1026 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



resistance Rz)is now switched in, and the resulting off-balance (repre- 

 senting the field modulation of conductivity) presented on the vertical 

 plates of an oscilloscope. The supply voltage is connected, via the high.j 

 bleeder resistance Ri , to the horizontal plates. The frequency of the 

 oscillator was chosen to be 25 cyc/sec, a value sufficiently low to obviate 

 lifetime difficulties; the peak-to-peak swing was generally 500 volts. 



During a field-effect measurement, the sample was also illuminated 

 with light chopped at 90 cyc/sec. This had the result of causing to be 

 presented on the oscilloscope screen a pattern such as that shown in 

 Fig. 3. The lower tilted line represents the (dark) field effect curve; the 

 vertical separation represents the photoconductivity, as modified by the 

 applied field. Measurements were made of the mean vertical separation, 

 and of the slopes of the upper and lower lines (by reading gain settings). 



During a surface photo-voltage measurement, the gold electrode was 

 disconnected from the high-voltage supply, and connected to a high- 

 impedance detector, similar to that used in the work of Brattain and 

 Bardeen.^ A value for the chopped light intensity was chosen to give a 

 contact potential change that was generally not more than 5 mV. A 

 simultaneous measurement of the photoconductivity was also made. 



The gas cycle was similar to that described by Brattain and Bar- 

 deen.^ Some variations were made in it to try to spread out the rate of 

 change with time so that the data could be obtained without large gaps. 

 The cycle used was: (i) sparked oxygen 1 min, (ii) dry O2 , (iii) mixture 

 of dry and wet O2 , (iv) wet O2 , (v) wet No , (vi) a mixture of dry and 

 wet N2 , (vii) dry O2 , (viii) dry O2 , triple flow, and (ix) ozone normal 

 flow. The normal rate of gas flow was about 2 liters per minute; the wet 

 gas was obtained by bubbling through water (probably about 90 per 



Fig. 3 — Picture of field effect-photoconductivity pattern, as observed on 

 oscilloscope. Dark curve at the bottom. , 



