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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.
Using a "ball and stick" figure, it is easy to visualize the atoms and the two pairs of electrons they share. A completely full valence band and a completely empty conduction band would be demonstrated with all bonds complete, as in the figure above. Each atom, "ball", would be connected to another one by two bonds, "sticks", on each side of it.
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.
The broken bond that creates the free electron leaves behind a positively charged vacancy that behaves like a new entity which we call the hole. A hole can move through the lattice as an electron does and it is also a carrier. It could be shown as a bond, "stick", breaking in one place and replacing a missing bond in another that is adjacent.
A point defect is when an atom is missing or an impurity is present in the crystal structure. Using the bonding model it is obvious since a "ball" and a few "sticks" would be missing in the ordered array of the crystal lattice.
The bonding model can also be used to show what occurs when the semiconductor is doped with donors or acceptors. When a donor is present and takes the place of an atom, the extra electron becomes a carrier as in the previous case, but in this case there isn't a broken bond left behind. If the dopant is an acceptor, it takes the place of an atom but, since it's missing an electron, it takes it from a neighboring atom, which in turn is left with a broken bond and steals an electron from another atom and so on and so forth. This situation results in a hole being the carrier.
Many differences exist between the bonding model and the energy band model. One is that the bonding model is a two dimensional model, it can show the position of the carriers in the x- and y-axis, whereas the energy band model is only one dimensional, displaying the movements in only the x-axis. Another is that the bonding model does not model the electron energies and the energy band model does.
The bonding model qualitatively describes the semiconductor lattice better, but it is impractical for quantitative analysis, since the analysis can get more complicated mathematically. Although the bonding model is rarely used, it helps us visualize what each individual electron is doing. Therefore, being able to understand the bonding model, drawing it and interpreting it, is one of the first steps needed to understand future concepts.
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