Hello, I look at two books in solid state physics one is Kittel and other Mayer, as well as the internet. Unfortunately, I didn't find any separate word such as (Fermi Zone). I think this is only a level and zone is defined by Brillouin and Fermi is the level which will be define by its spin. However, it will be a good idea if you look at Introduction to Solid State Physics C.KITTEL.