Photoconduction extrinsic

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. 

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.

Solutions of chlorophyll have been shown to be photoconductive even in the absence of extrinsic acceptors [256]. In acetonitrile as solvent, ions were formed from two molecules in the triplet state. In petroleum ether, red light yielded ions from dimers if the concentration exceeded 10"4 M, the dimerization point [257]. In chlorophyll solutions containing ascorbic... 

There is much still to be understood about the photoconductivity of a-Si H. However, the measurements confirm that recombination through defects is the main mechanism, particularly when their concentration is high. Extrinsic effects further complicate the interpretation of photoconductivity. For example, surface recombination can dominate when the bulk recombination rate is low. These effects can arise from either the excess defects at the surface or from the band bending, which causes a field induced separation of the electron and hole distributions. Contacts, which are almost invariably non-ohmic, also modify the photoconductivity, in particular, the response time. 

Pulse photoconductivity experiments on polydiacetylenes have established that charge carriers move along the chains, with the intrinsic chain motion, for up to 1 mm distance before trapping at some extrinsic defect. The intrinsic motion is that of the SWAP. 

There are cases where, in absorption measurements, the sample itself can be used as an extrinsic photoconductor, once provided with electrical contacts. This is illustrated in the specific case of germanium co-doped with acceptor couples (Ga, Zn), (Zn, Cu) and (Cu, Hg). The ionization energy of Ga is 11.3 meV, and those of the double acceptors, when neutral, are 32.9 meV (Zn), 43.2 meV (Cu) and 91.6meV (Hg). The continuous photoconductivity... 

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. 

Fig. 3 Schematic illustration of acceptor doping that gives rise to extrinsic hole conductivity and long-wavelength photoconductivity in metal phthalocyanines...

The phthalocyanines are still of some interest, and Ayers has interpreted the photoelectrochemistry of copper phthalocyanine in terms of the band-model for semiconductors. Menzel et alf have studied the photoconductivity and electrical field-induced fluorescence quenching in metal-free phthalocyanine films. Doping of the film increases charge-carrier photogeneration, and it seems likely that the mechanism is extrinsic, involving field-assisted exciplex dissociation. These are complicated materials, which are difficult to purify, and the mechanisms of carrier generation in polycrystalline and amorphous films are still obscure. The porphyrins and monolayer assemblies of chlorophyll on SnOj electrodes 2 72,273 ijggjj studied. 

Since the SPV method has been intensively used for diffusion length measurement in undoped a-Si H, we shall discuss the theory of the method in some detail. The approach is to contrast and compare the theory to that already given for conventional semiconductors. The differences arise from two basic facts (1) Undoped a-Si H is a photoconductive semi-insulator rather than an extrinsic semiconductor (2) The thickness of the surface space-charge region (the surface barrier) may be comparable to the diffusion length, whereas in the SPV theory for conventional semiconductors it is assumed that W c Lp. 

Intrinsic and Extrinsic. Photoconductivity can be observed in virtually all semiconductors. Intrinsic photoconductivity requires the excitation of a free... 

Extrinsic photoconductivity occurs when an incident photon, lacking sufficient energy to produce a free hole-electron pair, can produce excitation at an impurity center in the form of either a free electron-bound hole or a free hole-bound electron, see Fig. 2.2b. The long wavelength limit of an extrinsic photoeffect is thus given by... 

Analysis. The basic expression describing either intrinsic or extrinsic photoconductivity in semiconductors under equilibrium excitation (i.e., steady state) is... 

The second photon effect of general utility is the photovoltaic effect. Unlike the photoconductive effect, it requires an internal potential barrier with a built-in electric field to separate a photoexcited hole-electron pair. Although it is possible to have an extrinsic photovoltaic effect, see Ryvkin [2.32], almost all practical photovoltaic detectors employ the intrinsic photoeffect. Usually this occurs at a simple p — n junction. However, other structures employed include those of an avalanche, p—i — n, Schottky barrier and heterojunction photodiode. There is also a photovoltaic effect occuring in the bulk. Each will be discussed, with emphasis on the p—n junction photoeffect. 

Consider first the simple extrinsic photoconductor. Here the sample is a semiconductor containing a single impurity level, the source of the free electrons (or holes) present in the sample. Thus the fluctuation in the number of the free carriers arises from the fluctuation in the generation and recombination rates through that level. If it is assumed that the temperature is so low that very few of the extrinsic centers are thermally ionized (which is valid for most extrinsic cooled photoconductive infrared detectors), then the short circuit g—r noise current and the open circuit g — r noise voltage which appear only in the presence of a bias current Ig, are given by... 

The basic theory of photovoltaic and photoconductive detectors shall be presented in Section 4.1 in a unified form convenient for intercomparison of the two effects and of the various detector materials. Then Sections 4.2,4.3, and 4.4 shall cover photovoltaic, intrinsic photoconductive, and extrinsic photoconductive detectors, respectively, each of these sections including first a subsection in which the general theory of Section 4.1 is specialized to that class of detector, and then a subsection in which specific materials suitable for that class of detector are evaluated in terms of the theory. Finally in Section 4.5 we will draw some conclusions about the status and prospects of photovoltaic and photoconductive infrared detectors. Symbols used in this chapter which are not defined in the text are defined in Table 4.1. 

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