Semiconductors

Gallium arsenide
Titanium dioxide
Carbon nanotubes
Graphene

Semiconductors are materials that fall between conductor and insulator. Semiconductors have band gaps and therefore are not traditional “conductors”  but the band gap is not big enough to be a proper insulator. This property is actually extremely useful and a large portion of current technology uses semiconductors.  Semiconductors, like silicon, are used in solar cells and solar panels. If an incoming ray of light, or photon, has energy higher than the band gap of silicon (1 eV) then an electron will have enough energy to move from the valence band to the conduction band.  Silicon semiconductors are also used in computers. The silicon will not conduct low level electrons but allows higher energy electrons to pass, giving the binary “1” signal.

Semiconductors can be doped, or have small amounts of impurities added, to give them specific characteristics. Most commonly, semiconductors are doped to make either negative type (n-type) or positive type (p-type) semiconductors. N-type semiconductors have excess electrons whereas p-type have a deficiency of electrons. When p-type and n-type semiconductors are placed next to each other it helps establish directed flow in a device. This is called a p-n junction. This flow is important in transistors and diodes. Most people are familiar with diodes, especially light emitting diodes or LEDs. Diodes consist of a simple p-n junction. This p-n junction allows the current to flow in one direction but not the other. A diagram of how diodes and p-n junctions function can be seen in figure 3. In solar cells, once an electron is moved to the conduction band in the absorption layer it creates a positive “hole” in the valence band and a negative charge in the conduction band. These charges need to flow somewhere or they will recombine and the photon’s energy will be wasted. P-type and n-type semiconductors are layered into solar cells to allow the electron to flow towards the p-type materials and the electron hole to flow towards the n-type material creating a current and electricity. This is a simply written but more detailed description on how semiconductors work.

http://electronics.howstuffworks.com/diode.htm

Silicon is used because the synthesis is well known and already done by a number of large factories and the materials are earth abundant. Pure silicon is made through the Czochralski process, where SiO2 or other Si sources are melted at extremely high temperatures, then a rod is dipped into the molten silicon and pulled out slowly. As the silicon cools it creates a pure single crystal of silicon that can be cut into wafers and made into microchips or solar panels.