r decreasing the charge to one-half its original value. This behavior has been first attributed to shallow traps situated to the vicinity of the valence band. Their apparent mobility upeff varies therefore strongly with the temperature [Srou-76], [Srou-77b], [Boes-75], [Curt-77], [Lera-85]: Upeff = (-E;/kT) In [Hugh-75b], an example of parameter set is given: Upeff = 20cm2/v.exp(-0.6gv/kT) for thick thermal SiO2 film (862 nm). III-21 3.6.2. INTERPRETATION In fact, there is a broad distribution of activation energies, and it has been shown that the shallow traps were intrinsic to SiO2 in the sense that the hole becomes self-trapped due to its own electrostatic potential on oxygen atoms constituting the network (polaron effect, [Mott-77, McLe-77]). This is probably why this behavior is SO universal among the various kinds of oxides tried, although a slight dependency of activation energies has been found to depend on process [Lera-85], with high-temperature annealed oxides having smaller t1/2, higher hole mobility and lower activation energies. This might be related to the hopping mechanism between sites, favored by denser oxides [Lera-89a]. The figure 15 summarizes the main transport and trapping mechanisms in SiO2 by showing the importance of the spatial and energy distribution of traps. DEEP HOLE TRAPPING NEAR THE Si/SiO2 INTERFACE 1.1 eV INTERFACE TRAPS RESULTING FROM ELECTRON-HOLE PAIRS INTERACTION OF HOLES GENERATED BY IONIZING RADIATION POLY-Si HOPPING TRANSPORT OF HOLES THROUGH LOCALIZED STATES IN SiO, 2 BULK Free hole self-trapping hopping transport Figure 15. Simplified illustration of transport and trapping mechanisms of holes in SiO2 [McLe-77]. A global hole movement occurs as determined by this trap-controlled mobility under the electrical field toward electrodes. This movement has been measured after irradiations delivered in pulsed mode with X-ray or electron flash machines, and modeled by describing the transport equations in the presence of traps distributed in volume and in energy [Boes-74], [Boes