Guard ring voltage and breakdown voltage of the reverse biased $p^+$-n junction diode with a single floating ring are extensively studied using two dimensional device simulation. A self-consistent solution of Poisson and current continuity equations with impact ionization model allows simulating of the complete reverse characteristics for planar junction devices.
It is shown that the guard ring potential is determined from the requirement of the balance between the current generated thermally and that due to thermionic emission over the barrier formed between anode and floating ring. Furthermore, the sensitivity of the guard ring potential as well as the breakdown voltage with respect to the parameter variation such as $Si-SiO_2$ interface charge $Q_{ss}$, temperature and minority carrier recombination times, are also studied. the results indicate that the breakdown voltage variation is very sensitive to $Q_{ss}$ charge variation, while rather insensitive to the variations of temperature and minority carrier recombination time. Simple explanation for this is given from the guard ring potential dependent barrier height.
We also find that the role of $Q_{ss}$ on smaller breakdown voltage is twofold. Firstly, it increases the floating guard ring potential which increases the maximum field formed between anode and guard ring. Secondly, it directly increases the maximum electric field formed near the main junction. Above results should be considered properly for the optimum design of the junction termination in power semiconductor devices.
