The Heaviside step-function, Θ, determines the depletion region, which is the cross-sectional area of the NW where donor ionization can take place. Discussed further at the end of this section, the depletion region denotes areas of the NW where the donor electrons have energies larger than the Fermi energy, E F , for ionization to occur (cf. Chapter 6 of Ref. ). Within HADOKEN, the number density, n D , is scaled by a typical carrier density of n D,18 = 10 18 cm -3 , allowing us to concisely express the source term, S D , in units of eV in the n_D_func.m m-file to give: