he charges present. The actual charge quantity and, more important, the actual charge profile depends not only on the trap profile, but also on the evolution of the internal field when irradiation proceeds. This is why a comprehensive understanding of these kinds of oxides must use two sources: - The dependence of the image charge on the dose level and the irradiation bias (in the following, such a dependence with bias will be called "bias spectrum"). The calculation of the electrical field present from minimal hypotheses, compatible with the experimental spectrum and fitting dose dependence. In other words, the method for modeling the oxide trapping properties consists of heuristically resolving the inverse problem of determining a trap spatial profile from spectra of charge versus bias obtained at various doses. For this to occur, one must have in hand a simple, flexible tool to simulate charge response at any bias and any dose, given a population of traps characterized by their spatial distribution and cross-section. For this to happen, we present here the CEA- TRAPPOXR code version 4, whose aims are to exercise the oxide-trapped charge ( Vot) response of devices under the assumption of 1-dimensional finite-difference treatment, but with the advantage of a fast treatment and user-friendliness provided by the principal mathematics platforms of the moment (MatlabTM, PV-Wave™ or run-time on personal or portable computer [Lera-99b]. 4.2.5. THE EFFECTS OF GATE BIAS: THE CASES OF TRAPPING IN VOLUME AND AT INTERFACE In the following, we take the example of a Field Oxide transistor of 300 nm thickness. The voltage shift in Volt/kilorad can be seen in fig. 30 and 31 for the two models, different trap locations and variation of the hole mobility. One can see the difficulty of the V-Model to account for the asymmetry of the bias spectrum for the uniform distribution of traps. Hole mobility must be taken equal to electron mobility to show asymmetry versus bias. III-50 300 A silicon side Figure 30. Simulation results for J