Regarding the typology of traps, the Poole–Frenkel effect is described to occur for positively charged trap sites, i.e. for traps that are positive when empty and neutral when filled, in order for the electron to experience a Coulombic potential barrier due to the interaction with the positively charged trap. Donors or acceptors sites and electrons in the valence band will also exhibit the Poole–Frenkel effect as well. On the contrary, a neutral trap site, i.e. a site that is neutral when empty and charged (negatively for electrons) when filled, will not exhibit Poole–Frenkel effect. Among the others, Simmons has proposed an alternative to the classical model with shallow neutral traps and deep donors, capable to exhibit a bulk-limited conduction with a Schottky electric field dependence, even in presence of a Poole–Frenkel conduction mechanism, thus explaining the "anomalous Poole–Frenkel effect" revealed by Ta 2 O 5 and SiO films. Models there exist that consider the presence of both donor and acceptor trap sites, in a situation called compensation of traps. The model of Yeargan and Taylor, for example, extends the classical Poole–Frenkel theory including diverse degrees of compensations: when only one kind of trap is considered, the slope of the curve in a Poole–Frenkel plot reproduces that obtained from Schottky emission, in spite of the barrier lowering being twice that for Schottky effect; the slope is twice larger in presence of compensation.