The background carrier concentration (i.e. carrier concentration in nominally undoped layers) significantly depends on growth conditions. Thus, this effect can not be considered automatically.
However, the user can manually specify non-zero concentration of donors in nominally undoped layers. Let us assume that the structure contains undoped GaN layer, and user knows that in thick undoped GaN layers the electron concentration is of 1e16. Then the user can specify the donor concentration of ~2e16 (the value to be fitted to provide electron concentration of 1e16 in thick layers).
