ene-86], [Shan-91]). When one comes to analytical approximations, sigmoid curves can be suggested, such as, for X and photons: Y(E)=1+12 where m and Ec are coefficient that depends on the type of radiation. Table 5 gives examples of fitting parameters. Photons Co-60 rays X rays 1.17-1.33 MeV 10 keV From [Flament95] Ec (MV/cm)=0.65 m = 0.9 From [Oldh-81/83/85] Table 5. Coefficients for the empirical function of non-recombination Y(E). More specifically to take into account a possible residue Y at zero field and temperatures above 150 K (as evoked by [Ausm-86]): III-19 or (other approximate expression): and Eo Y Y . Ec with Y is about 0.05 at room temperature [Farm-73], [Lera-89a]. 3.6. SECOND STEP: MOTION OF THE HOLES 3.6.1. BASIC OBSERVATIONS From the point of view of electron and hole processes, one can consider that, whatever the duration of irradiations, it can be decomposed into an infinite series of small quasi- instantaneous elementary irradiations. This method of the "pulsed response" allows a more didactic statement of the successive processes that follow the creation of electron-hole pairs (fig. 14). The use of <<flash X or linac irradiation machines provides conditions to measure the real impulse response of MOS and to suitably model the successive phenomena [Hugh-75a, Hugh-75b, Curt-77, Lera-85, Lera-89a, Peyr-91]. DURING DURING AND AFTER BEFORE IRRADIATION IRRADIATION +-+-+-+-+_+ ++-+-+-+-+ IONIZATION DRIFT, DIFFUSION AND TRAPPING RECOMBINATION Figure 14. Generation and the trapping of charge in the insulator of MOS structures. Contrarily to semiconductors where the free carrier mobilities of electrons and holes have similar magnitudes, they differ here by several orders of magnitude. For electrons, III-20 mobility Me of about 10 has been reported [Hugh-75] from photocurrent measurements. On the contrary, the positive charge due to holes is slowly evacuated, allowing one to define an effective mobility derived from the time-of-flight of the moving holes [Boes-75, Curt-77]: with t1/2 the time needed fo