The Intro of Photoresistor
Description
A photoresistor or light dependent resistor (LDR) is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor.
A photoresistor is made of a high resistance semiconductor such as metallic sulfide, selenide and telluride. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.g. silicon. In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities, also called dopants, and added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (i.e., longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.
Applications
- Consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks
- Dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction
- Infrared astronomy and infrared spectroscopy
Features
Voltage current characteristic
It is the relationship between the DC current through an electronic device and the DC voltage across its terminals is called a current–voltage characteristic of the device.
Photoresistor voltage current characteristic is approximate to linear and has no saturated phenomenon. It is limited by dissipation power, when use it, the voltage on both ends of photoresistor cannot exceed the highest working voltage.
Photoelectric characteristic
The relationship between light current and illuminance of photoresistor is called photoelectric characteristic. Because it presents nonlinearity, so photoresistor is not suitable to be detecting component. It is used as switch photosensor in automatic control.
Frequency characteristics
When photoresistor under a pulse illumination, it takes a long time for light current to achieve steady value. When light suddenly disappeared, the current will not turn to zero immediately. This shows photoresistor has delay characteristics. Due to the different material, photoresistor delay characteristics are different, so their frequency characteristics are also not the same.
Temperature characteristic
Photoresistor, as well as other semiconductor devices, it is influenced the bigger by the temperature, when the temperature rises, its dark resistance will decline. The change of the temperature also has very big effect on spectral characteristics.