Stored Charge Method of Transistor Base Transit Analysis

A base layer transit time analysis has been made for high-frequency transistor base donor distributions. Transit time is defined as stored charge per unit emitter current. The emphasis on the stored charge/current ratio is particularly pertinent to high-frequency performance and facilitates qualitative analyses. The analysis applies to a p-n-p transistor in which the base donor density at the emitter (which specifies emitter breakdown voltage and emitter capacity for an alloyed emitter) and total number of donors per unit area of the base (which determines base resistance and emitter to collector punch-through voltage) are specified. It is shown that shorter transit times result with retarding fields since smaller base thicknesses are possible. It is thus shown that a built-in field is of lesser importance in determining transit time than is base thickness.