272 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



response and have been successfully operated with light modulated 

 at a frequency of several megacycles. The decay time of the output 

 voltage of germanium cells is of the order of 10 ^sec and increases 

 strongly at lower temperature. 1 



L oad line R L 

 IE 



Fig. (5-1)33. Voltage-current characteristic of a photo- 

 galvanic cell. 



i vWAA 



The influence of the temperature upon the open-circuit voltage 



is considerable; the voltage decreases rapidly with increased tem- 

 perature. The influence of the tem- 

 perature upon the short-circuit cur- 

 rent is much less pronounced. Figure 



tt£ c-- j? p < ft L < (5-1)35 shows the variation with 



temperature of the current output 

 for different load conditions. Tem- 

 porary (reversible) fatigue and the 

 permanent reduction of sensitivity 

 in photogalvanic cells have been ob- 

 served by some authors, while others 

 have experienced a variation of less 



than 1 per cent in the course of a year. 



The application of an auxiliary voltage and a load resistance in 



series with the photosensitive barrier layer changes the operation 



1 M. Becker and H. Y. Fan, Phys. Rev., 78, 301 (1950). 



Fig. (5-1)34. Equivalent circuit of 

 a photogalvanic cell: R s , series 

 resistance; C, capacitance; R P , 

 parallel resistance of the cell; R L , 

 load resistance. 



