ng irradiation two charge trapping mechanisms can take place in the oxide for each type of carrier. The continuity equations (for holes in the valence band and for electrons in the conduction band of silica) can thus be written by taking into account the generation term, the flux gradient and both trapping phenomena. Using a one-dimensional formulation, and noting simply p the hole density at interest for this deep trapping model, we get: (24) nt(E) jn(x,t) [NTN(x)-n(x,t)] nr(E) jn(x,t) pt(x,t) where: n(x,t) and p(x,t) are respectively the density of free electrons in the conduction band and that of free holes in the valence band, go is the density of electron-hole pairs generated per rad(SiO2), represents the dose rate [rad(SiO2).s`1], Y(E) is the probability of escaping initial recombination (also called the yield function), nt(x,t) and pt(x,t) are respectively the density of trapped electrons and trapped holes, t designates the time evolved since the beginning of irradiation, and q represents the charge of the electron. Note that current densities are defined III-24 here as currents of particles (in cm 2.su and not currents of charges (in A.cm-2. By replacing the value obtained for Po in the steady state, and provided that E < tox/upo p0 = 106 V cm-1 for tox=10 nm, we have: So, under these assumptions only, and for X > p0 E, the equation of hole trapping simplifies and becomes apparently similar to the equation for electrons: (27) - pt(E) jp(x,t) [NTp(x)-p,(x,t)] - pr(E) jp(x,t) nt(x,t) The only difference lies in the equation for currents, in which the mobility (and the diffusivity) is defined by a specific law, strongly depending on temperature as recalled before. 3.7.3. EQUATIONS RELATIVE TO TRAPPED CARRIERS To be able to derive equations for trapped carriers, we must first assume that there only exists one trapping level for holes and one level for electrons in the bandgap of the insulator, and that these traps are deep enough SO that we can neglect carrier detrapping during irradiation, at the temperature