 of the experiment, which is certainly an oversimplification. The continuity equations for trapped electrons and trapped holes can then be written: (28) jn(x,t) [NTN(x)-nt(x,t)] - pr(E) nt(x,t) (29) jp(x,t) [NTp(x)-p(x,t)] - nr(E)_jn(x,t) p,(x,t) The first term of each equation corresponds to the trapping of free carriers. This mechanism depends both on the number of "candidates" (brought by the current), and on the number of available trapping sites (e.g. NTN - nt). The second term accounts for the recombination of already trapped charges with free carriers of opposite sign. The trapping of a free charge on a trapped carrier of opposite sign does not a priori depend on the direction of its displacement. Therefore, only the absolute values of each current are taken into account. In the presence of electric field (coming from the space-charge + from the applied voltage), one expects a cross-section variation in Eox [Ning-76], [Kran-87], [Esco-95a]. 3.7.4. EQUATION RELATIVE TO THE ELECTRIC FIELD In order to simplify the equations, the internal electric field in the oxide will be written E, although it should more correctly be written E(x,t) since it depends both on the space coordinate and on time. Indeed, E does evolve with the build-up of the trapped charge, both in time and space. The evolution of the field is obtained from the Poisson equation, which for this one- III-25 dimensional model yields: where OX represents the dielectric permittivity of silica. This equation links the local concentrations of trapped carriers to the local field value E(x,t). Free carriers are not taken into account because during irradiation, their density rapidly becoming negligible compared with the trapped charge density. 3.7.5. EQUATIONS RELATIVE TO THE CURRENTS The expressions of the electron and hole currents must now be defined. Each current density of particles can be written in its very general form, as the sum of a drift term and of a diffusion term. It follows: (31) jn(x,t)=-n(x,t)nE(x,t)-I = - n(x,t) (32) i - where Dn and D