Improved impact-ionization modelling and validation with pn-junction diodes

Impact-ionization at low and high electric field as well as the temperature dependence has to be modeled well in order to improve the predictive capability of TCAD tools. The high field behavior is of particular interest for ESD protection devices with low breakdown voltages which are used to protect ICs made with modern technologies. In this paper, the model for estimating the impact-ionization proposed by Valdinoci with the parameters of Reggiani has been examined with diodes of various breakdown voltages. It was found that the experimental breakdown voltages of the diodes are underestimated using that model. The cause was traced back to the overestimation of the electron impact-ionization coefficient at high electric fields. By adjusting the model parameters to the experiments of Van Overstraeten and Grant, who measured the impact-ionization coefficient in silicon for fields up to 7.7 × 10⁵ V/cm, we extend the model´s validity to high fields. With the new parameter set, a much better agreement to the measured breakdown voltages is obtained. As a check for the temperature dependence of the impact-ionization, the diodes were further investigated under 100 ns transmission line pulses (TLP). The measured high-current I-V characteristic is well reproduced by simulations using the new model, as opposed to the well-established model based on Chynoweth´s law. Both the failure level and the damage location are well predicted by the simulation.