COMBINED MEASUREMENTS ON ETCHED GERMANIUM SURFACES 1027 



cent r.h.) and the mixture of dry and wet was obtained by letting ap- 

 proximately one-half the gas flow bubble through H2O. In carrying out 

 the experiment, it was found convenient to carry out alternately a com- 

 plete cycle of field effect and surface photo-voltage measurements. The 

 values of the photoconductivity at equivalent points in successive cycles 

 could be compared, in order to check that no systematic error was intro- 

 duced by this procedure. 



In addition to the foregoing, the folloAving measurements Avere made: 



1. All dimensions were determined. 



2. The resistivity of the sample was found, and also the body life- 

 time, on another specimen cut from the same crystal. 



3. The amplitude of the voltage swing was measured. 



4. The amplifiers in the field effect circuit were calibrated. 



5. The capacity of the germanium-gold condenser was determined 

 (by a substitutional method). The value obtained was larger than 

 that calculated from the parallel-plate formula, because of the edge 

 effects. 



6. A standard square-wave voltage was introduced into the surface 

 photo-voltage circuit, in order to calibrate the high-impedance detector. 



7. At several points in the cycle, the fundamental mode lifetime of the 

 sample was determined by the photoconductivity decay method. This 

 calibrated the 90 cyc/sec photoconductivity measurements, without the 

 necessity for a knowledge of the light intensity. 



IV. RESULTS 



Measurements were made on two samples: one n-type, 22.6 ohm cm 

 (X = 0.345), the other p-type, 8.1 ohm cm (X = 17.7). The body life- 

 time for both samples was greater than 10~ sec, so that for slices of the 

 thickness used (0.025 cm. or less), and for values of s in the range en- 

 countered, body recombination may be ignored. 



Results of typical field-effect runs for the two samples are indicated 

 in Tables I and II. The first column in each table gives the time in 

 minutes from the beginning of the cycle at which the measurements 

 were made. The second column shows the "effective mobility," dAG/d'2, 

 obtained from the observed (dark) field effect signal voltage AFi (see 

 Fig. 3) by use of the formula: jueff = wh^AVi/Ipo^CVapp , where w is the 

 width of the slice, t the thickness, / the dc flowing through it, po the re- 

 sistivity, C the capacity of the germanium-gold condenser, and Fapp 

 the voltage applied across it. The third column shows the mean value of 

 8{ = Ap/ni), obtained from the mean photoconductivity signal voltage 



