1276 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



fxi,c = constant of feedback generator used to characterize modulation 

 of do base spreading resistance. 



03 - 2ir/ = angular frequency in radians. 



oj^ = 2irfa = 2D/W = alpha or current transmission cutoff frequency 

 in radians. 



2.3 Minority Carrier Admittances 



An admittance representation of minority carrier diffusion is a way of 

 writing the dependence of the diffusion currents on the junction poten- 

 tials. To obtain this dependence analytically, the minority carrier den- 

 sities on both sides of each of the two depletion layers (emitter and 

 collector) are assumed to be exponential functions of the junction volt- 

 ages. This exponential dependence is a result of the normal thermal dis- 

 tribution of hole and electron energies. The carrier diffusion currents 

 are computed directly from the gradients of the minority carrier den- 

 sities at the depletion layer surfaces. Since the gradients of the carrier 

 densities are affected by many conditions besides the junction voltages, 

 additional assumptions are necessary. Their nature and pertinence may 

 be seen from consideration of the normal operation of a junction 

 transistor. 



The three principal regions of a junction transistor, the emitter, the 

 base or control, and the collector, are indicated in Fig. 1. These regions 

 are separated by transition regions in which the conductivity type 

 changes either gradually or abruptly from p-type to n-type. Roughly 

 coincident with these transition regions are the emitter and collector 

 depletion layers across which the emitter and collector voltages appear 



EMITTER BASE COLLECTOR 



11 n 



EMITTER 



DEPLETION 



LAYER 



Fig. 1 — p-n-p transistor. 



