Modelling IQE in nanowires with surface depletion

Hi

I'm completely new to COMSOL and want to see how band bending and recombination vary with doping and optical excitation in semiconductor nanowires.

I'm trying to define a continuous surface trap density of 'rectangular' distribution but am getting the error 'Undefined variable. - Variable: comp1.semi.Eg0'.

Does anyone have any ideas what is going wrong? Also general advice/links to using the surface trap feature in COMSOL would be appreciated.

Thanks