Extrinsic detectors

Extrinsic detectors, 22 180 Extrinsic fiber-optic sensors, 11 148 Extrinsic photoconductors, 19 138 Extrinsic semiconductors, 22 236-237 Extrinsic wastes, 10 68 Extruded food packaging, 18 45 Extruded lead-copper alloys, 14 776 Extruded lead-tellurium alloys, 14 778 Extruded rigid foam, 23 404-405 Extruders... 

Depending on the nature of the semiconductor material, photoelectric detectors can be classified as intrinsic or extrinsic detectors. Intrinsic photoelectric detectors are pure semiconductors, whereas in extrinsic photoelectric detectors some impurities are added to the semiconductor during the fabrication process. Energy diagrams showing the processes activated by photo-excitation in these two kinds of photoelectric detectors are shown in Figures 3.11 (a) and 3.11(b) for intrinsic and extrinsic detectors, respectively. 

On the other hand, in extrinsic detectors, electrons or holes are created by incident radiation with photons of energy mnch lower than the energy gap. As can be observed from Fignre 3.11(b), the inclnsion of impnrities leads to donor and/or acceptor energy levels within the semicondnctor gap. Thns, the energy separation between these impnrity levels and the valence/condnction bands is lower than the energy... 

The performance of the radiation detectors depends on their intrinsic properties, temperature and external conditions of use. They can be compared by using a factor of merit D, known as the detectivity, equal to the inverse of the NEP for a detector with unit area used with an electrical band-width A/ of 1 Hz and expressed in cm Hz1/2 W-1. When a value of D is indicated for a thermal detector, it is considered to be independent of the radiation frequency and the time modulation frequency is assumed to be adapted to the intrinsic time constant t, of the detector. For a photoconductive detector, D peaks at a radiation frequency very close to the band gap for an intrinsic detector or to the ionization energy of the relevant centre for an extrinsic detector and decreases steadily at lower energies. 

We start from the phenomenological expression for the current of a photocon-ductive detector, described in literature as the fundamental equation of photoconductivity [7]. This expression is valid both for intrinsic and extrinsic detectors in stationary state and is given as... 

Fig. 1. Photoexcitation modes iu a semiconductor having band gap energy, E, and impurity states, E. The photon energy must be sufficient to release an electron (° ) iato the conduction band (CB) or a hole (o) iato the valence band (VB) (a) an intrinsic detector (b) and (c) extrinsic donor and acceptor...

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

Fig. 9. Spectral sensitivity of detectors where the detector temperatures in K are in parentheses, and the dashed line represents the theoretical limit at 300 K for a 180° field of view, (a) Detectors from near uv to short wavelength infrared (b) lead salt family of detectors and platinum siUcide (c) detectors used for detection in the mid- and long wavelength infrared. The Hg CdTe, InSb, and PbSnTe operate intrinsically, the doped siUcon is photoconductive, and the GaAs/AlGaAs is a stmctured supedattice and (d) extrinsic germanium detectors showing the six most popular dopants.

Information on ionization energies, solubiUties, diffusion coefficients, and soHd—Hquid distribution coefficients is available for many impurities from nearly all columns of the Periodic Table (86). Extrinsic Ge and Si have been used almost exclusively for infrared detector appHcations. Of the impurities,... 

Figure 3.11 Photo-excitation mechanisms in (a) intrinsic and (b) extrinsic photoelectric detectors.

It is evidently insufficient to consider only the response of a detector when analysing its usefulness for a particular application. It is generally necessary to analyse both intrinsic and extrinsic noise signals and compare them with the response. The result of this comparison can be expressed in many different ways. One of the most useful is the noise-equivalent power nep which is the power of an rms signal input (in watts) required to give a response equal to the total rms noise voltage AVN. Then ... 

In the extrinsic technique, light from a suitable source travels along the FO to the distal end where an immobilized sensing layer is located. Reflected, scattered or emitted light returns from the sample by a second fiber or by bifurcation of the original fiber. The emitted light is interpreted at the detector and is a measure of the concentration of the analyte of interest. The simplest FO biosensor uses absorbance measurements to determine any changes in... 

Low-carbon silicon must also be used for the fabrication of extrinsic infrared detectors to prevent the formation of carbon-... 

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. 

The method of selectivity measurement is based on the fact that in any chemical procedure for a given species A any interference caused by the presence of an extrinsic species B will always, at least as observed from the detector s point of view, appear as (positive or negative) pseudo A. Therefore, the extent of this interference can be quantified by the selectivity coefficient, kAB, defined for any FIA method by the equation ... 

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