ransistor tox(x) Silicon Figure 80. 2D-representation into a continuum of transistors [Bris-96]. The simulated structure is decomposed into elementary transistors (ashed region), delimited by the field lines. Here, tox(x) is computed from the elementary oxide capacitance Cox(x), and can be interpreted as the length of the field lines in the bird's beak of a non-irradiated device (fig. 80). The function tox(x) is then related to the bird's beak shape. Cox(x) is calculated by the 2D code with the Gauss' along the field lines [Esco-95b]. III-89 Vox(x) is the oxide voltage drop. Qsc(x) is the net charge per unit area in the silicon layer. Qsc(x) is given by: (113) = where u is the distance along the field line at the abscissa X doping at the abscissa X. Using these method and a 2D-version of the TRAPPOXR code (version 2.0 issued with University of Montpellier under the name "ACCESS" [Esco-95a], [Esco-95b]), we obtain the threshold voltage at the abscissa X, and then, integrating over all the elementary transistors, a good description of the edge leakage current can be obtained [Bris-96]: 50 krad 40 krad 30 krad Simulation 10-10 Experiment 20 krad 10-11 10 krad 10-12 10-13 Figure 81. Variations of IDS as a function of VGS. The device has been irradiated at VGs=5 V. Additionally, this simulation has obtained new insights. As trapping depends both on the non- recombination yield Y(E) and on the oxide thickness, a maximum is reached at a point where the electric field lines are the longest (fig. 83). It finally gives access to the lateral threshold voltage versus the position on the edge (fig. 84). III-90 Bird's beak region Gate oxide Thick oxide X (um) Figure 82. Simulated Qot(x) at 50 krad with VGS=5 V during irradiation [Briss-96]. Bird's beak region Gate oxide Thick oxide X (um) Figure 83. Variation of Y(E) and tox(x) along the Si / SiO2 interface [Bris-96]. Bird's beak region 0 krad 10 krad 20 krad 30 krad 40 krad 50 krad Gate oxide Field oxide X (um) Figure 84. Evolution of the threshold voltage along the interface