a-88, Pail95c]). As recalled in table 3, the ratio of trapped electrons to trapped holes ranges from some percent to one half [Fleet-92], table 2, adapted). However, it is likely that a coherent modeling should require the two components. These data were interpreted in terms of wet and dry process, as in fig. 11. Ratio / f (0.16 + 0.06) (0.48 + 0.09) OXIDE THICKNESS (nm) Wet Gate Dry Gate Figure 11. Trapped electrons/trapped holes ratio for various processes [Flee-92]. III-16 In some specific cases, such as oxides used as substrate insulator for Silicon-On-Insulator under negative bias during irradiation, this ratio can be superior to 1, meaning that the net charge is negative in this case [Lera-88]. However, we shall consider as a first approach valid a first sight that the net charge is globally positive, as it has been considered for 30 years from the late 50's In table 4, electron and hole densities of traps are measured instead of trapped charges (according to a method described in [Pail-95a] and [Pail-99] and summarized tn section 3.14). As will be seen later, the two items generally differ because a quantity depicting capacity of traps to be filled (NTN and NTP) is not generally the same as another measuring the amount of traps filled (Qe or Qh). Type of oxide Thickness Range of dose Range of the NTH/NTP and Reference electric field method used SIMOX 380 nm 1 Mrad + and - 0.2 to 0.8 [Pail-93] 1 MV/cm Vt-Vg SIMOX 80 nm 1 Mrad + and - [Pail-95a] 1 MV/cm Vt-Vg UNIBOND TM 400 nm 1 Mrad + and - [Pail-95a] 1 MV/cm Vt-Vg Thermal 400 nm 1 Mrad + and - [Pail-93] annealed 1320°C 1 MV/cm Vt-Vg Table 4. Ratio of electrons trap NTN to hole traps NTP densities as measured under various circumstances. It can be seen that holes, although generally the most numerous, never come alone. In this course, this will be the guideline for further understanding. FIRST STEP: CREATION AND SEPARATION OF ELECTRON-HOLE PAIRS IN SIO2 Globally, electron and holes are created in equal quantities at a rate: with: and =2.27 g.cm-3. w=18 eV i