measurement and light controlled switches.
In summary, the main reason why the time response of photodiodes is shorter than that of photoresists is due to differences in their working principles, structures, and performance. Photodiodes are based on the photoelectric effect of PN junctions, with fast electron hole pair separation and high sensitivity, while photoresists are based on the photoconductive effect, with slower electron excitation and migration processes, and are greatly affected by environmental factors. These characteristics make photodiodes more suitable for applications that require fast response, while photoresists are suitable for situations that do not require high response speed.