12 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



0.024 



0.020 



0.016 



0.012 



-0.006 



0.004 



q: 



(J -0.004 

 < 



-0.008 



0.012 



- 0.016 



■0.020 



-0.024 



0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 

 C.R IN VOLTS 



Fig. 8 — Change of contact potential with illumination versus contact po- 

 tential, samples A and D etched. 



Not shown in Fig. 8 because of the scale used is the result that (Ac.p.)l 

 for n-type material does not increase indefinitely as c.p. decreases but 

 approaches a maximum value. Likewise — (Ac.p.)l for p-type approaches 

 a maximum as c.p. increases. Figures illustrating these results will be 

 discussed after the theory is presented. 



Some of the experimental details require discussion. It is necessary 

 to calibrate the ac response in terms of absolute potential change. This 

 can be done by comparing the ac reading with the dc reading when the 

 light signal is large. One can also do this by introducing a known square- 

 wave signal across the potentiometer in Fig. 1, and reading the ac signal 

 out. Both methods agree when allowance is made for variation of the 

 light signal with frequency. The latter response is almost flat from 25 

 to 300 cycles, but there is evidence for some very low frequency com- 

 ponents in the dc measurements of (Ac.p.)^ . When the light signal 

 goes through zero the signal is small and the dc value is difficult to read. 



