Electric field and collector depletion region are modulated by injected electrons when collector current density increases. Using a perturbation approximation, the collector current and base transit time models were derived in any injection region. The models were derived assuming the uniformly doped transistors, and then they were extended to the arbitrary doped transistors. The modulated electric field improves the base transit time for uniformly doped base while it degrades for Gaussian doped one. The collector depletion region is vanished when the collector current density increases, and neutral base region widening occurs. This is called Kirk effect and the related model is presented. It is also noted that the lateral voltage drop along the emitter-base junction cannot be neglected in high injection region. This induces the emitter current crowding at the emitter edge, which is also treated in this chapter.