d in the insulator. Depending on the process used to manufacture the oxide, this trapped charge can be distributed in different ways in the material. To determine the type of charge trapping taking place, a simple method consists in measuring the trapped charge for different biases applied during irradiation. We can thus evaluate how the electric field influences charge trapping, which gives information on the position of the centroid of the net trapped charge. In the next paragraphs, several typical cases are presented. 3.12.3. CASE OF CHARGE TRAPPING AT THE S1O2/S1INTERFACE This is the simplest case, and that most often used to interpret charge trapping in thermal oxides. III-38 The traps are assumed to be all located at the SiO/Si interface. During an irradiation under a positive bias (VG > 0), the generated holes flow toward the interface and can get trapped there. This trapping gives birth to an important image charge in the semiconductor, and therefore to a significant voltage shift of the irradiated structure. N+(x) Thickness Applied electric field Figure 23. Schematic representation of the electric field dependence of the measured voltage shift, in the case of an interfacial charge trapping (left). The corresponding trap distribution (NTP(x)) and charge distribution (x) in each case (E>0 and E<0) are also shown (right). On the other hand, during an irradiation under a negative bias, holes flow toward the gate, and only a very small fraction of them can get trapped at the SiO/Si interface. In this case, the image charge in the semiconductor is reduced and leads to a far less important voltage shift. Vt (Volts) Figure 24. Measured midgap voltage shift versus gate bias applied during irradiation, of a thermal oxide at a dose of 10 krad(SiO2) [Pail-94]. VG (V) One thus obtains a completely asymmetrical Vot(E) characteristic. This asymmetrical shape is typical of a distribution of hole traps close to the Si/SiO2 interface. Figure 23 illustrates this expected behavior, as well as the trap and charge distributio