 electrode corresponds to the anode, and the cathode corresponds to the Si/SiO2 interface. In this case, the field collapse occurs under the gate, and the region of strong recombination extends from the gate into the oxide bulk. The trapped charge is therefore found mostly in that region (of thickness W) where the field is not nil, as illustrated in fig. 34. Figure 34. Illustration of the field collapse effect under positive bias. Schematic representation of: left) the trapped charge density right) its contribution to the internal electric field. When the absorbed dose increases, while VG remains unchanged, the thickness of this trapping region decreases. This is because Q+ increases with dose, and thus its associated internal electric field contribution increases also. Since the integration of the net internal electric field must remain equal to VG , W must decrease accordingly. The region where the field is not nil has therefore a thickness of: III-53 (87) w - 2 ma Val When the trapped charge Q+ increases, W decreases. The calculation of the obtained voltage shift yields: (88) - After integrating, and inserting W, we obtain: (89) - Ynt.sat= This model predicts that for an irradiation under positive field, the voltage shift increases with increasing dose, beyond the applied bias, but in a sub-linear manner. 4.4.2. CONSEQUENCES IN THE CASE OF AN IRRADIATION UNDER NEGATIVE GATE BIAS When a negative bias is applied on the gate electrode of a MOS structure, the anode corresponds to the surface of the silicon, and the cathode is the gate electrode. Figure 35. Illustration of the field collapse effect under negative bias [Boes-91]. Schematic representation of: left) the trapped charge density right) its contribution to the internal electric field . It follows that the internal field collapse effect now occurs near the SiO2/Si interface, and the region where the field is virtually nil extends from the interface into the oxide bulk. The trapped charge distribution and its contribution to the internal electric field are 