Photoconductivity
Encyclopedia
Photoconductivity is an optical
and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation
such as visible light
, ultraviolet
light, infrared
light, or gamma radiation.
When light is absorbed by a material such as a semiconductor
, the number of free electrons and electron hole
s changes and raises its electrical conductivity. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap
, or to excite the impurities within the band gap. When a bias voltage
and a load resistor
are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity of the material varies the current flowing through the circuit.
Classic examples of photoconductive materials include the conductive polymer
polyvinylcarbazole
, used extensively in photocopying (xerography
); lead sulfide
, used in infrared detection applications, such as the U.S. Sidewinder and Russian Atoll
heat-seeking
missile
s; and selenium
, employed in early television and xerography
.
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
such as visible light
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
, ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light, infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
light, or gamma radiation.
When light is absorbed by a material such as a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
, the number of free electrons and electron hole
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
s changes and raises its electrical conductivity. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
, or to excite the impurities within the band gap. When a bias voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
and a load resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity of the material varies the current flowing through the circuit.
Classic examples of photoconductive materials include the conductive polymer
Conductive polymer
Conductive polymers or, more precisely, intrinsically conducting polymers are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive...
polyvinylcarbazole
N-Vinylcarbazole
N-Vinylcarbazole is an organic compound used as a monomer in the production of poly, a photoconductive polymer used in the photoreceptors of photocopiers. Upon exposure to γ-irradiation, N-vinylcarbazole undergoes solid-state polymerisation....
, used extensively in photocopying (xerography
Xerography
Xerography is a dry photocopying technique invented by Chester Carlson in 1938, for which he was awarded on October 6, 1942. Carlson originally called his invention electrophotography...
); lead sulfide
Lead(II) sulfide
Lead sulfide is an inorganic compound with the formula Pb. It finds limited use in electronic devices. PbS, also known as galena, is the principal ore and most important compound of lead....
, used in infrared detection applications, such as the U.S. Sidewinder and Russian Atoll
Vympel K-13
The K-13 is an short-range, infrared homing air-to-air missile developed by the Soviet Union. It is similar in appearance and function to the American AIM-9 Sidewinder from which it was reverse-engineered...
heat-seeking
Infrared homing
Infrared homing refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track and follow it. Missiles which use infrared seeking are often referred to as "heat-seekers", since infrared is just below the...
missile
Missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...
s; and selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
, employed in early television and xerography
Xerography
Xerography is a dry photocopying technique invented by Chester Carlson in 1938, for which he was awarded on October 6, 1942. Carlson originally called his invention electrophotography...
.
Applications
- Solar cellSolar cellA solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
s - Photoconductive antenna
- BetavoltaicsBetavoltaicsBetavoltaics are generators of electrical current, in effect a form of battery, which use energy from a radioactive source emitting beta particles . A common source used is the hydrogen isotope, tritium...
- Atomic batteriesAtomic batteryThe terms atomic battery, nuclear battery, tritium battery and radioisotope generator are used to describe a device which uses the emissions from a radioactive isotope to generate electricity. Like nuclear reactors they generate electricity from atomic energy, but differ in that they do not use a...
- PhotoresistorPhotoresistorA photoresistor or light dependent resistor is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor or CdS device, from "cadmium sulfide," which is the material from which the device is made and that actually exhibits the...
s