Sec 



5-1] 



RA DIA TION TRA NSD UCK RS 



265 



the illumination, but rises gradually after the excitation is applied 

 and decays gradually after the excitation is removed. The time con- 

 stants can vary for different photoconductors from microseconds to 

 several minutes; also both time constants decrease with the level of 

 illumination and increase with the temperature. A typical charac- 

 teristic^ a commercial cadmium sulfide cell is shown in Fig. (5- 1)29. 

 Rise and decay times of photoconductor currents are not identical 



10 100 1,000 



Time ofter excitation is applied, msec 



10 100 1,000 



Time otter excitation is removed, msec 



Fig. (5-1)29. Rise and decay of a photoconductive cell (from RCA Tube Manual, 

 Type 6957; by permission). 



with the lifetime t of the free carriers but are usually much longer. 

 They are determined by the time required for filling or emptying the 

 lower levels of energy. 



Noise in photoconductors is intrinsically very small and consists of 

 photocurrent noise and thermal-agitation noise. In addition to these 

 two components, noise can result from thermally generated dark 

 currents. The contribution of this source to the total noise level is 

 complex; it is possible that the thermally generated "dark" electrons 

 contribute comparatively little to the noise level. It is generally 

 observed, however, that noise in photoconductors can be larger than 

 that caused by the sources mentioned above and that the mean 

 square noise current per unit band width is not equally distributed 

 over the entire frequency spectrum but is proportional to 1//, i.e., 

 decreases with increased frequency. It is likely that the source of 

 this excess noise is in the barrier surfaces formed primarily at the con- 

 tact electrodes. Shulman, Smith, and Rose 1 have succeeded in re- 

 ducing the noise from cadmium sulfide crystals by orders of magni- 

 tude by replacing the silver contact electrodes with indium or gallium 



1 C. I. Shulman, R. W. Smith, and A. Rose, Phys. Rev., 92, 857 (1953). 



