Detector, photoconductive

Hg Cd Te is an example of a ternary detector, in which the value of x controls the cutoff wavelength. Photoconductive detectors are generally simpler to couple to low noise amplifiers photodiodes generally have lower power consumption because these have no external bias, and better high frequency performance (15,16). 

The photoconductive detector is primarily used in the visible-infrared region rather than the ultraviolet—visible range. 

The cadmium chalcogenide semiconductors (qv) have found numerous appHcations ranging from rectifiers to photoconductive detectors in smoke alarms. Many Cd compounds, eg, sulfide, tungstate, selenide, teUuride, and oxide, are used as phosphors in luminescent screens and scintiUation counters. Glass colored with cadmium sulfoselenides is used as a color filter in spectroscopy and has recently attracted attention as a third-order, nonlinear optical switching material (see Nonlinear optical materials). DiaLkylcadmium compounds are polymerization catalysts for production of poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVA), and poly(methyl methacrylate) (PMMA). Mixed with TiCl, they catalyze the polymerization of ethylene and propylene. 

H.S. Sommers, Jr., Macrowave-Based Photoconductive Detector Robert Sehr and Rainer Zuleeg, Imaging and Display... 

Peter R. Bratt, Impurity Germanium and Silicon Infrared Detectors E.H. Pulley, InSb Submillimeter Photoconductive Detectors... 

W.F.H. Micklethwaite, The Crystal Growth of Cadmium Mercury Telluride Paul E. Petersen, Auger Recombination in Mercury Cadmium Telluride R.M. Broudy and V.J. Mazurczyck, (HgCd)Te Photoconductive Detectors M.B. Reine, A.K. Sood, and T.J. Tredwell, Photovoltaic Infrared Detectors M.A. Kinch, Metal-Insulator-Semiconductor Infrared Detectors... 

Figure 2.24 Photoelectric detectors. Photovoltaic detectors measure the flow of electrons displaced by the absorption of radiation. Photoconductive detectors measure the changes in conductivity caused by the absorption of radiation.

There are two classes of photoelectric detectors photoconduction detectors and photodiodes. 

Fignre 3.12 shows the operational scheme of a photoconduction detector. The incident light creates an electrical current and this is measured by a voltage signal, which is proportional to the light intensity. This proportional relation is provided by the fact that, in most photoconduction detectors, the density of carriers in the steady state is proportional to the number of absorbed photons per unit of time that is, proportional to the incident power. 

Figure 3.13 The spectral dependence of the specific detectivity for several photoconduction detectors. The values corresponding to a typical thermopile and to a typical piroelectric detector are also shown.

We now turn to photoconductive and photodiode detectors, both of which are semiconductor devices. The difference is that in the photoconductive detector there is simply a slab of semiconductor material, normally intrinsic to minimize the detector dark current, though impurity doped materials (such as B doped Ge) can be used for special applications, whereas by contrast, the photodiode detector uses a doped semiconductor fabricated into a p-n junction. 

The difference between a photoconductive detector and a photodiode detector lies in the presence of a thin p-doped layer at the surface of the detector element, above the bulk n-type semiconductor. Holes accumulate in the p-layer, and electrons in the n-type bulk, so between the two there is a region with a reduced number density of carriers, known as the depletion layer. The important effect of this is that electron-hole pairs, generated by photon absorption within this depletion layer, are subjected to an internal electric field (without the application of an external bias voltage) and are automatically swept to the p and n regions, and... 

Poly Chlorinated Biphenyls. The photoconductivity detector provides good responses for polychlorinated biphenyls separated by GPC. The normal matrix components are detected by RI and UV detectors while the polychlorinated species show high responses in the electrochemical detector (Figure 8). ... 

Figure 8. Gel permeation chromatograms with refractive index, UV 254, and photoconductivity detectors. Key top, 2 -chlorohiphenyl and bottom, decachlorobiphenyl.

The most important use of CD films for many years was to make PbS and PbSe films for photoconductive detectors [10]. Such detectors, made by CD, are still in use today, although they are facing competition from photovoltaic 111-V detectors. It should be noted that for good photosensitivity, air-annealing of the CD films is carried out, and this annealing treatment is connected with partial oxidation of the PbS and PbSe surfaces. 

The first apparent report in the open literature of CD PbSe for photoconductive detectors was in 1949 [53], The PbSe was deposited from a solution of PbAci and selenourea onto a predeposited (from PbAci and thiourea) layer of PbS. The PbS layer acted as a seed layer, presumably to obtain faster deposition (it was noted that the PbSe deposition was much slower than that of PbS). The photoconductivity of this film exhibited a broad maximum between 3 and 4 p,m, giving a reasonable response out to beyond 4.5 p,m (PbS drops off at 3 iJim). 

Mixed compositions are of interest mainly because they allow tuning of the semiconductor properties (most commonly bandgap and, therefore, spectral sensitivity). This is useful for various device applications. Photoconductive detectors, where a certain spectral sensitivity range is desired, is probably the main application that drove many studies on CD of ternary semiconductors. 

The detector output signal is generated by a current preamplifier for photovoltaic detectors, such as InSb, and by a simple detector bias circuit shown in Fig. 4 for photoconductive detectors, such as PbS and Hg Ge. The voltage signal derived from the bias circuit is normally preamplified and forwarded to a phase-sensitive synchronous detector usually embodied in a lock-in amplifier (Stewart, 1970 Blass, 1976b). 

Fig. 4 Simple photoconductive detector bias circuit. RD is the detector resistance, (/ D)0 the equivalent detector resistance at zero signal (i.e., background flux only on detector). RL is the load resistance, V0 the signal at (RD)0, Vsig the signal voltage at Rd.V0= VbJRD)(J[RL + (RJ0l Vsie(t) = Vba,RDl(RL + rd)> and Fba. is the battery voltage.

A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

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