modeling SNAPBACK ESD in cadence spectra environment

hi..
every one....

i am doing my thesis on characterising and modeling of snapback esd protection...now i am facing a problem with modeling. i.e. i am not able to find the SNAPBACK ESD spice models..so, could any one help me for this modeling or can any one say how to develop a new model in order to be use in my modeling..


thankq.....