 levels of irradiation, there is the loss of functionality of the circuit which contains the transistor. At the level of integrated circuit, these effects provoke loss of performance, increase of the power consumption, then circuit failure. This increase of leakage of various kinds that happens equally in other types of technology as CMOS, leads notably to increases of power consumption, contributes to drops of the bipolar transistor gain, and increases of the noise and the offsets of linear circuits. THE BASIC PICTURE FOR HOLE AND ELECTRON TRAPPING Although the sketches of mechanisms that occur from the initial interaction until the "permanent trapping" are numerous, complex and not fully understood up to now, it can be crudely described in a few words as follows: if a transistor is exposed to high energy ionizing irradiation, electron- hole pairs are created uniformly throughout the oxide. Immediately after their electrons X-Rays generation, electrons drift rapidly out of the oxide toward the negative electrode (within picoseconds) and electron holes drift toward the other traps electrode depending on the electric field direction (fig. Recombinatio 9). However, even before Charged Traps electrons leave the oxide, traps Neutral Trap some fraction will recombine holes with holes. The fraction of holes that survives initial Electric Field(V>0) recombination available for either trapping escaping toward Figure 9. Illustration of charge transport and trapping in the electrodes. MOS structure for positive gate bias [Herv-93]. III-12 3.3. CHARGE CONTAINED IN THE INSULATOR AND THRESHOLD VOLTAGE SHIFT OF MOS TRANSISTOR For a standard thermal oxide, the drift of the threshold voltage is related to the density of charge present in the insulator by: 0 and tox the abscissa of gate and silicon interface, respectively )=q[p(x)-n(x)] the net volume charge (charge density) present in the oxide Cox the the gate oxide capacitance per unit area The net trapped charge density resulting from the irradiation can have many causes: (i)