774 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



2.2 Donor-Free and Donor-Doped Contacts 



The qualitative picture of the conventional formed contact given 

 above has been substantially supported by the work of Valdes, who ob- 

 served a large increase in floating potential near the reverse biased col- 

 lector after the forming pulse and a substantial p-region in the bulk of 

 the germanium after forming. 



Experiments have been directed to a comparison of the properties as 

 diodes and collectors, between two kinds of points. Phosphor bronze 

 points of the type used as transistor collectors, and beryllium copper 

 points, normally used as emitters, were investigated. Thus a direct com- 

 parison can be made between donor-doped and donor-free points which 

 have been given similar forming pulses. The forming pulses were of the 

 capacitor discharge type, with voltage and RC values similar to those 

 used in conventional transistor forming. The points used were of the 

 cantilever variety, and the n-germanium was zone-leveled material in 

 the 3 to 4 ohm-cm range. Two points were supported in a double-ended 

 micro-manipulator which allowed freedom of movement in 3 dimensions 

 for each point. 



2.2.1 Potential Probes 



Conventionally, point-contact transistors are made on a superoxol- 

 etched wafer. This etch leaves a rough surface which is unsuitable for 

 accurate potential probing. Some measurements were made of the float- 

 ing potentials on this kind of surface, but accurate results were difficult | 

 to obtain. In a later section it is shown that the kind of etch used in 

 surface preparation can have profound effects on the degree of forming 

 obtained. However, it is shown that forming characteristics of an "aged" 

 CP4-etched surface are quite similar to the superoxol surface. Thus this 

 kind of surface was used, since its topographical uniformity allows very 

 reproducible results in the measurement of floating potentials. 



Fig. 2 is a comparison of the floating potentials for the two kinds of 

 transistor points examined. The log-log plot shows the magnitude of the 

 floating potential, Vp , near the reverse biased collector as a function of 

 r, the distance of the probe from the collector measured between centers 

 of the two points. The bars represent the uncertainty in measurement of 

 the linear distance. Three curves are shown. The lowest Curve I repre- 

 sents the potential near a Be-Cu point formed with a conventional form- 

 ing pulse. Curve II is a plot of the potential near a similarly formed phos- 

 phor bronze point, while Curve III represents data obtained using such 

 a point more heavily formed. In all cases the magnitude of the floating 

 potential decreases inversely as the distance from the point, and is given 



