GS curve in the linear region, or of the (ID) 1/2 VS VGS curve in the saturation region. Another method, which is similar to that used for capacitor studies, is to determine the gate voltage for which the surface potential of the silicon substrate equals 2 B, which corresponds to the onset of strong inversion [McWh-86]. Figures 21 and 22 show typical I-V curves obtained on an n-MOS transistor, before and after irradiation. On fig. 22, the position of the "threshold current" and "midgap current" is defined as those values of IDS for which the surface potential of silicon equals 2 B and B respectively. The amplitudes of the threshold and midgap voltage shifts caused by irradiation are also shown in fig. 22. III-37 The calculated "midgap current" is usually very low, thus the midgap voltage is obtained by extrapolating the I-V curves to the low current level required, and by determining the voltage corresponding to that "midgap current". Except for the extrapolation step, this method is very similar to the high-frequency C-V method. Using the same assumption, Vot is equal to the midgap voltage shift ( Vmg), whereas the threshold voltage shift ( Vth) is equal to the sum of Vot and Vit. Consequently, the voltage shift due solely to the interface trapped charge, can be determined by the stretch-out of the I-V curve (i.e. by difference Vth Vmg). Ids (mA) Post-rad Pre-rad MOS P MOS N VGS (V) Figure 21. Drift transistor Figure 22. Typical sub-threshold I-V curves of characteristics in the case of only the an n-MOS transistor revealing the trapping of positive charge in the contribution of interface states. oxide. Other techniques have been developed to determine V ot ot and Vit, especially the dual-transistor techniques, combining threshold voltage measurements with either mobility measurements or charge pumping techniques. The reader is referred for instance to [Wino-92] and [Autr-99] for more details. 3.12.2. BEHAVIOR IN THE LOW DOSE REGIME After a low dose irradiation of a silicon oxide, a net positive charge is create