- Semiconductors act as insulators at low temperatures and conductors at higher temperatures.
- Conduction occurs at higher temperature because the electrons surrounding the semiconductor atoms can break away from their covalent bond and move freely about the lattice
- The conductive property of semiconductors forms the basis for understanding how we can use these materials in electrical devices.
The bond structure of a semiconductor determines the material properties of a semiconductor. One key effect are the energy levels which the electrons can occupy and how they move about the crystal lattice. The electrons in the covalent bond formed between each of the atoms in the lattice structure are held in place by this bond and hence they are localized to the region surrounding the atom. These bonded electrons cannot move or change energy, and thus are not considered "free" and cannot participate in current flow, absorption, or other physical processes of interest in solar cells. However, only at absolute zero are all electrons in this "stuck," bonded arrangement. At elevated temperatures, especially at the temperatures where solar cells operate, electrons can gain enough energy to escape from their bonds. When this happens, the electrons are free to move about the crystal lattice and participate in conduction. At room temperature, a semiconductor has enough free electrons to allow it to conduct current. At or close to absolute zero a semiconductor behaves like an insulator.
When an electron gains enough energy to participate in conduction (is "free"), it is at a high energy state. When the electron is bound, and thus cannot participate in conduction, the electron is at a low energy state. Therefore, the presence of the bond between the two atoms introduces two distinct energy states for the electrons. The electron cannot attain energy values intermediate to these two levels; it is either at a low energy position in the bond, or it has gained enough energy to break free and therefore has a certain minimum energy. This minimum energy is called the "band gap" of a semiconductor. The number and energy of these free electrons, those electrons participating in conduction, is basic to the operation of electronic devices.
The space left behind by the electrons allows a covalent bond to move from one electron to another, thus appearing to be a positive charge moving through the crystal lattice. This empty space is commonly called a "hole", and is similar to an electron, but with a positive charge.
The most important parameters of a semiconductor material for solar cell operation are:
- the band gap;
- the number of free carriers (electrons or holes) available for conduction; and
- the "generation" and recombination of free carriers (electrons or holes) in response to light shining on the material.
More detail on these properties is given in the following pages.