1286 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



the sum of the equilibrium barrier potential and whatever part of an 

 applied external potential is not lost in IR drops in the p and n regions. 

 For reverse bias conditions, the sum of the equilibrium potential and 

 the entire applied potential is almost always a satisfactory approxima- 

 tion, but for equilibrium and forward bias conditions, the barrier po- 

 tential may be computed more easily from the minority carrier den- 

 sities immediately adjacent to the depletion layer. 



The equilibrium barrier potential may be calculated by 



„ kT . Ui Pi 



Up p. 



in which Un , Pp and Ui , pi are, respectively, equihbrium and intrinsic 

 carrier densities in the two regions. The total depletion region potential 

 for a forward biased emitter junction may be calculated by the same 

 expression but the hole density in the base region side of the emitter 

 depletion layer is not the equihbrium value pn but rather 



_ WqJj 



which is simply a modified form of the equation for diffusion current. 

 Because of the large difference in the voltages across the barriers, emitter 

 depletion region capacitance per unit area is ordinarily 2-20 times larger 

 than the collector depletion region capacitance per unit area. It should 

 not be assumed that emitter depletion region capacitance is the more 

 important. It contributes but a small fraction of the emitter admittance, 

 while the collector depletion layer capacitance contributes greatly to 

 collector admittance. 



S.Jf- Summary 



A one-dimensional study of the small signal transmission properties of 

 the junction transistor shows two terminal-pair short circuit admittances 

 which are closely proportional to the dc bias currents. The input and 

 output admittances of this set are shunted by depletion layer capaci- 

 tances which are essentially passive circuit elements. A major feature 

 of the diffusion current admittances is the presence of a single frequency 

 determining factor, the alpha cutoff frequency. Comparison of the dif- 

 fusion or storage capacitances with the depletion region capacitances 

 shows that the collector depletion region capacitance is usually much 

 larger than its storage capacitance, while at the emitter the reverse is 

 true. This results from the fact that the output and feedback minority 



