 to the silicon bulk. The most interesting case occurs when the transistors are biased at or above threshold voltage. In this case, the interface states are filled in by carriers belonging to silicon, as depicted in the figure 66 below. During the functioning of the MOS transistor in conduction (beyond the threshold voltage), these states are charged negatively in the case of the NMOS with electron carriers channel, and positively in the case of the PMOS with hole channel. MOS N MOS P @@@@@@eeee @@0@@@@000 AVIT>0 AVIT<0 Figure 66. Second component VIT of the drift of the due threshold voltage to the presence of interface states. CONSEQUENCE FOR MOBILITY REDUCTION IN SILICON It has been recognized that mobility can be strongly affected by interface states. Let us recall that a direct consequence is the decrease of the current drive capability of the transistors. This effect is very detrimental to Power MOS transistors, as for instance, the ON resistance (RDSon) is affected. [Gall-84] developped a simple formula based on scattering of carriers in silicon by the surface charges [Sun-80], firstly depending on Qit, always proved valid since then [Zupa-93]. This model has been then extended to Qot by simply adding a supplementary term: (109) it it ots with: 1) it and ot having the dimension of a cross-section. 2) Qit the charge of the interface states (closely related at threshold voltage to the average Dit the interface state density by: Qit = Vit /Cox Dit - B , where B is the Fermi energy). Qots the portion of the oxide-trapped charge residing close to the Si/SiO2 interface. III-77 it is nearly identical for N and P channels, for instance, it (0.8+0.2) 10-1 cm2 for NMOS [Gall-84] and (0.9+0.2) 10-1 cm2 for PMOS [Gall-85]. These values compare well with [Sext- 85]), determined as (0.7+0.13) 10-12 cm². 7.3. INTERFACE STATE BUILD-UP Interface states generally manifest themselves about 100s after an instantaneous irradiation but some can appear only after 105 to 106 S (for a review, [Schw-94] can be referred to). Several