Devices that respond to incident ultraviolet, visible, or infrared electromagnetic radiation by giving rise to an output signal, usually electrical. Based upon the manner of their interaction with radiation, they fall into three categories. Photon detectors are those in which incident photons change the number of free carriers (electrons or holes) in a semiconductor (internal photoeffect) or cause the emission of free electrons from the surface of a metal or semiconductor (external photoeffect, photoemission). Thermal detectors respond to the temperature rise of the detecting material due to the absorption of radiation, by changing some property of the material such as its electrical resistance. Detectors based upon wave-interaction effects exploit the wavelike nature of electromagnetic radiation, for example by mixing the electric-field vectors of two coherent sources of radiation to generate sum and difference optical frequencies. This discussion concentrates on photon and thermal detectors. See also: Nonlinear optical devices
The most widely used photon effects are photoconductivity, the photovoltaic effect, and the photoemissive effect. Photoconductivity, an internal photon effect, is the decrease in electrical resistance of a semiconductor caused by the increased numbers of free carriers produced by the absorbed radiation. The change in resistance is measured by passing a bias current through the photoconductor and measuring the change in resistance between the illuminated and dark state. See also: Photoconductive cell
The photovoltaic effect, also an internal photoeffect, occurs at a pn junction in a semiconductor or at a metal-semiconductor interface (Schottky barrier). Absorbed radiation produces free hole-electron pairs which are separated by the potential barrier at the pn junction or Schottky barrier, thereby giving rise to a photovoltage. This is the principle employed in a solar cell. See also: Photodiode; Photovoltaic cell; Photovoltaic effect; Semiconductor diode; Solar cell
The photoemissive effect, also known as the external photoeffect, is the emission of an electron from the surface of a metal or semiconductor (cathode) into a vacuum or gas due to the absorption of a photon by the cathode. The photocurrent is collected by a positively biased anode. Internal amplification of the photoexcited electron current can be achieved by means of secondary electron emission at internal structures (dynodes). Such a vacuum tube is known as a photomultiplier. Internal amplification by means of an avalanche effect in a gas is employed in a Geiger tube. See also: Geiger-Müller counter; Light amplifier; Photoelectric devices; Photoemission; Photomultiplier; Phototube
Role of materials
Semiconductors are key to the development of most photon detectors. These materials are characterized by a forbidden energy gap which determines the minimum energy that a photon must have to produce a free hole-electron pair in an intrinsic photoeffect. Since the energy of a photon is inversely proportional to its wavelength, the minimum energy requirement establishes a long-wavelength limit of an intrinsic photoeffect. It is also possible to produce free electrons or free holes by photoexcitation at donor or acceptor sites in the semiconductor; this is known as an extrinsic photoeffect. Here the long-wavelength limit of the photoeffect is determined by the minimum energy (ionization energy) required to photoexcite a free electron from a donor site or a free hole from an acceptor site. In a given semiconductor, the long-wavelength limit of an extrinsic photoeffect exceeds that of the intrinsic photoeffect. In either the intrinsic or extrinsic photoeffect, the ability to detect a photon is described by the quantum efficiency, which is the ratio of the photoexcited carriers produced to the number of incident photons. Quantum efficiencies of 0.5 are typical of many photon detectors. See also: Semiconductor
Free electrons and holes can also be thermally excited by lattice vibrations (phonons) in the semiconductor, whose magnitude is a function of the detector temperature. Detection of a few photoexcited free carriers in the presence of numerous thermally excited carriers is difficult. The problem is eased by cooling the detector in order to reduce the thermally excited carrier background. Long-wavelength infrared photon detectors having spectral responses extending to 10 micrometers have small forbidden energy gaps or small donor or acceptor ionization energies. Accordingly, cooling the detector becomes more important for long-wavelength detectors. Use of liquid nitrogen, which boils at 77 K, or mechanical refrigerators (cryocoolers) is required for photon detectors whose spectral response extends to 10 μm. Even longer-wavelength spectral responses require cooling below 77 K. See also: Cryogenics
The usual way to provide a given spectral response for a photon detector is to synthesize a semiconductor whose forbidden energy gap or donor or acceptor ionization energy meets the long-wavelength limit requirement. The semiconductor material most frequently used in long-wavelength military systems is mercury cadmium telluride, wherein the mercury-to-cadmium ratio determines the forbidden energy gap.
