What are semiconductors?
In short, semiconductors are materials whose electrical properties lie somewhere between those of conductors and insulators. At low temperatures, semiconductors resist the flow of current, thus behaving like insulators. At high temperatures, however, semiconductors behave more like conductors, allowing current to flow. Silicon is the semiconductor material that is most commonly used in the production of electronics, largely due to its natural abundance. To explain the general properties of semiconductors, the silicon crystal is taken under consideration.
The Silicon Crystal Structure
Solid state electronics arises from the unique properties of silicon and germanium, each of which has four valence electrons and which form crystal lattices in which substituted atoms (dopants) can dramatically change the electrical properties. Silicon crystallizes in the same pattern as diamond, in two interpenetrating face centered cubic primitive lattices.
Figure 1. Crystalline Silicon
The lines between silicon atoms in the lattice illustration indicate nearest neighbor bonds. The cube side length for silicon is 0.543 nm. Germanium has the same diamond structure with a cell dimension of 0.566 nm.
Every silicon atom has four neighbouring atoms, each containing 4 electrons in its outermost shell. The bonding of the silicon atoms is estalished by the pairing of electrons, forming covalent bonds. All the electrons in the outer shell are used in bonding so that no free charge carriers are available.
Figure 2. Simple diagram of the silicon outermost shell
Although all the electrons are tightly bound together, an electron can be set free if energy such as heat is applied to the silicon crystal. In this case, electrons will become free to move, turning the silicon into a conductor. Once this electron moves, a hole is left in its place. This hole can also "move", as bound electrons subsequently exchange its position, and the hole thus has the characteristics of a positive charge. The creation of free electrons and holes is refered to as charge carrier generation.
Figure 3. Charge carrier generation in silicon
If a free electron comes into contact with a hole, the electron 'falls' into the hole. Both the electron and the hole diappear, causing a release of energy. This process is refered to as (charge carrier) recombination.
Figure 4. Charge carrier recombination
The number of charge carriers available for conduction increases with temperature since the increase in energy allows more electrons to break free from the silicon atoms. The amount of available charge carriers per volume of material is refered to as the intrinsic carrier concentration: where n is the concentration of conducting electrons, p is the concentration of holes, and ni is the intrinsic carrier concentration. The intrinsic carrier concentration is dependent on temperature and type of material.
For silicon ni = 1.08 . 1010 cm-3 (at room temperature)
Electrons and Holes in an electric field
In the presence of an electric field, the charge carriers available for conduction (excited electrons and holes) are forced to move, thus resulting in electric current. The electrons and holes move in opposite directions.
Figure 5. Electrons and holes move in different directions
The electrical properties of semiconductors can be modified by the introduction of impurity atoms into the crystal lattice, a process know as doping. The impurity atoms, dopants, contain 1 more (N-type doping) or one less (P-type doping) electron in their outer shell than a silicon atom. The result in both cases is an increase in carrier concentration, thus increasing the conductivity of the semiconductor material. The versatility of doped semiconducter materials is why semiconductors form the foundation of modern electronics.
Other Semiconductor Materials
Apart from silicon, there are many other materials that have semiconductor properties. Some of these materials include:
- Cuprous Oxide (Cu2O) : Used for low voltages. This material has been put in industrial use since 1925.
- Selenium: Used to make rectifiers
- Germanium: Can only be used for temperatures of up to 90° C
- Gallium Arsenide (GaAs): Used for very fast components, of up to 300V
- Silicon Carbide (SiC): very hard crystals that are difficult to produce and handle, can be used at high temperatures, high voltage, and high frequencies. Some argue that this is the material of the future.
- Gallium nitride (GaN): Very similar to Silicon Carbide
- Diamond (C): All the properties of diamond are excellent, but its commercial use is not foreseeable.
- Indium Phosphide (InP), Indium Arsenide (InAs), Cadmium Sulfide (CdS), Copper Sulfide (CuS), Copper Indium Diselenide (CuinSe2 ): normally used for optoelectronics.
Organic Semiconductors are also going to gain importance in the future.
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