The semiconductors are elements that perform the function of conductors or insulators selectively, depending on the external conditions to which they are subjected, such as temperature, pressure, radiation and magnetic or electric fields.
In the periodic table 14 semiconductor elements are present, among which are silicon, germanium, selenium, cadmium, aluminum, gallium, boron, indium and carbon. Semiconductors are crystalline solids with medium electrical conductivity, so they can be used dually as a conductor and an insulator.
If they are used as conductors, under certain conditions they allow the circulation of electric current, but only in one direction. In addition, they do not have a conductivity as high as that of conductive metals..
Semiconductors are used in electronic applications, especially for the manufacture of components such as transistors, diodes, and integrated circuits. They are also used as accessories or complements to optical sensors, such as solid state lasers, and some power devices for electrical power transmission systems..
At present, this type of element is being used for technological developments in the fields of telecommunications, control systems and signal processing, both in domestic and industrial applications..
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There are different types of semiconductor materials, depending on the impurities they present and their physical response to different environmental stimuli..
They are those elements whose molecular structure is made up of a single type of atom. Among this type of intrinsic semiconductors are silico and germanium.
The molecular structure of intrinsic semiconductors is tetrahedral; that is, it has covalent bonds between four surrounding atoms, as presented in the image below.
Each atom of an intrinsic semiconductor has 4 valence electrons; that is, 4 electrons orbiting in the outermost shell of each atom. In turn, each of these electrons forms bonds with the adjacent electrons..
In this way, each atom has 8 electrons in its most superficial layer, thereby forming a solid bond between the electrons and the atoms that make up the crystal lattice..
Due to this configuration, electrons do not move easily within the structure. Thus, under standard conditions, intrinsic semiconductors behave like an insulator.
However, the conductivity of the intrinsic semiconductor rises whenever the temperature increases, since some valence electrons absorb heat energy and separate from the bonds..
These electrons become free electrons and, if properly directed by an electric potential difference, can contribute to the flow of current within the crystal lattice..
In this case, the free electrons jump into the conduction band and go to the positive pole of the potential source (a battery, for example).
The movement of the valence electrons induces a vacuum in the molecular structure, which translates into an effect similar to that produced by a positive charge in the system, which is why they are considered as carriers of positive charge..
Then, an inverse effect occurs, since some electrons can fall from the conduction band to the valence shell, releasing energy in the process, which is called recombination.
They conform by including impurities within the intrinsic conductors; that is, by incorporating trivalent or pentavalent elements.
This process is known as doping and its purpose is to increase the conductivity of materials, to improve their physical and electrical properties..
By substituting an intrinsic semiconductor atom for an atom of another component, two types of extrinsic semiconductors can be obtained, which are detailed below.
In this case, the impurity is a trivalent semiconductor element; that is, with three (3) electrons in its valence shell.
The intrusive elements within the structure are called doping elements. Examples of these elements for P-type semiconductors are boron (B), gallium (Ga) or indium (In).
Lacking a valence electron to form the four covalent bonds of an intrinsic semiconductor, the P-type semiconductor has a gap in the missing bond.
This makes the passage of electrons that do not belong to the crystalline lattice through this hole that carries a positive charge..
Due to the positive charge of the bond gap, these types of conductors are designated by the letter "P" and, consequently, they are recognized as electron acceptors..
The flow of electrons through the holes in the bond produces an electric current that circulates in the opposite direction to the current derived from the free electrons.
The intrusive element in the configuration is given by pentavalent elements; that is, those that have five (5) electrons in the valence band.
In this case, the impurities that are incorporated into the intrinsic semiconductor are elements such as phosphorus (P), antimony (Sb) or arsenic (As).
Dopants have an additional valence electron that, having no covalent bond to bind to, is automatically free to move through the crystal lattice.
Here, the electric current circulates through the material thanks to the surplus of free electrons provided by the dopant. Therefore, N-type semiconductors are considered electron donors..
Semiconductors are characterized by their dual functionality, energy efficiency, diversity of applications and low cost. The salient characteristics of semiconductors are detailed below.
- Its response (conductive or insulating) may vary depending on the sensitivity of the element to lighting, electric fields and magnetic fields in the environment..
- If the semiconductor is subjected to a low temperature, the electrons will remain united in the valence band and, therefore, no free electrons will arise for the circulation of electric current.
On the other hand, if the semiconductor is exposed to high temperatures, thermal vibration can affect the strength of the covalent bonds of the element's atoms, leaving free electrons for electrical conduction..
- The conductivity of semiconductors varies depending on the proportion of impurities or doping elements within an intrinsic semiconductor.
For example, if 10 boron atoms are included in a million silicon atoms, that ratio increases the conductivity of the compound a thousand times, compared to the conductivity of pure silicon..
- The conductivity of semiconductors varies in a range between 1 and 10-6 S.cm-1, depending on the type of chemical element used.
- Composite or extrinsic semiconductors can exhibit optical and electrical properties considerably superior to the properties of intrinsic semiconductors.An example of this is gallium arsenide (GaAs), used predominantly in radio frequency and other optoelectronic applications..
Semiconductors are widely used as raw material in the assembly of electronic elements that are part of our daily lives, such as integrated circuits..
One of the main elements of an integrated circuit are transistors. These devices fulfill the function of providing an output signal (oscillatory, amplified or rectified) according to a specific input signal.
In addition, semiconductors are also the primary material of diodes used in electronic circuits to allow the passage of electrical current in only one direction..
For diode design, P-type and N-type extrinsic semiconductor junctions are formed. By alternating electron donor and carrier elements, a balancing mechanism is activated between both zones.
Thus, the electrons and the holes in both zones intersect and complement each other where necessary. This occurs in two ways:
- The transfer of electrons from the N-type zone to the P zone occurs. The N-type zone obtains a predominantly positive charge zone.
- There is a passage of electron-carrying holes from the P-type zone to the N-type zone. The P-type zone acquires a predominantly negative charge.
Finally, an electric field is formed that induces the circulation of the current in only one direction; that is, from zone N to zone P.
In addition, by using combinations of intrinsic and extrinsic semiconductors, devices can be produced that perform similar functions to a vacuum tube that contains hundreds of times its volume..
This type of application applies to integrated circuits such as microprocessor chips that cover a considerable amount of electrical energy..
Semiconductors are present in electronic devices that we use in our daily lives, such as brown line equipment such as televisions, video players, sound equipment; computers and cell phones.
The most widely used semiconductor in the electronics industry is silicon (Si). This material is present in the devices that make up the integrated circuits that are part of our day to day.
Silicon germanium alloys (SiGe) are used in high-speed integrated circuits for radars and amplifiers of electric instruments, such as electric guitars..
Another example of a semiconductor is gallium arsenide (GaAs), widely used in signal amplifiers, specifically for signals with high gain and low noise level..
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