As previously stated, there is an energy gap between the conduction and valence bands for semiconductors. However, valence electrons can ""absorb" heat or light
energy, to enable them to "jump up" into the conduction band. Even at typical room temperatures, many electrons will have acquired sufficient energy to jump up
into the conduction band and if the temperature increases, so will the number of electrons in the conduction band.
Each electron that moves to the conduction band, leaves behind a vacant position or hole in the valence band, (i.e. an unoccupied electron position in the covalent bond that it "escaped" from).
This process is called electron hole pair generation.
The free electrons from electron hole pairs, enable current to flow in the semiconductor when an external voltage is applied. The holes in the valence band also allow electron movement within the valence band itself and this also contributes to current flow. The net effect is that heat increases the conduction properties of a pure semiconductor, due to electron hole pair generation. The conductance, however, will still be very low, compared to a typical metal conductor. This is due to the relatively low number of electron hole pairs generated, compared to the vast number of free electrons in a conductor. For this reason, the currents produced in pure semiconductor will typically only be in the region of micro-Amps.
The movement of valence electrons due to holes in the valence band, is complicated. An electron moves into a unoccupied electron position, leaving its previous position vacant. Another electron then takes up this position, and so on. A simplified one dimensional version of this is illustrated below. Even in this simple illustration, it is easier to consider the movement of the gap (or hole), than the movement of the electrons. We can imagine this hole to be a virtual positively charged particle, (positive because it moves in the opposite direction to electrons) .
At first this may seem a strange idea, but later you will see that by considering the movement of these virtual particles, it is much easier to describe how semiconductor devices work.
When an external voltage is applied, the negatively charged electrons in the conduction band, will move towards the positive terminal and the positively charge holes in the valence band, will move towards the negative terminal.