The ƒT of bipolar transistors with thin lightly doped bases

The ?_T of a bipolar transistor is normally increased by decreasing the base doping density and width. At very small widths and densities this improvement may not occur because injection modulation of the base charge at normal operating currents then increases emitter junction resistance, r_e, and so makes the emitter time constant, r_e. C_te, larger. Simple expressions are given for r_e and ?_T in this situation and the theory is compared with measurements on sets of transistors in which ?_T is found to be largely independent of impurity charge in the base.