HOLE CONCENTRATION AND POINT CONTACTS 



477 



terior of the floating end of the filament is small. The small drop which 

 does exist results from the difference in mobility between electrons and 

 holes. Almost all of the potential difference between the probe and the 

 right end is the floating potential, Vcf, across the barrier layer of the 

 probe point. 



Pearson's data are plotted in Fig. 3. The data can be fitted by an equa- 

 tion of the form (16) with ^ = 0.5. 



The difference in potential between a floating point contact and the 

 interior which exists under non-equilibrium conditions explains anoma- 

 lously high values of probe potential which were sometimes observed by 

 Shockley and by Brattain in the vicinity of an emitter point operating in 

 the forward direction. As an example of a case in which the effect is 



Table I 



Measurements of probe potential, Vp/, at a contact on an etched germanium surface 

 .005 cm from a second contact carrying a current /. The conductance of the probe point 

 is Gp. The voltage drop across the probe contact, Vp/ — Vi, at zero current is calculated 

 from Vpf - Vi = 2.5{kT/e) log (Gp/Go). Data from W. H. Brattain. 



/ 



amps 



2.0 X 10-» 



1.0 



0.5 



0.2 



0.1 

 -0.1 

 -0.2 

 -0.5 

 -1.0 



mhos 



8.3 X 10-* 



5.0 



3.3 



2.2 



1.7 



1.2 



1.2 



1.25 



1.35 



VJI 

 ohms 



35 

 51 

 64 

 75 

 90 



large, some data of Brattain are given in Table I for the experimental 

 arrangement of Fig. 4. Two point contacts were placed about .005 cm 

 apart on the upper face of a germanium block. The surface was ground 

 and etched in the usual way. A large-area, low-resistance contact was 

 placed on the base. The potential, Vp, of one point, used as a probe, 

 was measured as a function of the current flowing in the second point. In 

 this case, the potential on the probe point is produced in part by the 

 Vcf term and in part by a potential, V i, in the interior which comes from 

 the IR drop of the current flowing from the emitter point to the base 

 electrode. Reasonable values are obtained for 7, from measurements of 

 Vp if a correction for Vc/ is properly made. 



The first column of Table I gives the current and the second column 

 the probe potential, Vp, measured relative to the base. The third column 

 gives values of Vp/I. In the reverse direction (negative currents) Vp/I 



