Ionic collisions are a statistical process, the ions with same initial momentum do not undergo same collision history and R_P is the depth at which most ions stop. R_P (straggle) is the standard deviation of the curve along the direction of the incident ion and R_T (lateral straggle) is the standard deviation parallel to the surface. Because of the spread of ions at right angles to its incidence, the lateral straggle is also known as transverse straggle. R_T is of importance in determining the doping distribution near the edge of a window which is cut in a mask.

The transverse straggle can be ignored if the width of the window is large compared to the junction depth. When the beam of ions hits the wafer, the concentration of ions in the substrate becomes a function of the depth of the ions. First approximation of this function is a Gaussian curve of the form