Here, N can be approximated as the difference in positively charged donor and negatively charged acceptor ions and 𝜖 is the permittivity of electric field. We also include an addition constant n0, to account for any additional local charge density that hasn't been observed in SIMS. In our case, we can assume the acceptor ion density to be negligible and take the depth profile of phosphorus and local charges as the overall charge density, given by equation .