An electron energy diagram Is drawn below Black represents l

An electron energy diagram Is drawn below. Black represents levels filled with electrons and grey represents empty energy levels This material is a. When hooked to a battery, some electrons in the lower band will. When hooked to a battery, some electrons in the upper band will. i.conductor, move. move ii.semi conductor in the dark, not move, move. iii.semi conductor in the light, move. move. iv.semi-conductor in the light, not move, move. v.insulator, not move, not move. b) Explain your reasoning for your answrr abeve in a lew sentences Include in your answer the fundamental idea behind why some electrons can move when a voltage is applied and why some cannot

Solution

a) The answer is (iv)

The material is a semiconductor in the light.
When hooked to a battery some electrons in the lower band will not move.
When hooked to a battery some electrons in the upper band will move.

b) As is seen in the above figure, there are two bands with an energy gap in between. The lower band (called valence) is alomst filled with few available states. The upper band (conduction band) is partiallty filled. This represents a semiconductor which is light. The photon energy has cause some electrons from the valence band to be excited into the conduction band. For a metal, the conduction and valence band overlap. For an insulator the energy gap is huge and electrons from the valence band cannot be excited into the conduction band. Hence the energy band diagram represents a semiconductor in light. Note that in dark, the electrons do not have enough energy to cross the band gap and go into the conduction band.

When hooked to a battery the electrons in the lower band do not move. This is beacuse they are tightly bound to the nucleus of the atom.

The electrons in the conduction band however are free electrons and are free to move. Hence when hooked to a battery, the electrons drift due to the applied electric field.