Detector types photoconductive

Although not specifically delineated, the volume is also divided into three general sections. The first addresses the full spectrum of infrared detectors and contains a limited coverage of all the material presented in subsequent chapters. It serves as an introduction to the volume and presents to the reader an overall view of the present state of the infrared technology art. It also serves as the mortar between the more in-depth discussions which follow. The midsection. Chapters 3,4, and 5, is a detailed analysis of those detector types which are most widely used today thermal, photoconductive/photovoltaic and photoemissive. 

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. 

The most common approaches to sulfonylurea determinations involve high-performance liquid chromatography (HPLC). The earliest reported methods utilized normal-phase liquid chromatography (LC) with photoconductivity detection this type of detector demonstrated undesirably long equilibration times and is no longer... 

The materials and design of the various photoelectric detectors available are such that the absorption of radiation results in the displacement of electrons and hence in the development of a potential difference between two electrodes. The main types of photoelectric detectors may be classified as either photovoltaic or photoconductive (Figure 2.24). 

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... 

Detectors are similar in type to those for the far-infrared, namely thermocouples, bolometers, Golay cells or photoconductive semiconductors. 

As shown in Table 25-2, there are two general types of transducers one type responds to photons, the other to heat. All photon detectors are based on the interaction of radiation with a reactive surface either to produce electrons (photoemission) or to promote electrons to energy states in which they can conduct electricity (photoconduction). Only UV, visible, and near-IR radiation possess enough energy to cause photoemission to occur thus, photoemissive detectors are limited to wavelengths shorter than about 2 p.m (2000 nm). Photoconductors can be used in the near-, mid-, and far-IR regions of the spectrum. 

Impurity photoconductivity (extrinsic photoconductivity) is a type of absorption measurement where the detector is the sample itself. Classical photoconductivity occurs when the absorption of an electron or of a hole takes place between a discrete state and a continuum, where it can contribute to the electrical conductivity. When the final state of a discrete transition is separated from the continuum by an energy comparable to k T at the measurement temperature, the electron or the hole in this state can be thermally ionized in the continuum and give rise to photoconductivity at the energy of the discrete transition. This two-step process, which is temperature-dependent, is known as photo-thermal ionization spectroscopy (PTIS) and is discussed in more detail later in the section on extrinsic photoconductors. 

IR transducers are of three general types (1) pyroelectric transducers. (2) photoconducting transducers, and (3) thermal transducers. The tirst Ls found in photometers, some FTIR spectrometers, and dispersive speclropholometers. Photoconducting transducers arc found in many F TIR inslrumonis. Thermal detectors arc found in older dispersive instruments but arc too slow to be used in FTIR spectrometers. 

The type of detector used in an FT-IR spectrometer is highly dependent upon the bandwidth (i.e. the spectral frequencies), the modulation rate of the interferometer, and the intensity of the radiant flux. Several types of detectors are used in the infrared regions photoconductive, photovoltaic, bolometers, pyroelectric and Golay cells. A detailed discussion of detectors may be found elsewhere.12 In general, the photovoltaic and photoconductive detectors can be used in the near- and mid-infrared regions as rapid response, high sensitivity detectors. Usually the bandwidths are limited and will not cover the total ran passed by the beamsplitter. Examples of such detectors are given in Table I. As can be seen from the... 

Thus the zinc blende structure semiconductors can be useful for intrinsic photoconductive detectors. Compounds such as InSb have been used as intrinsic photoconductors [4.20], as well as for photovoltaic detectors, but greater versatility of wavelength response is possible with the Hg j tCd Te alloy system. The Hgi j.Cd,Te alloys have received considerable development effort in recent years and are the most prominent intrinsic photoconductor materials they will be analyzed in this subsection. The development of Hg, Cd Te has concentrated almost entirely on n-type material since it provides high photoconductive gain however, p-type Hg, Cd,(Te crystals may be useful for intrinsic photoconductive detectors also [4.21]. 

Fig. 4,11. G RA product vs carrier concentration for n-type Hgo gCdo 2Te intrinsic photoconductive detector at 77 K...

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... 

Single crystals of Hg,, Cd Te are grown by several different methods [4.42]. Almost regardless of the growth method or composition x in the above range, undoped ( pure ) crystals which are n-type at low temperatures have an extrinsic electron concentration near 10 cm" this is a relatively low carrier concentration for a semiconductor, and it is one of the major reasons for the success of Hg, j.Cd Te as a photoconductive infrared detector material. However, undoped crystals often turn out p-type with or have... 

Heterodyne detectors in the microwave and millimeter regions (hv< kT) include square-law mixers such as the crystal diode detector [7.93], the InSb photoconductive detector [7.94-96], the Golay cell [7.95], the pyroelectric detector [7.95], the metal-oxide-metal diode, and the bolometer [7.87]. The latter three types of detectors have also been used successfully in the middle infrared (at 10.6 pm) [7.97-100]. For this type of detector Johnson noise generally predominates, and the input SNR is given by [7.100]... 

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