Sec. 5-1] RADIATION TRANSDUCERS 271 



The sensitivity of photogalvanic cells can be fairly high; that of 

 commercial selenium cells is of the order of 1 mA/lumen for small 

 load resistances. The open-circuit output voltage of selenium cells, 

 at high levels of illumination, can attain values from 200 to 600 mV. 

 The sensitivity of cuprous oxide cells is only about one-tenth that 

 of selenium cells. The power output is about 10~ 8 watt/cm 2 at a light 

 flux of 0.01 lumen and increases to 10~ 5 watt/cm 2 at 10 lumens. In 

 the same light-flux interval the internal resistance decreases from 

 10 5 to about 10 3 ohms/cm 2 of cell area. 



The spectral response of selenium cells extends from about 250 

 to 750 m/z, with a maximum response at about 570 m/u; occasionally 

 values for the threshold wavelength of 850 to 1,200 m/ji and a 

 maximum response at 720 m^ are found. 1 Selenium cells respond 

 also to X rays. 2 The spectral response of the cuprous oxide front- wall 

 cell extends from less than 300 to 630 m/u with a maximum at 520 

 m/i; that of the cuprous oxide back-wall cell from 570 to 1,400 m/u 

 with a maximum response near 630 m/x. The response of germanium 

 cells is primarily in the infrared part of the spectrum (maximum 

 response 1,500 m/z, threshold about 1,700 m//) but extends into the 

 visible region. The threshold wavelength of silicon is 1,200 m//. 

 The quantum yield in photogalvanic cells is in the vicinity of 1. 



The electrical (voltage-current) characteristic of the photogal- 

 vanic cell is complicated by the nonlinearity of the system. The 

 internal resistance, i.e., the resistance of the barrier layer, depends 

 upon the current density and therefore upon the illumination and 

 the load resistance. Hence, impedance matching is possible only 

 under a given level of illumination. A voltage-current characteristic 

 (relative values only) is shown in Fig. (5-1)33. The base curve A 

 denotes the rectifier characteristic of the system without illumina- 

 tion. By plotting the resistance-load line, the output voltage, 

 current, and power as well as the internal resistance can be found 

 in the usual way. 



The generation of an emf within the cell takes place a short 

 time after the exposure to light (of the order of 10 -6 sec). However, 

 the build-up of the output voltage is delayed by the large internal 

 capacitance of the barrier-layer system. The equivalent circuit of 

 a photogalvanic cell is shown in Fig. (5-1)34. The capacitance of 

 selenium cells is very high, of the order of 0.01 to 0.1 farad/cm 2 , and 

 is less for cuprous oxide cells. Germanium cells have a much faster 



1 Kellermann, loc. cit. 



2 B. Lange and P. Seleny, Naturwissenschajten, 19, 639 (1931); K. Scharf 

 and O. Weinbaum, Z. Physik, 80, 465 (1933). 



