The different “conduction phases” of a semiconductor material are analogous to traffic on a road. (A) If the Fermi level is somewhere in the conduction band, then there are many states for electrons to occupy and they can flow smoothly, just as traffic flows smoothly on an empty road. (B) If all of the states in the valence band are full and the conduction band is empty, there is no room for motion, just as in a traffic jam. (C) If the Fermi level is in the conduction band, then there are just a few empty states for electrons to move into, and these empty states (or “holes”) can travel through the material carrying positive charge in the opposite direction of the electron flow, just as a gap in traffic can move backwards through a line of cars on a busy road.The different “conduction phases” of a semiconductor material are analogous to traffic on a road. (A) If the Fermi level is somewhere in the conduction band, then there are many states for electrons to occupy and they can flow smoothly, just as traffic flows smoothly on an empty road. (B) If all of the states in the valence band are full and the conduction band is empty, there is no room for motion, just as in a traffic jam. (C) If the Fermi level is in the conduction band, then there are just a few empty states for electrons to move into, and these empty states (or “holes”) can travel through the material carrying positive charge in the opposite direction of the electron flow, just as a gap in traffic can move backwards through a line of cars on a busy road.