Extrinsic photoconductor

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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. 

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... 

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... 

We shall consider next the theory of condition 2 for an extrinsic photoconductor, then the theory of G RA for condition 3, and finally the dependence of t on material parameters. We shall analyze the geometrical model of Fig. 4.8 and assume a simple energy level model of an n-type extrinsic semiconductor consisting of a photoionizable donor level and a compensating acceptor level properties of a corresponding p-type model would be analogous. This material is extrinsic as both a photoconductor and semiconductor. 

Sweepout effects can occur in extrinsic photoconductors also [4.26,27], but we shall not explicitly include them in this model they are more difficult to understand, but not generally as important in practice as in intrinsic photoconductors. The essential result is that whereas sweepout of carriers can limit the... 

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... 

As indicated earlier, condition 4 is not easily satisfied by these detectors. Let us require that f=2.5a so that if >0.9, use maximum value for the shallow dopant of this example at which the ionization energy nearly equals the low doping level value then f 2.5 x 10 cm is the required detector thickness. We had used this thickness in the above evaluation of condition 2. Thus even in this "best case" estimate, the Si extrinsic photoconductor must be about 50 times thicker than the comparable Hg<, 8Cdo.2Te intrinsic photoconductor. 

The theory of the electronic energy levels of deep dopants in Si is far more difficult and is not yet fully developed. The electrons are much more tightly bound to these impurities than to shallow impurities, and the Mott transition is not observed. Also, the solubilities of these dopants in Si are generally much lower than those of the shallow dopants, so that solubility limits the concentration of a deep dopant which can be achieved in Si for use as an extrinsic photoconductor. 

The carrier mobility is determined mainly by impurity scattering at the low temperatures of extrinsic photoconductor operation. Furthermore, in the relatively uncompensated Si of most interest for detectors, scattering by the neutral majority dopant atoms will dominate. It can be shown that the neutral impurity scattering mobility is given in Si by [4.65]... 

Fig. 6.8a and b. The use of CCD registers for TDI. (a) IR active CCD register (monolithic focal plane array with photogeneration in the register), (b) Direct injection with Si parcel in series out CCD register hybrid with IR photodiodes or monolithic with silicon IR extrinsic photoconductors... 

The use of an extrinsic photoconductor with direct injection has the advantage that dc gain can enhance I and therefore g, but gain saturation due to sweepout will limit ac gain to 1/2 at frequencies near f [6.30]. The capacitance C of an extrinsic photoconductor can be an order of magnitude lower than for a photodiode, which will lead to a higher /. With an extrinsic photodetector, crosstalk problems must be considered for the detector thicknesses necessary to provide reasonable quantum efficiency, and even for the 3-5 pm window operating temperatures will tend to be below 50 K. 

The photoconductor, as shown in Fig. 7, depends upon the creation of holes or electrons in a uniform bulk semiconductor material, and the responsivity, temporal response, and wavelength cutoff are unique to the individual semiconductor. An intrinsic photoconductor utilizes across-the-gap photoionization or hole-electron pair creation. An extrinsic photoconductor depends upon the ionization of impurities in the material and in this case only one carrier, either hole or electron, is active. The same is true for a quantum-well photoconductor, in which electrons or holes can be photoexcited from a small potential well in the narrower band-gap regions of the semiconductor. The quantum efficiency for the structure in the figure is determined by the absorption coefficient, o, and may be written 2isrj = (l — / )[ — where R is the reflection coefficient at the top surface. Carriers produced by the radiation, P, flow in the electric field and contribute to this current flow for a time, r, the recombination time. The value of the current is... 

In semiconductors doped with donor or acceptor atoms, optical transitions between discrete levels of these atoms and conduction or valence band are possible. Since these levels are within the band gap, close to the conduction band or valence band, respectively, this extrinsic photoexcitation is already possilbe at low photon energies (Fig.4.77b). Because of these low excitation energies the donor or acceptor levels can easily be ionized by thermal excitation which would decrease the population density and therefore the absorption coefficient. Such extrinsic photoconductors which are sensitive in the infrared up to wavelengths of about 30 ym, have to be operated at low temperatures to prevent thermal ionization. The absorption coef-... 

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