Photoconductive detectors operation

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. 

W. L. Eiseman, J. D. Merriam, and R. F. Potter, Operational Characteristics of Infrared Photodetectors P. R. Bratt, Impurity Germanium and Silicon Infrared Detectors E. H. Putley, InSb Submillimeter Photoconductive Detectors... 

A short time later, Levin s group modified step-scan FT-IR spectrometer to operate with a mid-IR MCT FPA detector. Unlike most MCT detectors used in FT-IR spectrometers, which operate in the photoconductive (PC) mode, the pixels of MCT FPA detectors operate in the photovoltaic (PV) mode. As noted in Section 1.2.2, the cut-off wavenumber of narrow-band PC MCT detectors is about 750 cm" . The PV detector elements used in MCT FPA detectors have the same high sensitivity as narrow-band PC MCT detectors, but the cut-off wavenumber is higher, at about 850 cm" . 

Besides the BRN, there are additional sources of noise due to the physical nature and operation method of the detectors. Most bolometers and photoconducting detectors are basically resistors, and they display at their terminals voltage fluctuations due to the random motion of the electric charges within this resistor. The corresponding noise is called Johnson noise or thermal noise. The voltage fluctuations Vn at the terminal of a resistor R at temperature T for an electrical band width A/ is ... 

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

Figure 7 illustrates a block diagram of the various parts that make-up a commercial photoconductivity detector in HPLC-PCD operation (142). 

Related to the problem of detector operating temperature is that of its electrical power dissipation. A photoconductive detector must always carry a primary current / and therefore dissipate l R of electrical power, whereas a photovoltaic detector can be operated without a bias. The trend toward mechanical or electrical cooling, occurring for practical reasons, makes it necessary that a detector dissipate a minimum of electrical power and thereby heat itself as little as possible. 

The left side of (4.88) is plotted vs N — N in Fig. 4.10 for several possible operating temperatures we have used the values of B and NJg given in Appendix F, as well as t = 5 x 10 cm to be consistent with condition 4 below. By comparing these curves with that for Hg gCd jTe in Fig. 4.10 and with the abscissa of Fig. 4.1, we see that this extrinsic Si photoconductive detector requires considerably lower operating temperatures than the intrinsic photoconductor for comparable performance. This result is a well-known disadvantage of an extrinsic photoconductor [4.31]. The curves for a p-type example would lie... 

All semiconductor detectors operate on the photo-conductive principle, namely the inner photoelectric effect. However, it is normal to use the term photo-conductive detector for those that rely on the incident photons to change the conductivity in the bulk of the photoconductive layer. When a p-n junction is present these are called junction detectors or photodiodes. Photodiodes are generally subdivided further into photovoltaic (i.e., operating without bias voltage) and photoconductive (i.e., operating with a bias voltage). The mode of operation usually depends on the requirements for the detector electronics. 

The use of ACRT at SELEX Sensors and Airborne Systems Infrared Ltd also made possible the fabrication of early 2-dimensional arrays of photodiodes with a high degree of uniformity of response. This type of material was also used to demonstrate nonequilibrium detector operation. The quality and costs of epitaxial material are improving continuously but it will be some years yet before bulk-grown material ceases entirely to be produced, at least for long-wavelength photoconductive applications. 

Chapter 4 discussed the general characteristics of photon detectors, provided some details for two classic detector types simple photoconductive (PC) and photovoltaic (PV) detectors, and described general detector operation. That information will suffice for many users of most IR detectors, but we need to at least acknowledge some of the many other photon detectors now available. 

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