 However, the existence of further donor levels is discussed, even if they are supposed to be entirely filled for every field and temperature regime, and thus to not furnish any conduction carrier (this is equivalent to state that the additional donor levels are placed well below the Fermi level).The calculation made for the trap depth reduction is a one-dimensional calculation, resulting in an overestimation of the effective barrier lowering.In fact, only in the direction of the external electric field the potential well height is lowered as much estimated accordingly to the Poole-Frenkel expression.