POINT-CONTACT TRANSISTOR SURFACE EFFECTS 



781 



2.3 Under-Formed and Over-Formed Contacts 



One of the problems encountered in the large-scale manufacture of 

 point-contact transistors is the variation in the forming yield. Thus, 

 forming to a specified criterion of transistor performance does not always 

 result in a uniform product. Although considerable care may be taken to 

 ensure uniformity of all bulk properties and forming technique, a large 

 variation may be encountered in the output characteristics of the tran- 

 sistors. In Section 4, a prime factor in determining the efficiency of form- 

 ing is shown to be the chemical history of the germanium surface. Un- 

 controllable variations in surface conditions may therefore often account 

 for much of the variations in results of a specific forming technique. 



Such variations often manifest themselves merely as differences in 

 degree, but may show up as differences in kind, takmg the form of anoma- 

 lous output characteristics. These have been classified by L. E. Miller^^ 

 into three qualitatively different phenomena. The first of these, referred 



COLLECTOR 



PLATING 



INHIBITED 



IN THIS AREA 



BY MASKING 



COPPER 



PLATED 



AREA 



VERY HEAVY PLATE 

 UNDER COLLECTOR 



EMITTER 



- UNPLATED 

 AREA UNDER 

 EMITTER 



GERMANIUM 



TO BASE 



OHMIC 



CONTACT 



5 



SCALE IN MILS 



UNIT OPERATED AT LOW le; PLATED 0.25'Vo CUSO4, 20 SECONDS 

 le = 0.5 MA, Vc = -20V, Oi - 0.1 



Fig. 8 — Flow geometry for a low alpha point-contact transistor. 



