Question

(a) Sketch the bond model and (b) the energy band model illustrating a broken Si-Si bond for each model. Label Ec and Ev for the energy band model.

Answer 1

a)

b)

shows a two-dimensional representation of a silicon crystal. We have to show it in two dimensions as that is what paper and chalkboards allow. The circles are silicon atoms and the lines represent valence bonds formed by valence electrons. Note that each atom has eight lines emanating from it, representing the eight shared electrons that hold the crystal together.

The bonding model is mostly used to give a basic idea of what the individual electrons are doing in the semiconductor. It takes advantage of the fact that the structure of a polycrystalline or crystalline material is uniform and virtually identical throughout. It demonstrates the actions of an electron with only a few samples, very much like looking at it through a microscope.

When an electron gains enough energy, either through thermal vibrations or some other mechanism, it breaks free from its native atom and can move around, thus transporting its charge within the crystal lattice. This free electron is shown as a broken bond and/or a negative charge leaving the atom, and this identifies it as a carrier of charge.
{{{ Fig. 2 Bonding model diagram showing bond broken by thermal energy}}}

The broken bond that creates the free electron leaves behind a positively charged vacancy that behaves like a new entity that we call the hole. A hole can move through the lattice as an electron does and it is also a carrier