 noted above) emitter area emitter perimeter base width Wdep surface depletion width Db base diffusion constant In this formula, S and Wdep are function of dose in a complex manner, because both the trapping properties of the base-emitter spacer insulator and the doping profile in the base are involved. For modeling this behavior, pioneering work has been done by numerically asserting the effects of the trapped charges in the depletion layer by [Schr-96] (aiming at calculating the effective s(D) [Kosi-94] and Wdep(D) as a function of Nox for a given doping profile). A second step has been to model and compute the trapped charge itself and the interface states component, which are a complex function of time and dose-rate [Grav-98]. A third effect, attributed to hydrogen release by irradiation, affecting the boron doping level, has recently been noticed and should probably be taken into account [Witz-98]. As a consequence of these recent observations, it is likely that modeling of radiation effects in base-emitter spacer oxide requires to take into account not only electron and holes, but three charge carriers: e, h, and H under the form of H+, their motion, trapping, neutralization and chemical reactions producing interface states and boron dopant passivation. Note that due to the small electric field applied by the VBE bias, most of the charge and proton motion is driven by diffusion and the effect of the fringing fields [Pers-97] established by the built-in base-emitter voltage and the depletion layer. Consequently, codes must be self-consistent with Poisson's equation. Such work is only at its beginning, and, consequently, complete modeling of effects in bipolar transistors is not sufficiently mature to be presented at the present time. Anyway, a sure thing derived from this formula is that the degradation is very sensitive to the PAe ratio, as pointed out in [Peas-74] [Mess-92]. In one hand, transistors are traditionally designed to maximize perimeter-to-area ratio, in order to reduce the base spreading resist