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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



ward during the plating pulse. The copper deposits to within the order 

 of a diffusion length from the emitter point. Fig. 7(b) shows the pattern 

 obtained by plating the region near a forward biased formed collector. 

 Here again the copper has deposited over practicallj^ all of the base 

 wafer surface, except for a much smaller hemispherical region near the 

 collector point. 



By adjustment of the plating time and solution concentration, the 

 almost radial field in the bulk germanium under a reverse-biased col- 

 lector point can be detected. Under similar conditions, an emitter point 

 biased to the same voltage shows a plating pattern similar to that of Fig. 

 6(b), with little evidence of the radial field. This would be expected 

 from the potential plots shown in Fig. 2. 



These techniques serve merely to illustrate graphically the differences 

 in the two types of contact. Although both points when formed give 

 rise to a formed region in the bulk germanium of similar size and shape, 

 the diode characteristics of the junction under the donor-doped point 

 are degraded. 



The plating technique may also be adjusted to allow sensitivity to the 

 current flow pattern in a transistor with both points biased to operating 

 values. The example shown in Fig. 8 demonstrates visually the bulk 

 nature of the current flow in the point contact transistor. Here the cop- 

 per plates out on the negative regions of the crystal and is noticeably 

 absent from the regions of high hole density under the emitter point. In 

 the region to the left of the collector indicated by the arrow, the plating 

 is partially obscured by masking. The size of the copper-free region under 

 the emitter point may be i-educed to substantially zero for the same /, 

 by increasing the bias applied to the collector. 



(a) t^^^, 



COLLECTOR POINT (BEFORE FORMING) "^"-S COLLECTOR POINT {AFTER FORMING) 



Fig. 7 — The effect of forming and current flow in point-contact collectors. 



