Photoconduction current

We can find the potential at which a free electron appears by measuring the threshold of external photoemission E h. However, the ionization is not always accompanied by electron emission. We can consider the ionization event to have occurred if the electron is transferred to the conductivity band. The corresponding ionization potential 7C equals the energy needed to transfer an electron to the bottom of the conductivity band. It is found experimentally by measuring the threshold of photoconduction current. In crystalline insulators /c can be found from the limit to which the energy series for the Wannier-Mott179 exciton converges. 

Exposure and latent image formation. The sensitized photoreceptor is exposed to a light and dark image pattern in the light areas the surface potential of the photoconductor is reduced due to a photoconductive discharge. Since current can only flow perpendiculai to the surface, this step produces an electrostatic-potential distribution which replicates the pattern of the image. 

Probe measurements in silane discharges have been reported [296,297]. Apparently, no difficulties were experienced, as the deposited amorphous silicon layer on the tip was sufficiently photoconductive. For typical silane discharge conditions values for are found to be between 2 and 2.5 eV. Electron densities are around 1 x 10 cm - [296]. Probe measurement in the ASTER system failed due to strong distortions of the probe current, even after following cleaning procedures. 

Thermally stimulated hole current in poly-N-vinylcarbazole shows distinct maximum at around 5°C and another large current above 100°C. This 5°C maximum is due to 0.56 eV hole traps of 7 x 1015 cm 3 density. Photoconductivity in poly-N-vinylcarbazole increases appreciably when irradiated with UV-light in air at room temperature and this increase accompanies the formation of 0.56 eV hole traps. The nature of this traps has been discussed. 

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. 

We now turn to photoconductive and photodiode detectors, both of which are semiconductor devices. The difference is that in the photoconductive detector there is simply a slab of semiconductor material, normally intrinsic to minimize the detector dark current, though impurity doped materials (such as B doped Ge) can be used for special applications, whereas by contrast, the photodiode detector uses a doped semiconductor fabricated into a p-n junction. 

Some polymeric materials become conductive when illuminated with light. For instance, poly(A -vinylcarbazole) is an insulator in the dark, but when exposed to UV radiation it becomes conductive. Addition of electron acceptors and sensitizing dyes allows the photoconductive response to be extended into the visible and near-IR (NIR) regions. In general, such photoconductivity is dependent on the material s ability to create free-charge carriers, electron holes, through absorption of light, and to move these carriers when a current is applied. 

Selenium has many industrial uses, particularly electronic and solid-state applications, which have increased phenomenally in recent years. This is attributed to its unique properties (1) it converts light directly to electricity (photovoltaic action) (2) its electrical resistance decreases with increased illumination (photoconductivity) and (3) it is able to convert alternating current to direct current. 

When a semiconductor is bombarded with photons equal to or greater in energy than the band gap, an electron-hole pair is formed. The current that results is a direct function of the incident light intensity. Photoconductive devices consist of a p-n junction called a photodiode, or a p-i-n junction commonly used in a photodetector, an... 

The detector output signal is generated by a current preamplifier for photovoltaic detectors, such as InSb, and by a simple detector bias circuit shown in Fig. 4 for photoconductive detectors, such as PbS and Hg Ge. The voltage signal derived from the bias circuit is normally preamplified and forwarded to a phase-sensitive synchronous detector usually embodied in a lock-in amplifier (Stewart, 1970 Blass, 1976b). 

The charge transport in amorphous selenium (a-Se) and Se-based alloys has been the subject of much interest and research inasmuch as it produces charge-carrier drift mobility and the trapping time (or lifetime) usually termed as the range of the carriers, which determine the xerographic performance of a photoreceptor. The nature of charge transport in a-Se alloys has been extensively studied by the TOF transient photoconductivity technique (see, for example. Refs. [1-5] and references cited). This technique currently attracts considerable scientific interest when researchers try to perform such experiments on high-resistivity solids, particularly on commercially important amorphous semiconductors such as a-Si and on a variety of other materials... 

The photoconductive gain G as measured from saturated current-voltage curves is of the order of unity in several dyes 3,50,51) .g. in malachite green G =0.2, pinacyanol G=0.37 and merocyanine A 10 7 G=0.6. 

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

Fig. 1. The relationship of the sample and the various pieces of equipment necessary to automatically measure the current, and the parallel and perpendicular voltages. A standard contact configuration is shown in the main drawing, whereas a van der Pauw configuration is shown in the inset. The lock-in amplifier is used in ac photoconductivity measurements. [From Look and Farmer (1981), copyright by The Institute of Physics.]...

This work will attempt to demonstrate the importance of adsorbed oxygen on many properties of zinc oxide, namely the conductance, fluorescence, photoconductance, and the adsorption of hydrogen. The contribution, potential, and current of each of these studies to a more complete understanding of the adsorption process will be discussed. 

When a semiconductor is illuminated, electrons may be excited into the conduction band and/or holes into the valence band, producing photoconductivity. This excited condition is not generally permanent, and when the illumination ceases, the excess current carriers will decay, or recombine. The average time which a photoelectron remains in the conduction band is termed the lifetime. As the lifetime increases, the photocurrent, for a given intensity of illumination, increases. 

The traditional source in IR absorption spectroscopy is a glowing rod or wire heated by the passage of an electric current the hot body emits radiation over a continuous frequency range. The radiation is dispersed using a prism NaCl, which is transparent over much of the IR region, is commonly used for IR prisms and windows. The sample may be a solid, liquid, or gas. Various detectors are used the most common are thermocouples, photoconductive materials such as PbS, bolometers (which are temperature-dependent resistors), and the Golay cell (which uses the thermal expansion of a gas contained in a chamber). 

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