Difference between revisions of "Photoresistor" - New World Encyclopedia
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A photoresistor is made of a high-resistance [[semiconductor]]. If light falling on the device is of high enough [[frequency]], [[photon]]s absorbed by the semiconductor give bound [[electron]]s enough energy to jump into the [[conduction band]]. The resulting free electron (and its [[electron hole|hole]] partner) conduct electricity, thereby lowering [[Electrical_resistance|resistance]]. | A photoresistor is made of a high-resistance [[semiconductor]]. If light falling on the device is of high enough [[frequency]], [[photon]]s absorbed by the semiconductor give bound [[electron]]s enough energy to jump into the [[conduction band]]. The resulting free electron (and its [[electron hole|hole]] partner) conduct electricity, thereby lowering [[Electrical_resistance|resistance]]. | ||
| − | A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, eg. 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 added, which have a ground state energy closer to the conduction band — since the electrons don't have as far to jump, lower energy photons (that is, 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, there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor. | + | A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, eg. 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" added, which have a ground state energy closer to the conduction band — since the electrons don't have as far to jump, lower energy photons (that is, 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. |
== Cadmium sulfide cells == | == Cadmium sulfide cells == | ||
Revision as of 03:00, 27 April 2007
A photoresistor is an electronic component whose resistance decreases with increasing incident light intensity. It may also be referred to as a light-dependent resistor (LDR), photoconductor, photocell, or electric eye.
How it works
A photoresistor is made of a high-resistance semiconductor. 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, eg. 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" added, which have a ground state energy closer to the conduction band — since the electrons don't have as far to jump, lower energy photons (that is, 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.
Cadmium sulfide cells
Cadmium sulfide (or cadmium sulphide, CdS) cells rely on the material's ability to vary its electrical resistance according to the amount of light striking the cell. The more light that strikes the cell, the lower the resistance. Although not accurate, even a simple CdS cell can have a wide range of resistance, from less than 100 ohms (Ω) in bright light to in excess of 10 MΩ in darkness. Many commercially available CdS cells have a peak sensitivity in the region of 500 - 600 nanometers (nm). The cells are also capable of reacting to a broad range of frequencies, including infrared (IR), visible light, and ultraviolet (UV). They are often found on street lights as automatic on/off switches. They were once even used in heat-seeking missiles to sense for targets.
Applications
Photoresistors come in many different types. Inexpensive cadmium sulphide cells can be found in many consumer items such as camera light meters, clock radios, security alarms, street lights and outdoor clocks. They are also used in some dynamic compressors to control gain reduction. At the other end of the scale, Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.
Circuit symbol
Below is symbol for a photoresistor as used in some circuit diagrams.
See also
- Infrared
- Optoelectronics
- Phototransistor
- Semiconductor
- Transistor
- Ultraviolet
ReferencesISBN links support NWE through referral fees
- Harrison, Ian. 2004. The Book of Inventions: How'd They Come Up With That?. Washington, DC: National Geographic. ISBN 0792282965.
- Hunter, Lloyd P., ed. 1970. Handbook of Semiconductor Electronics: A Practical Manual Covering the Physics, Technology, and Applications of Transistors, Diodes, and Other Semiconductor Devices in Conventional and Integrated Circuits. 3rd ed. New York: McGraw-Hill. ISBN 0070313059.
- Turner, Rufus P. 1980. Solar cells and photocells. 2nd ed. Indianapolis, IN: H.W. Sams. ISBN 0672217112.
External links
- Photocell Sensor. Making Things. Retrieved April 26, 2007.
- How do the streetlights turn on automatically at night? howstuffworks.com. Retrieved April 27, 2007.
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