n 



8 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



bias in the form of a condenser discharge pulse. Care must be taken 

 to avoid changes in contact characteristics resulting from the plating 

 pulse. The deposit of copper does not appear instantly after pulse ap- 

 phcation, but may require several seconds before becoming visible. At 

 the instant the deposit becomes visible, the plating solution is washed 

 ofi-. 



Fig. G(a) and 6(b) show the results of the plating operation on a formed 

 collector point and a formed emitter point. Both pulses were similar to, 

 though somewhat "heavier" than those usually used to form transistors. 

 These units were plated under the conditions illustrated in Fig. 5(a). 

 The floating potential in the vicinity of the reversed bias point can be 

 measured as a function of the distance, r, from its center, using an aux- 

 iliary tungsten point. Qualitatively this potential is shown as a function 

 of the distance, r, in Fig. 5(b). In this case most of the drop in magnitude 

 of the potential appears within a radius, r, less than 0.002 inches, pro- 

 vided surface conductivity is small. The conductivity of the plating solu- 

 tion is kept small to ensure that the potential distribution in the ger- 

 manium is not altered by presence of the solution. Under these 

 conditions, it is assumed that, although copper ions in solution are at- 

 tracted towards the highly negative regions of the germanium, the main 

 current flow is through the germanium, except for regions of high poten- 

 tial gradients. In these regions some of the current will be carried by 

 ions in the solution, by -passing the region. If the formed region bound- 

 ary is a sharp p-n junction, one would expect a plating pattern as ob- 

 served in Fig. 6(b) and 6(d), as is observed with the donor-free emitter 

 point. For the more complicated structure produced by forming the 



COLLECTOR POINT 



MASKING 



FORMED REGION 

 BOUNDARY 



CU SO4 SOLUTION 



MASKING 



n-Ge 



17" 



■'-BASE CONTACT 



-V(r) 

 (b) 



Fig. 5 — Experimental conditions for copixT ])lating. 



