We adopt a self-consistent real space Kerker method to prevent the divergence from charge sloshing in the simulating transistors with realistic discrete dopants in the source and drain regions.The method achieves efficient convergence by avoiding unrealistic long range charge sloshing but keeping effects from short range charge sloshing.Numerical results show that discrete dopants in the source and drain regions could have a bigger influence on the electrical variability than the usual continuous doping without considering charge sloshing.Few discrete dopants and the narrow geometry create a situation with short range Coulomb screening and oscillations of charge density in real space.The dopants induced quasilocalized defect modes in the source region experience short range oscillations in order to reach the drain end of the device.The charging of the defect modes and the oscillations of the charge density are identified by the simulation of the electron density.