Why electrical current is possible only with the presence of energy BANDs, rather than discrete energy levels.
Change of energy would mean change of energy level (principal quantum number), as far as I understand, electrical current is due to jump of electrons within degenerate states, that's why electrical current possible in metals and not in insulators (without changing of energy level).The electron can change its state by acquiring energy from somewhere.
An energy level that can get occupied by an electron.What is meant by the state then?
The number of electrons compared to the number of available energy levels.And what determines when the band is fully packed and when not?
The gap does not have to exist.As far as I understand, electrons can be in one of the energy bands which are separated by energy gaps (Kronig Penney model).