the trapped charge density. (bottom) Profile of the internal electric field. Cases a), b), c), d) are described in the text. At the beginning of irradiation, a positive trapped charge is created uniformly in the bulk (case b). The electric field induced by the positive trapped charge tends to reduce the applied field at the anode, and to increase it at the cathode. The internal electric field, initially uniform in the oxide (no trapped charge), varies now linearly in the oxide, from anode to cathode (case b). When the trapped charge ( i.e. qp,(x)dx) becomes sufficient, it cancels the action of the field at the anode (case c). By assuming that the charge is uniformly trapped, we can calculate the value of the trapped charge density (Q+) for which the field at the anode becomes nil: it is that density which produces a voltage shift equal to the applied voltage. It yields: III-52 (85) - and thus (86) charges.cm3/Volt/(100nm)2 If we assume that this charge is spread nearly uniformly in the oxide, the equivalent surface density of trapped charge is: Qtot= tox.Q+ = 4.2 10 11/Volt/100 nm, which is a very usual value encountered in oxides. Therefore, the field collapse is a rather usual situation, in oxides of thickness above 100 nm. For this trapped charge density, the separation of electron-hole pairs is small in the anode region since the electric field there is nil. On the other hand the field is increased at the cathode, and the generated electrons, separated in this strong field region, are swept to the low field region, where they recombine with the existing trapped holes. The space charge in this region then tends to disappear, which leads to an extension of the zero field region (cases C and d). Let us now examine what are the consequences of the internal field collapse effect described above on the characteristic voltage shifts obtained under positive and negative bias. 4.4.1. CONSEQUENCES IN THE CASE OF AN IRRADIATION UNDER POSITIVE GATE BIAS When a positive bias is applied on the gate of a MOS structure, this