1032 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



Table VI — 8.1 ohm cm p-type 



The first problem is the constructing, from the experimental results, 

 of the curve relating AG and S. The experiments provide a series of pic 

 tures like Fig. 3, each one corresponding to a different chemical environ- 

 ment, and so to a different Y. At each of two succeeding pictures of this 

 sort one knows (i) the vertical displacement (photoconductivity) be- 

 tween the dark and light field effect curves; and (ii) the mean difference 

 in the dark and light slopes, and hence the rate of change of photocon- 

 ductivity with applied field, and therefore with S. The problem is to de- 

 duce the horizontal displacement (in 2) between the two pictures. 



A corrrection must first be made for the fact that the ambient changes 

 2 uniformly on both surfaces, whereas the applied field induces charge 

 only on the lower surface, plus fringing effects.* The correction is applied 

 by taking the difference in slopes (lUei* — /Jen), and multiplying this by 

 (2/1.27), where the number 1.27 is deduced for the given geometry from 

 the standard edge-effect formula. This having been done, it is now pos- 

 sible to take the revised pictures and piece them together to form tA\() 

 smooth curves (Fig. 4). The process of assembling such a diagram de- 

 termines the horizontal and vertical distances, and therefore the change 

 of 2 and AG, between successive experiments. 



This argument may be given analytically as follows. First notice that 

 the photoconductivity ^•oltage in the absence of field (AT% in Fig. 3) is 

 proportional to (1/s). The application of a voltage between the gold 

 and the germanium induces some charge density 2 at each point on the 

 germanium surface, 2 being (due to fringing effects) a complicated 

 function of position. At each point (1/s) is changed by an amount 

 2[(i(l/s)/(/2]. This causes the photocondu(!tivity in the presence of field 



We are indebted to W. L. Brown for l^ringing this to our attention. 



