I have learned that in intrinsic semi-conductor when electrons leave the atom they leave a hole (that can move) . I also learned that when electrons leave the atom of a donor in n-type semi-conductors it doesn't create a hole and I can't understand why . From what I read , the reason for this is that these electrons don't exist in the valence band but exist in the donor level/band but I still don't understand why don't the electrons leave holes in the donor level/band. When Electrons leave the donor bands , another electron can replace it ( if the electron that left was being able to get to this donor band then any other electron should be able to get to this donor band and replace the electron that left)
Holes are vacancies in the valence band that effectively behave as positively charged particles. On the other hand, donor states do not form a band: these state usually originate from the impurity atoms randomly dispersed through the crystal, at a density much lower than that of the host atoms. Of course, an electron leaving a donor site leaves behind a positively charged state, which contributes to scattering and thus must be incorporated in any realistic calculations of conducting properties. Such a state can also occasionally trap an electron, in which case it is referred to as a trap, and at extremely low temperatures all the n-electrons end up localized in the donor states (or similarly the holes become localized in the acceptors of a p-semiconductor).
To summarize: we do not call them holes, because in this context term hole has a more specific meaning than just an empty vacancy that can trap an electron.