Base spreading resistance of square-emitter transistors and its dependence on current crowding

The internal DC and small-signal low-frequency base-spreading resistances of a square-emitter transistor have been calculated analytically as a function of base current. Simplifying assumptions result in a nonlinear differential equation with a simple explicit solution. The validity of the assumptions has been checked by exact numerical device simulation. The error of the analytical approximation has a maximum of only 11% at negligible current crowding (r/sub Bi/-r/sub Bi,0/=r/sub s//32 instead of the exact value r/sub s//28.6). In the total operating range of interest, the error of the analytical expression for r/sub Bi/ can be dropped to negligible values if the exact value of r/sub Bi,0/ is inserted.<
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