I'm not an expert in chemistry but I hope it to be a straight forward question.
The situation is, as shown in the figure below, I'm trying to etch a semiconductor in KOH solution (4M concentration) at room temperature. There is a negative sheet charge with a density of P (C/cm2) located at the bottom surface which repels the OH- from accessing the semiconductor. However, the etching can progress along the sidewall until the distance (d) is too close and the diffusion force and electric field reach an equilibrium. I'd like to know how to calculate this distance d. Thanks in advance.