Biasing photoconductors

A biased photoconductor under the influence of magnetoconcentration for the case of low magnetic fields was investigated by Malyutenko and Teslenko [397, 398]. They denoted the effects of Lorentz force to carriers in such a device the transversal sweep-out ( a semiconductor with induced anisotropy of conductivity ). Their work included both theoretical and experimental investigation, and the magnetic induction they utilized did not exceed 0.2 T. 

A strain-biased antiparallel domain configuration is induced in the plane of a memory-type PLZT (7/65/35 with a grain size of 1.5 /mi) plate approximately 60 fim thick, as shown in Fig. 8.17(a). A strain of 10 3 is achieved by bonding the PLZT to a thicker plate of transparent Perspex and bending the combination so that the ceramic is in tension in the convex surface. Combining a photo-conductive layer, e.g. polyvinyl carbazole, CdS, ZnS or selenium, with the PLZT plate allows an optical pattern to be electrically stored, read and erased. Transparent ITO electrodes sandwich the photoconductor-PLZT combination. 

Hole Transport in PMPS. In the experiments with layered structures (20) and visible excitation (to which PMPS is transparent), transient currents were observed only when the top electrode was negatively biased with respect to the substrate. The substrate was composed of a visible photoconductor (charge generation layer) overcoated aluminum ground plane. When the polymer top surface was directly (intrinsically) photoexcited with pulsed 337-nm excitation, current transit pulses were observed only when the top electrode was positively biased. Therefore, under the experimental conditions described, only hole transient transport could be directly observed. Transit pulses were nondispersive over a wide range of temperature. Figure 14 illustrates the relative increase in dispersion with decreasing temperature. In addition, no evidence for anomalous thickness dependence at the transit time was obtained, even at the lowest temperature. 

After development, the toner pattern is transferred to a sheet of paper placed face-to-face with the photoconductor. To overcome the electrostatic and van der Waals forces holding the charged powder to the photoconductor surface, an electric field is applied through the paper, e.g. by means of a corona spray or a biased, conformable semiconductive roller (Figure 12). The paper must present an electrically blocking interface to the toner. 

The minority carrier sweepout effects have been observed in n-type Hg, jjCd,(Te by several investigators [4.22, 23]. The speed of response of the photoconductor is improved by biasing into the sweepout mode, as expected, and sweepout is thus a useful effect for controlling detector response time. An... 

Since extrinsic silicon photoconductor material has high resistivity at cryogenic temperatures it can be used to form the substrate of an accumulation mode CCD as shown in Fig. 6.11. With an accumulation mode MIS structure the gates are biased so that majority carriers are stored and transferred down the insulator semiconductor interface. Local potential wells are formed under the gates however the dynamics of the charge transfer process will be very different from those for an inversion mode device since with an accumulation mode device the transverse electric fields will extend all the way to the back contact instead of being confined to the depletion region of an inversion mode structure. 

For material with Kttle carrier trapping, the response time is determined by carrier lifetime. As device dimensions get smaller, however, the response time can be altered by the nature of the metal contact and the method of biasing. With nonaUoyed contacts to the MSM device, Schottky barriers can be formed at the metal semiconductor interfaces, and the depletion region extends across the length of the photoconductor. In this case, the device operates more or less like a photodiode with r replaced by t , and its MB value becomes 1/t (see Eq. (9.19)). 

Figure 5.12.3 shows circuits most commonly used with photon detectors. The bias circuit illustrated in (a) is well-suited for photoconductors. The signal voltage is measured across a load resistor. The simple circuit in (b) is appropriate for a PV detector without an external bias. The more complex circuit in (c) is for a reverse-biased PV detector. In this circuit, which resembles (a) to some extent, an external voltage is applied across the detector and a load resistor arranged in series. 

Low-impedance detectors may operate better if they are biased with a current instead of a voltage. These devices typically change their effective resistance with exposure to photons (photoconductors and bolometers, for example). Biasing with a constant current results in a change in voltage with a change in photon flux, which... 

Fourthly, the detectors operate with small biases which must be maintained accurately for correct detector operation. For example, the voltage transients on a detector used with a conventional switched multiplexer would be sufficient to break down a far infrared photoconductor. To maintain the detector bias requires a readout that is powered continuously. 

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