1278 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



and the resulting electrostatic potential keeps each type of carrier in 

 its own region. The potential appears across a thin layer separating the 

 two regions. In this depletion layer, the hole density is lower than in the 

 p-region and the electron density is lower than in the n-region, and there 

 is a net charge density. Acceptor and donor atoms are not neutralized 

 by mobile charge as they are in the p- and n-regions, but instead serve 

 to terminate the field of the electrostatic potential. Application of ex- 

 ternal potential across the junction changes the electrostatic potential, 

 and by exposing more or fewer fixed (donor and acceptor in equal num- 

 ber) charges widens or narrows the depletion layer. 



The passive capacitance of this region is simply that of a parallel 

 plate condenser having a plate spacing equal to the layer thickness. 

 Calculation of this capacitance is explained in Section 3.0, following 

 the discussion of the minority carrier diffusion admittances. 



2.5 Base Spreading Resistance* 



ImpUcit in the one-dimensional analyses described above is the as- 

 sumption that the base region is everywhere at the same potential. 

 Actually, since the emitter and collector currents are not equal, current 

 must flow through the base region parallel to the junctions. Because the 

 base region has finite, rather than zero resistivity, this current produces 

 transverse voltage drop in the base region. 



It is assumed that the most important effect of these voltage drops is 

 the feedback produced between the base contact and the emitter junc- 

 tion. In consequence, each of the ohmic base resistances studied is de- 

 fined as the quotient of an average voltage between base contact and 

 emitter junction divided by the current producing it. The need for 

 defining more than one feedback base spreading resistance results from 

 the fact that the base current has two principal ac components. One of 

 these is the difference between the emitter and collector minority carrier 

 diffusion currents. The other is the collector depletion layer capacitance 

 current. The feedback effects of these two currents on the emitter jimc- 

 tion are the same only when the flow paths of the two currents through 

 the base region are the same. Consequently, the representation of base 

 resistance effects is somewhat more complicated in transistors where the 

 flow paths differ than in those where they are identical or nearly so. 



* The majority carrier resistance of the base region for base current flow- 

 parallel to the junctions. The word "spreading" was suggested by the base con- 

 tact geometry of Fig. 2(a) and readily distinguishes this resistance from the 

 "base resistance" of the familiar tee network, which was long believed to be 

 identical with it. It is not. See Sections 5.2 and 5.3. 



