Above, you see the energy level diagram for the third type of material, a semiconductor. (We use silicon; purists may rightly complain that pure
silicon is not truly a semiconductor, but we hope they will let us slide for a moment.) Its properties lie between those of insulators and
conductors. As the diagram suggests, it takes less energy than with an insulator for an electron to move from the valence to the conducting
band. At room temperature, it takes 1.12 eV.
To contrast the differences, the diagram below shows energy band diagrams for the three types of material side by side by side. You can see
how the energy gaps differ by the type of material.
Semiconductor
Energy bands
·Medium gap between bandsThree types of material
Contrast between energy gaps
36.12 - Mobile electrons and holes
A hole: is caused by the departure of an
electron and is positive.
In a physics topic like direct current circuits, an electric current is described as a flow of
electrons, since it is moving electrons that are the charge carriers in a copper wire.
However, semiconductor engineers think about the flow of current slightly differently.
We will use a silicon atom or two to explain.
Here is a traditional discussion of covalent bonds as taught in first-year chemistry: A
single silicon atom has four electrons in its valence band, but it “wants” to have eight
there. When it is part of a solid piece of silicon, it forms covalent bonds with neighboring
atoms. It “shares” electrons with neighboring silicon atoms, and by sharing, it fills its
valence band with eight electrons. You see a symmetrical, “satisfied” silicon atom in the
diagram of Concept 1.
Now let us consider what happens when an electron makes a jump from the valence
band to the conduction band. It might do so as the silicon increases in temperature and
the internal energy of the material increases.
When an electron makes this jump, a silicon atom has lost its valence electron. The
valence band now has an opening, called a hole. The diagram in Concept 2 shows the
“missing” electron as a hole. A hole is positive since it is caused by the departure of an
electron. The number of protons in the region now exceeds the number of electrons by
one.
Holes are crucial in semiconductors. Why? Because they provide a place for electrons
to flow to. They provide natural “landing spots” for mobile electrons.
However, there is more to it than that. When considering semiconductors, the flow of
electrons is crucial, as electrons constitute the electrical current in a conductor.
However, equally important and real in the eyes of semiconductor engineers is the flow
of holes. In the animation in Concept 3, we illustrate some moving electrons and the
resultant motion of a hole. Refresh your browser screen if you did not see this animation
yet and wish to do so.
You may consider hole movement as being akin to a bubble moving through a fluid. A
Silicon in equilibrium
Valence band is filled
Silicon with an electron in the
conduction band
Valence band is “missing” an electron
A hole is created
·Holes are positive