When an electron

Thermal collision means that the electrons have already acquired sufficient energy due to the room temperature (300 K) and it moves freely thus colliding with lattice atoms and transferring its energy to other electrons in the atom.

At room temperature, some of the electrons occupying the highest energy level in the valence band will acquire enough energy (greater than 1.1 eV) and hence they jump to the conduction band from the valence band before any collisions take place. Also some excited electrons (energy less than 1.1 eV) occupying lower energy levels in the valence band collides with other excited electrons occupying the highest energy level in the valence band. Due to this collision, there is the exchange of 0.026 eV which makes these electrons to jump from higher energy level of valence band to the conduction band. The thermal collision as well as already applied room temperature are the reason for this movement of electrons.