4 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



niques employed in the fabrication of grown junction transistors. Fig. 

 2(b). However, a much simpler technique has been evolved. If the sur- 

 face concentration of the donor diffusant is maintained below a certain 

 critical value, it is possible to alloy an aluminum wire directly through 

 the diffused n-type layer and thus make effective contact to the base 

 layer, Fig. 2(c). Since the resistivity of the original silicon wafer is one 

 to five ohm-cm, the aluminum will be rectifying to this region. It has 

 been experimentally shown that if the surface concentration of the 

 donor diffusant is less than the critical value mentioned above, the 

 aluminum will also be rectifying to the diffused n-type region and the 

 contact becomes merely an extension of the base layer. The n-layers 

 produced by diffusing from elemental antimony are below the critical 

 concentration and the direct aluminum alloying technique is feasible. 



-n + TYPE DIFFUSED LAYER 



-p-TYPE DIFFUSED LAYER 



n + 



n+ 



-ALUMINUM WIRE 



p + ALUMINUM DOPED 

 REGROWTH LAYER 



n-TYPE 



(b) 



,^- ALUMINUM WIRE 



P + ALUMINUM DOPED 

 , REGROWTH LAYER 



^M'nY ^-i-r 



n-TYPE 



(c) 



Fig. 2 — ■ Schematic illustralioii of (a) double diffused n-p-n wafer, (b) angle 

 section method of making base contact, and (c) direct alloying method of making 

 base contact. 



