Semiconductors are materials that can behave as drivers or as insulators depending on various factors, such as electric field, magnetic field, radiation, pressure or temperature. There are two types, intrinsic semiconductors and extrinsic semiconductors. The extrinsic semiconductors are obtained from the intrinsic ones and are essential in the electronics industry.
semiconductors
The elements that behave as semiconductors are the following:
- Cadmium (Cd)
- Aluminum (Al)
- Gallium (Ga)
- Boron (B)
- Indian (In)
- Silicon (Yes)
- Carbon (C)
- Germanium (Ge)
- phosphorus (P)
- Arsenic (As)
- Antimony (Sb)
- Selenium (Se)
- Tellurium (Te)
- Sulfur (S)
The most used is Silicon, followed by germanium and, less, sulfur. Pure crystals of these elements are considered intrinsic semiconductors. and in them a double electrical current is generated when they are subjected to an electrical differential.
In the crystal structure, atoms are linked together by covalent bonds in what is known as the valence band. Under certain circumstances, some of the electrons can absorb the energy needed to escape the valence band and move into the so-called conduction band. The following image outlines this situation in Silicon (tetravalent, each atom joins four others):
The leaving electrons leave a electron hole in the valence band, which favors electrical conduction. Free electrons also favor electrical conduction and both electrons and holes are called carriers. The energy for this to occur is different in each material. Si at room temperature requires an electrical differential of 1.12 eV. This same energy is released in the recombination process, which is the opposite process, when an electron falls from the conduction band to the valence band.
If the temperature is kept constant, there comes a time when the recombination and exit of electrons equalizes and the concentration of electrons in the conduction band (negative charges – n) equals the concentration of holes in the valence band ( positive charges – p).
In an intrinsic semiconductor, two electrical currents are generated: one by the movement of electrons in the conduction band and another by movements of electrons in the valence band that can jump to nearby holes. The current in the valence band is opposite to the electric field and of much lower intensity than the current in the conduction band.
Extrinsic semiconductors
Extrinsic semiconductors are obtained by a process known as doping and which consists of introduction of impurities (dopants) in a controlled way in intrinsic semiconductors. Depending on the dopant used, P-type semiconductors (positive) or N-type semiconductors (negative) can be obtained.
N-type extrinsic semiconductors
N-type extrinsic semiconductors are those obtained by the addition of dopants with more valencies than the starting intrinsic semiconductor. In the case of silicon, which is tetravalent, pentavalent dopants are used, for example phosphorus. In each phosphorus atom there will be one electron without forming a bond. This electron can jump into the conduction band but does not leave any holes.so these dopants are said to be electron donors and there will be more negative charges (electrons) in the conduction band than positive charges (holes) in the valence band.
The conductivity of the material is greatly increased, up to 24,100 by adding just one donor atom for every 1,000 silicon atoms. In the following image we can see a scheme of this type of semiconductors:
P-type extrinsic semiconductors
In which P-type extrinsic semiconductors, the situation is the opposite of that in N-type. Elements with fewer valences than the starting intrinsic semiconductors are used. In the case of Silicon, trivalent dopants are used, for example Boron. These dopants are said to be electron acceptors, because there are holes in the valence band where electrons can jump to absorb energy instead of doing it to the conduction band. A net balance of positive charges (holes) is generated in the valence band greater than negative charges (electrons) in the conduction band.
Applications
Semiconductors have an infinite number of uses and applications, for example, they are essential in the manufacture of diodes (including LEDs), electronic devices or solar panels. Some of the most widely used semiconductors are:
- thermistors: conductivity depends on temperature
- pressure transducers: the application of pressure to this type of semiconductor causes the energy gap between the conduction band and the valence band to narrow and increase conductivity.
- Rectifiers (pn-type junction devices): n and p-type semiconductors are joined (pn junction) and in doing so the electrons are concentrated in the n-type junction and the holes in the p-junction, this electronic imbalance creates a voltage in the junction that is used as a rectifier.
- Bipolar Junction Transistors: These transistors are generally used in CPUs (central processing units) of computers for the efficiency in giving a quick response to the commutation.
- field effect transistors: They are frequently used for store information in memory of the computers.