illustrated in fig. 35. By symmetry, the cancellation of the field at the interface is obtained for the same density of trapped charge as previously, but in this case the charge is distributed over a thickness W under the gate electrode. The calculation of the associated voltage shift in this case can be written : (90) - III-54 which, after integration, and after inserting W, leads to: Vot,sat = VG This model therefore predicts that with a negative applied voltage on the gate electrode, the voltage shift associated to the oxide trapped charge saturates at a value Vot,sat = VG when dose increases. 4.4.3. CONCLUSIONS This simple model of the "field collapse effect" has the advantage of yielding correct orders of magnitude. It agrees quite well with experimental data, provided we can measure voltage shifts due solely to the charge trapped in the oxide. As a matter of fact, the charge trapped in interface states is absolutely not taken into account in this model. In the case where interface trapping is important, this model is incomplete and must be revised. 4.4.4. NUMERICAL ILLUSTRATIONS USING THREE DIFFERENT CODES A significant number of 1D codes have been developed in-house these last 20 years, mainly for research use of the individual or the team itself. A new development is the appearance of 2D and 3D codes treating the trapping in oxides, inserted as modules into existing suites like ATHENA (now ORCHID) and DESSIS. To take examples, we can use here two commercial numerical tools (code A being commercialized by ISE, and code B by SILVACO). In order to simplify comparison, we adopt a uniform profile of traps. The Field Collapse model, anticipated by [Boes-76], and extended by [Hugh-85] and [Boes-91], is globally verified whatever the J- or V-model options used. However, this study demonstrates the striking effect of Field Enhancement (F.E.) that necessarily exists along with the Field Collapse to ensure the external boundary voltage imposed (gate bias), as pointed out in [Boes-91]. Also well demonstrated is the n