alue for which the silicon surface potential equals 0 and B. Potential B is the bulk potential given by: III-36 where q is the elementary charge of an electron, k is the Boltzmann's constant, T is the absolute temperature, NA is the substrate doping concentration, and ni is the intrinsic carrier concentration. The gate voltage applied to reach Cfb (resp. Cmg) is the flatband voltage V fb (resp. migdap voltage Vmg). The traps at the Si/SiO2 interface are amphoteric, which means that their charge state can change depending on the value of the surface potential of silicon. The traps thus can be positive, neutral, or negative. Traps in the lower half of the Si bandgap are predominantly donor- like (i.e. they "give off" an electron when the Fermi level at the interface is below the trap energy level), and traps in the upper half of the bandgap are essentially acceptor-like (i.e. they "accept" an electron when the Fermi level at the interface is above the trap energy level). The most widely accepted assumption (well confirmed by recent results [Flee-92]) is that interface traps are approximately charge neutral at midgap [Kim-88, Wino-88, McWh-52]. With this assumption, the midgap voltage shift ( Vmg) caused by irradiation is only due to the oxide trapped charge ( Vot), while the flatband voltage shift takes into account both the oxide trapped charge and the charge trapped on interface traps between flatband and midgap. CHF (PF) Figure 20. Typical high frequency (1MHz) C-V curves of a 0 krad MOS capacitor before and after a 10 Mrad(SiO2) irradiation. The structure was biased at VG = 0 V during irradiation. VG(V) In the case of a MOS transistor, the most usual technique consists in recording the current- voltage curve (I-V curve), usually the drain current DS versus gate-to-source bias VGS, at a given drain-to-source bias VDS either in the linear region (small VDS) or in the saturation region (large VDS). The threshold voltage of a transistor is determined basically from the intercept with the voltage axis of the ID VS V