A photoresistor, also known as a light-dependent resistor (LDR), is a type of semiconductor device that exhibits a change in its electrical resistance based on the intensity of incident light. It is a passive component, which means it does not require an external power source to function. The operation of a photoresistor relies on the principles of semiconductor physics.
The basic structure of a photoresistor consists of a semiconductor material, typically cadmium sulfide (CdS) or cadmium selenide (CdSe), that is sandwiched between two metal electrodes. The semiconductor material's resistance is inversely proportional to the intensity of light falling on it: as light intensity increases, the resistance decreases, and as light intensity decreases, the resistance increases.
The operation of a photoresistor can be summarized in three main steps:
Incident Light:
When light falls on the surface of the photoresistor, photons from the incident light interact with the semiconductor material. This interaction excites electrons within the semiconductor, increasing their energy level. The amount of excitation depends on the intensity and wavelength of the incident light.
Electron-Hole Pairs:
The excited electrons gain enough energy to break free from their atomic bonds, leaving behind holes (positively charged locations) in the semiconductor material. These liberated electrons and holes are referred to as electron-hole pairs.
Conductivity and Resistance:
The presence of electron-hole pairs affects the electrical conductivity of the semiconductor material. In the dark or low-light conditions, the number of electron-hole pairs is relatively low, resulting in a higher resistance for the photoresistor. Consequently, only a small current can flow through the device when a voltage is applied across its terminals.
Conversely, when the photoresistor is exposed to bright light, more electron-hole pairs are generated due to increased photon absorption. This high number of charge carriers enhances the conductivity of the semiconductor, reducing the device's resistance significantly. As a result, a larger current can flow through the photoresistor for the same applied voltage.
Photoresistors are widely used in various applications, such as automatic lighting systems, light meters, camera exposure control, and dusk-to-dawn switches. When integrated into these systems, the photoresistor detects changes in light intensity and triggers corresponding actions, such as turning on or off lights, depending on the desired functionality.
It's essential to note that the specific characteristics of a photoresistor, such as sensitivity, resistance range, and response time, can vary based on its construction and the properties of the semiconductor material used.