264 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



Photoconductors can respond to radiation over a spectral range 

 extending from thermal radiation through the infrared, visible, ultra- 

 violet spectrum into the range of X rays and gamma rays; some of 

 the photoconductors also respond to bombardment with a rays and 

 electrons. The spectral response of a photoconductive transducer 

 drops sharply at longer wavelengths. The "cutoff wavelength" (i.e. , 

 either the wavelength at which the output is equal to the noise level 

 or, in the English literature, that wavelength at which the output is 

 decreased to 50 per cent of its maximum value) depends upon the 

 temperature. Cooling of the photoconductor generally increases its 

 response to longer waves. 



The radiation sensitivity of commercial photoconductive trans- 

 ducers is of the order of 300 /uA/ftW for a radiation of a wavelength 

 at which the photoconductor has a maximum sensitivity. The lumi- 

 nous sensitivity of photoconductive transducers varies widely, from 

 about 10~ 4 to 10 amp/lumen. The sensitivity depends, of course, 

 upon the applied voltage (and upon many other parameters), so that 

 comparisons of photoconductive devices on the basis of sensitivity 

 only are of little value. A standardized method to evaluate a photo- 

 conductive cell or to compare the performance of different cells has 

 been proposed by Jones. 1 



In general, the photoconductor current increases linearly with the 

 applied voltage. Polarization phenomena or barrier layers may 

 cause an apparent deviation from linearity. 2 An exponential increase 

 of current with the applied voltage can also be due to space-charge 

 limitation. 3 Nonlinearity of the exponential-increase characteristic 

 is particularly pronounced in photoconductive powders. Thomsen 

 and Bube 4 have found, in cadmium sulfide and cadmium selenide 

 layers produced from microcrystalline powder in a plastic solution, 

 that at low field strength the current varies with the fourth or fifth 

 power of the field, but that at fields greater than 4,000 volts/cm the 

 current-field relationship is linear. The apparent reason for this effect 

 is that at low field strengths the current is limited by the high resist- 

 ance of the plastic layers between the much less resistant photo- 

 conductor granules. This barrier effect is reduced as the voltage 

 increases and the voltage-current characteristic approaches a linear 

 relationship. 



The photocurrent does not follow instantaneously a variation of 



1 R. C. Jones, Rev. Sci. Instr., 24, 1035 (1953). 



2 R. W. Smith, and A. Rose, Phys. Rev., 92, 857 (1953). 



3 A. Rose and R. W. Smith, Phys. Rev., 92, 857 (1953). 



4 S. M. Thomsen and R. H. Bube, Rev. Sci. Instr., 26, 664 (1955). 



