1290 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



in the same simple way possible for the structure of Fig. 2(a), since the 

 base resistance between the collector circumference and the base contact 

 is not changed by collector depletion region thickness changes. 



The details of the base contact may change the magnitude of this 

 feedback greatly. In particular, poor placement of a bonded base contact 

 may sometimes result in very large values of fXic. A point of general 

 interest is that the sign of the feedback is determined directly by the 

 direction of base current flow^, while the phase is otherwise exactly that 

 of the collector voltage. 



4.5 Equivalent Circuit with Base Resistance Effects 



The base resistance effects discussed above can be combined with the 

 admittance circuits obtained in the one-dimensional study of Section 3.0 

 to give equivalent circuits for the three-dimensional structures of Fig. 2. 

 The resulting representations, shown in Fig. 5, reflect the geometry of 

 these structures since the base resistance elements are shown in a branch 

 through which the base current must flow to reach the base terminal. 

 All of the effects can be seen to give feedback, which is to say, coupling 

 from the output circuit of the collector to the input loop of the emitter. 

 This leads to complication of the electrical characteristics of the trans- 

 istor, whose characteristics would otherwise be given by the electronic 

 functions developed in the previous section. 



Fig. 5(a) is an equivalent circuit which represents the small signal 

 transmission properties of the structure of Fig. 2(a). It should be noted 

 that a single base resistance is needed, through Avhich all of the base 

 current passes. The effects of surface leakage are indicated by the ad- 

 mittance Yi from collector terminal to base resistance. The indicated 

 uncertainty of placement of this leakage effect with respect to n' reflects 

 the fact that the feedback resulting from the leakage depends on the 

 position of the leakage with respect to the base contact. 



The structure of Fig. 2(b) is represented by the circuit of Fig. 5(b). 

 The separate base resistances Vb^ and n^ mirror the physical fact that the 

 collector region is, on the average, closer to the base contact than is the 

 emitter region. Leakage effects may be added to this circuit in the same 

 manner as before. 



Although the circuits of Fig. 5 represent primarily small signal ac 

 transmission functions in the transistor, very similar representations 

 may be employed for large signal functions. The principal changes occur 

 in the diffusion admittances, which should be replaced by the diffusion 

 currents written as functions of the barrier voltages, and in the capaci- 



