- Models and various traps 1500 A silicon side location (hole mobility of 10-5 cm²/V.s, 2 see appendix). 1500 Agate side € 3000 Auniform distribution APPLIED VOLTAGE (Volts) Here can be seen that asymmetry factor AV ot h 7AV h+ is related to the trap distribution centroid Xp/tox, as predicted from the analytical model (section 3.9). The ratio is equal to 1/2 for a uniform distribution of traps, and becomes superior to 1 if the traps are concentrated near the gate. The situation is not SO clear in the case of the V-model, and becomes strongly dependent on the mobility of the holes, as depicted in fig. 31. 'V 3000 A uniform up=1E-5 V 3000 A Figure 31. uniform Simulation results for V- up=0.1 Models, uniform trap distribution and various V 3000 hole mobility values. uniform up=10 GATE BIAS (Volts) CASE OF AN UNHARDENED POWER MOS TRANSISTOR. As can be seen, agreement is excellent for a thickness of 90 nm and a U-shaped profile of traps with the following approximation made using three boxes: - a box near the gate (2.5 traps/cm on 45 nm) - a box mid of oxide (3.3 10 traps/cm 3 on 25 nm) - a box in the middle of the silicon interface (2.8 10 traps/cm III-51 Figure 32. Comparison of -0,14 the J-Model with experiment in -0,12 EXPERIMENTAL the case of a power MOSFET with a U-shaped distribution of J-MODEL 900 m1 traps. Vot-V spectrum: example -0,08 at 1 kilorad(SiO2). -0,06 -0,04 -0,02 GATE BIAS (V) With this triple location if traps, this case ressembles the multilayred insulator depicted above. THE FIELD COLLAPSE EFFECT Let us consider the case, illustrated schematically in fig. 33, of an oxide displaying a uniform density of hole traps, and for which electron trapping is first supposed to be negligible. Before irradiation, no trapped charge is present in the material; the internal electric field is therefore uniform in the oxide and equal to the applied field (case a). no trapped charge very low dose low dose high dose Figure 33. Illustration of the field collapse effect in an oxide (after [Boes-91]). (top) Profile of 