anged) and the analytical resolution gives [Pail-99]: Y(E) tox2 D The net total shift obtained for a positive applied field is then simply the sum of the contributions due to negative and positive trapped charges. We then obtain (with the mean free path for electrons): Vot,e+h tox 2 III-45 and symmetrically for negative bias: Vot,eth = Y(E) tox 2 We therefore come to a system of two non-linear equations with two variables, which must be solved to extract the values of n and p from experimental data. (In the literature, this type of method has been first applied to photocurrents [Bene-87, Penn-90, Penn-92, Boes-94]. We here consider a similar approach, based on voltage shifts). The main disadvantage of such models is that it only gives access to values of the tNT products. It is not possible to assess independently the capture cross-section and the trap density. For that purpose, it is necessary to use a simpler model, based on the following idea: although trapped charges are distributed in the bulk, what we experimentally measure is the net equivalent charge, projected at the Si/SiO2 interface. It is thus possible to assimilate this equivalent charge to a charge located at the Si/SiO2 interface, and to use a simplified model, presented in the following paragraph. 3.15.2. CASE OF INTERFACIAL DISTRIBUTION OF TRAPS Electron trapping in oxides is only revealed in the case of a high dose irradiation when a large negative bias is applied. Due to large discrepancies in the capture cross sections of holes and electrons, the corresponding net voltage shift can be separated into two exponential components given by (67). Each one can be fitted with its set of trap-related parameters. III-46 Figure 29. Oxide trapped charge V ot (Volts) contribution Vot as a function of dose for -1 MV/cm back gate applied field [Pail-95b]. The symbols refer to different SIMOX oxide variants: Thin oxide, Medium I, Medium II, Standard. E = -1MV/cm The solid lines correspond to the bi- exponential model fit. Dose (krad) Examples of such a trap p