An alternative method is to tailor a new material by building up numerous alternating layers of controlled thickness of two semiconductors which have compatible material properties (crystal structure, lattice constant, thermal expansion coefficient). Such structures are known as multiple quantum wells. The most frequently used pair of materials is gallium arsenide and aluminum gallium arsenide. By adjusting the aluminum-to-gallium ratio and the thicknesses of the layers during growth, the spectral response of the photoeffect exhibited by these structures can be tailored within broad limits. When such structures are employed for infrared detection, they are known as quantum-well infrared photodetectors (QWIPs). See also: Artificially layered structures; Crystal growth; Quantized electronic structure (QUEST); Semiconductor heterostructures
The choice of materials also plays a role in thermal detectors. The most widely used thermal detector is a bolometer, that is, a temperature-sensitive resistor in the form of a thin metallic or semiconductor film (although superconducting films are also used). Incident electromagnetic radiation absorbed by the film causes its temperature to rise, thereby changing its electrical resistance. The change in resistance is measured by passing a current through the film and measuring the change in voltage. Materials with a high temperature coefficient of resistance are desired for bolometers, a criterion which usually favors semiconductors over metals. See also: Bolometer
Another widely used thermal detector is based upon the pyroelectric effect. Here a change in temperature causes positive and negative charges to appear on opposite faces of thin samples of certain ferroelectric materials such as triglicine sulfate and strontium barium niobate. This charge can be detected by means of electrodes deposited on the faces. In contrast to a bolometer, a pyroelectric detector requires the incident radiation to be temporally modulated, which can be done by means of a radiation chopper. See also: Chopping; Pyroelectricity
A third thermal detector is the radiation thermocouple, formed by two junctions between two dissimilar metals or semiconductors. One junction, exposed to the incident radiation, is warmed by it; the other, shielded junction is not. A voltage generated by the temperature difference between the junctions is detected by an external circuit. See also: Thermocouple; Thermoelectricity
In contrast to photon detectors, the spectral response of thermal detectors is determined by the wavelength dependence of their absorption of electromagnetic radiation. If the absorption is invariant with wavelength, the spectral response is invariant with wavelength and is said to be flat.
Elemental and imaging detectors
Optical detectors can also be classified as elemental or imaging. An elemental detector averages any spatial variation in the incident radiation to produce an output signal characteristic of the average incident intensity. An imaging detector produces a time-varying output signal which bears a one-to-one relationship with the spatial variation of intensity falling on it. An example is a television camera tube known as a vidicon, which contains a broad-area thin photoconductive film upon which the radiant image of a scene is projected by a lens. An internal electron beam scanned across the film in raster fashion generates a time-varying electrical signal representing the spatial variation of film resistance due to the projected image. It is also possible to detect an image by mechanically scanning the scene across an elemental detector by using two moving mirrors, one to scan horizontally and the other vertically. Use of a linear array of detectors simplifies the system complexity by requiring scanning in one direction only. See also: Television camera tube
The applications of optical detectors can be organized according to the spectral interval within which they respond.
These find limited applications in laboratory instrumentation such as spectrometers and in astronomy. Industrial applications include the monitoring of gas-burner flames in large furnaces. Ultraviolet detectors are used also in dual-color (infrared and ultraviolet) missile-guidance schemes. The principal ultraviolet detector is the photomultiplier. See also: Ultraviolet astronomy
Visible radiation detectors have many applications, for example, in television cameras and camcorders. Solid-state arrays based upon silicon charged-coupled-device technology are widely used in simple, inexpensive high-performance imaging systems. Such arrays also find use in astronomy, where their sensitivity at low light levels is of key importance. They are used in industrial automation systems, including those employing machine vision. Electronic still photography employs imaging arrays to replace photographic film in solid-state cameras. See also: Camera; Charge-coupled devices; Computer vision
Detectors of infrared radiation are widely used in military and, to less extent, commercial night-vision systems. Both image intensifiers, which exploit photoemissive photocathodes responding to the low-level ambient illumination of the night sky at wavelengths out to about 1.1 μm, and thermal imaging systems, employing cryogenic linear and matrix (two-dimensional) arrays of long-wavelength photon detectors, are widely used. Uncooled matrix arrays of bolometers and pyroelectric detectors offer high performance in thermal imaging systems without the complexity and cost of cryogenic systems. See also: Infrared astronomy; Infrared imaging devices; Infrared radiation