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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



emission-limited current flow. Insufficient voltage may result in space- 

 charge limited operation. ' ° 



Three structures which may be used to obtain low ohmic base re- 

 sistance are sho^Mi in Fig. 4. Obviously, the base contact ring may be 

 placed arbitrarily close to the emitter, as in Fig. 4(a), so that the base 

 resistance is that of the region beneath the emitter. Since this is some- 

 what difficult, the ring may be placed at a distance from the emitter, 

 and the emitter imbedded in the base n-region as in Fig. 4(b), reducing 

 the resistance between the emitter periphery and the base ring at only a 

 small cost in alpha cutoff frequency. In addition, as shown in Fig. 4(c), 

 the n-region used may be of graded resistivity such as results from im- 

 purity diffusion from the surface. The large impurity concentration at 

 the surface minimizes both edge emission and radial base resistance. 



(a) CLOSE-SPACED RING 



(b) IMBEDDED EMITTER 



(C) IMBEDDED EMITTER - DIFFUSED SURFACE LAYER 

 Fig. 4 — Low-base resistance structures. 



