One of the most difficult and misunderstood concepts in the ideas to implementation section of the current NSW HSC Physics syllabus is band theory. Listed below are some tutorial points on this concept.
- Bands are not the same as the orbits of electrons around a nucleus (these are called shells).
- Bands refer to an entire material not a particular atom.
- The valence band is the set of possible energy states an electron can have if it is bonded to an atom. In a full valence band conduction is not possible as there are no states available for an electron to move to (no net flow of electrons).
- The conduction band is the set of possible energy states that an electron can have if it is free of an atom. In a full conduction band conduction is not possible.
- In a conductor at room temperature there is a partial overlap of the valence and conduction bands. The valence band is full and the conduction band is partially filled. Electrons can be moved easily by an applied electric field and enter nearby unoccupied states in the conduction band. In a semiconductor at room temperature there is a small energy gap between the valence and conduction bands. The valence band has some unoccupied states and some electrons occupy states in the conduction band. An electron in the valence band can gain thermal energy and enter the conduction band creating a hole in the valence band that behaves like a positive charge carrier. In an insulator there is a large energy difference between the valence and conduction bands. The valence band is full and the conduction band is empty.