Grouped by mechanism, photodetectors include the following devices:
Photoemission or photoelectric •
Gaseous ionization detectors are used in experimental
particle physics to detect photons and particles with sufficient energy to
ionize gas atoms or molecules. Electrons and ions generated by ionization cause a current flow which can be measured. •
Photomultiplier tubes containing a
photocathode which emits
electrons when illuminated, the electrons are then amplified by a chain of
dynodes. •
Phototubes containing a
photocathode which emits
electrons when illuminated, such that the tube conducts a current proportional to the
light intensity. •
Microchannel plate detectors use a porous glass substrate as a mechanism for multiplying electrons. They can be used in combination with a photocathode like the photomultiplier described above, with the porous glass substrate acting as a
dynode stage
Semiconductor •
Active-pixel sensors (APSs) are
image sensors. Usually made in a
complementary metal–oxide–semiconductor (CMOS) process, and also known as CMOS image sensors, APSs are commonly used in cell phone cameras, web cameras, and some
DSLRs. •
Cadmium zinc telluride radiation detectors can operate in direct-conversion (or photoconductive) mode at room temperature, unlike some other materials (particularly germanium) which require liquid nitrogen cooling. Their relative advantages include high sensitivity for x-rays and gamma-rays, due to the high atomic numbers of Cd and Te, and better energy resolution than scintillator detectors. •
Charge-coupled devices (CCD) are image sensors which are used to record images in
astronomy,
digital photography, and
digital cinematography. Before the 1990s,
photographic plates were most common in astronomy. The next generation of astronomical instruments, such as the
Astro-E2, include
cryogenic detectors. •
HgCdTe infrared detectors. Detection occurs when an infrared photon of sufficient energy kicks an electron from the valence band to the conduction band. Such an electron is collected by a suitable external readout integrated circuits (ROIC) and transformed into an electric signal. •
LEDs which are reverse-biased to act as photodiodes. See
LEDs as photodiode light sensors. •
Photoresistors or
Light Dependent Resistors (LDR) which change
resistance according to
light intensity. Normally the resistance of LDRs decreases with increasing intensity of light falling on it. •
Photodiodes which can operate in
photovoltaic mode or
photoconductive mode. Photodiodes are often combined with low-noise analog electronics to convert the
photocurrent into a voltage that can be
digitized. •
Phototransistors, which act like amplifying photodiodes. •
Pinned photodiodes, a photodetector structure with low
lag, low
noise, high
quantum efficiency, and low
dark current, widely used in most CCD and CMOS image sensors. •
Quantum dot photoconductors or
photodiodes, which can handle wavelengths in the visible and infrared spectral regions. •
Semiconductor detectors are employed in gamma and X-ray spectrometry and as particle detectors. •
Silicon drift detectors (SDDs) are X-ray radiation detectors used in x-ray spectrometry (EDS) and
electron microscopy (EDX).
Photovoltaic •
Photovoltaic cells or
solar cells which produce a
voltage and supply an
electric current when sunlight or certain kinds of light shines on them.
Thermal •
Bolometers measure the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. A
microbolometer is a specific type of bolometer used as a detector in a
thermal camera. •
Cryogenic detectors are sufficiently sensitive to measure the energy of single
x-ray, visible and
infrared photons. •
Pyroelectric detectors detect photons through the heat they generate and the subsequent voltage generated in pyroelectric materials. •
Thermopiles detect electromagnetic radiation through heat, then generating a voltage in
thermocouples. •
Golay cells detect photons by the heat they generate in a gas-filled chamber, causing the gas to expand and deform a flexible membrane whose deflection is measured.
Photochemical •
Photoreceptor cells in the
retina detect light through, for instance, a
rhodopsin photon-induced chemical cascade. • Chemical detectors, such as
photographic plates, in which a
silver halide molecule is split into an atom of metallic silver and a halogen atom. The
photographic developer causes adjacent molecules to split similarly.
Polarization • The
photorefractive effect is used in
holographic data storage. • Polarization-sensitive photodetectors use
optically anisotropic materials to detect photons of a desired
linear polarization.
Graphene/silicon photodetectors A graphene/n-type silicon heterojunction has been demonstrated to exhibit strong rectifying behavior and high photoresponsivity.
Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. Both the electrical and optical contributions of Si QDs enable a superior performance of the photodetector. == Applications ==