DESIGN THEORY OF JUNCTION TRANSISTORS 



1311 



formly over the entire area of the collector junction resulting in a feed- 

 back voltage at the emitter; n'i is defined as the ratio of this feedback 

 voltage to this current. 



Calculations 



The resistances n'l and rh2 for the transistor of Fig. 2(6) may be com- 

 puted with the help of three formulas which give the feedback voltages 

 for the three geometrical problems involved in this transistor. Each 

 expression gives the voltage V developed at electrode C by a current I 

 entering through electrode A and leaving through electrode B. The 

 formulas are in terms of sheet resistance ph/w (resistance per square in 

 ohms) and the radii involved. 



A-c 



BL 



T 



^ 



JB 



BC 



:b 



V=^^^n(ra/n)I 



V = lW^^ 



(c) 



(a) (b) 



Fig. 7 — Feedback voltage for three geometries. 



The simplest situation and its formula are shown in Fig. 7(a). Elec- 

 trodes A and C are the same and expression gives the resistance of an 

 annular ring to radial current flow. 



Fig. 7(b) also shows an annular ring, but the current I is introduced 

 uniformly over one of the flat surfaces, while the voltage V is measured 

 from outside edge to inside edge. 



Fig. 7(c) shows current introduced uniformly over the surface of a 

 disc, while voltage V is the average voltage developed along the surface 

 of the disc. It should be clear that electrodes A and C do not conduct 

 parallel to the disc surface (i.e., they are not equipot^ntials). 



Exarrfple 



The n's for the transistor of Fig. 2(b) will be calculated. 



A. Tb'i — The current I = (l-a)ie is assumed to originate uniformly 



