So far many hole trapping models have been developed to explain the experimentally observed large time constants and the bias dependence of stress and recovery [69, 40, 119, 120]. Such an approach, developed in the 1990s, is used in the model of Tewksbury which uses elastic hole trapping into preexisting traps [132]. However, the modeling by spatially distributed defect states in the oxide is only feasible in thick oxides, as in ultra-thin oxide layers elastic tunneling gives time constants in the millisecond regime [55] at the most. Moreover, elastic tunneling does not account for the anticipated temperature and bias dependence needed to explain the recent experimental findings [78]. Therefore, a new approach has to be found, i.e. a probably inelastic (thermally activated tunneling) mechanism [133, 134, 98].