Semiconductors have a memorial impact on our society. You determined semiconductor at the heart of microprocessor chips as well as transistors.
A semiconductor could be a component, typically a solid substance, that may conduct electricity below some rules however not others, creating it an honest medium for the manage of electrical current. … during a semiconductor material, the flow of holes happen during a direction opposite to the flow of electrons.
various types of semiconductor:
There are two basic categories of semiconductors: Extrinsic and Intrinsic. Intrinsic semiconductors are made of very pure materials and therefore they make very poor conductors. The intrinsic semiconductors have an equal number of negative and positive carriers called electrons and holes respectively. Am extrinsic semiconductor is one in which conductivity is improved by the adding of impurities, the doping process. Doping can produce two different categories of semiconductors: negative charge (n-type) or positive charge (p-type). Semiconductors are available as either a compound or an element. The most common and basic semiconductors are silicon and Germanium. Both silicon and Germanium semiconductors have a crystalline structure that is called the “diamond lattice.” This means that the atom is in the center and all of its neighbors are situated around the corners of the tetrahedron. There are many different pure element semiconductors as well as alloys and compounds. Compound semiconductors have some advantage in that they can provide a very broad range of mobilities and energy gaps. This simply means that materials have qualities and properties that match very specific requirements. For this reason, there are some semiconductors which are considered to be wide band gap semiconductors.
Solid-state materials are generally grouped into three classes: insulators, semiconductors, and conductors. The figureshows the conductivities σ (and the corresponding resistivities ρ = 1/σ) that are associated with some important materials in each of the three classes. Non-conductor, such as fused quartz and glass, have very low conductivities, on the order of 10−18 to 10−10 siemens per centimetre; and conductors, such as aluminum, have high conductivities, typically from 104 to 106 siemens per centimetre. The conductivities of semiconductors are between these extremes and are generally sensitive to temperature, illumination, magnetic fields, and minute amounts of impurity atoms. For example, the addition of about 10 atoms of boron (known as a dopant) per million atoms of silicon can increase its electrical conductivity a thousandfold .
The study of semiconductor equipment began in the early 19th century.he elemental semiconductors are those determined of single forms of atoms, like element (Si), semiconducting material (Ge), and tin (Sn) in column IV and selenium(Se) and atomic number 52 (Te) in column VI of the table. There are, yet, numerous compound semiconductors, which are composed of two or more elements. Gallium arsenide (GaAs), for example, is a binary III-V compound, which is a combination of gallium (Ga) from column III and arsenic (As) from column V. Ternary compounds can be formed by elements from three different columns—for instance, mercury indium telluride (HgIn2Te4), a II-III-VI compound.They can also be created by components from (2) columns, like Al metal chemical compound (AlxGa1 −xAs), which is a three III-V compound, where both Al and Ga are from column III and the subscript x is connected to the composition of the two elements from 100 percent Al (x = 1) to 100 percent Ga (x = 0). Pure silicon is the most important material for integrated circuit applications, and III-V binary and ternary compounds are most important for light emission.
The electrical characteristics of an intrinsic semiconductor are not that helpful. The feature can be changed by introducing some impurities in the intrinsic semiconductor like pentavalent and trivalent elements, then those material is called extrinsic semiconductor material. The operation of proposing impurities is called doping process.
Extrinsic semiconductor. … An extrinsic semiconductor which has been doped with electron donor atoms is called an n-type semiconductor, as a result of the bulk of charge carriers within the crystal area unit negative electrons.
Added to the semiconductor are two types of impurities. They are pentavalent and trivalent impurities. Pentavalent impurity atoms have 5 valence electrons. The various examples of pentavalent impurity atoms include Phosphorus (P), Arsenic (As), Antimony (Sb), etc.
Types of dopents in extrinsic semiconductor:
Crystals of semiconducting material and semiconductor area unit doped exploitation 2 kinds of dopants:
- Pentavalent (valency 5); like Arsenic (As), Antimony (Sb), Phosphorous (P), etc.
- Trivalent (valency 3); like Indium (In), Aluminium (Al),Boron (B) etc.
The reason behind using these dopants is to have similarly sized atoms as the pure semiconductor. Both Si and Ge enter to the fourth(IV) group in the periodic table. Hence, the choice of dopants is from the third and fifth(V) group. This ensures that size of the atoms is not much different from the fourth group. Hence, the trivalent and pentavalent choices. this dopants give rise to two types of semiconductors:
- n type
- p type
n type semiconductor:
An n-type semiconductor is created when pure semiconductors, like Si(Silicon)and Ge(germenium), are doped with pentavalent elements.
As can be seen in the image above, when a pentavalent atom takes the place of a Si atom, four of its electrons bond with four neighbouring Si atoms. However, the fifth electron remains freely bound to the procreator atom. Hence, the ionization energy required to set this electron free is very small. Thereby, this electron can move in the lattice even at room temperature.
To give you a better perspective, the ionization energy required for silicon at room temperature is around 1.1 eV. On the other hand, by adding a pentavalent impurity, this energy drops to around 0.05 eV.
It is important to remember that the number of electrons made available by the dopant atoms is independent of the ambient temperature and primarily depends on the doping level. Also, as the temperature rises, the Si atoms free some electrons and generate some holes. But, the number of these holes is very small. Hence, at any given point in time, the number of free electrons is much higher than the number of holes. Also, due to recombination, the number of holes reduce further.
when a semiconductor is doped with a pentavalent atom, electrons are the majority charge bearer. On the other hand, the holes are the minority charge carriers. Therefore, such extrinsic semiconductors are called n-type semiconductors. In an n-type semiconductor,
free electrons >> Number of holes .
By doping pentavalent element (Antimony), which has 5 electrons in the valence shell, four electrons of the shell make a covalent bond with four Silicon/Germanium atoms. The one electron remains un-bonded which is loosely connected to its parent atom can move freely. The pentavalent element is called a donor. A new type of material is gain through the process and is called N-Type Material.
It’s worth noting that the number of electrons and proton remain same in the N-Type Material, so, it remains electrically neutral.
p type semiconductor:
A semiconductor device is formed once power components are accustomed dope pure semiconductors, like Si and Ge. As can be seen in the image above, when a trivalent atom takes the place of a Si atom, three of its electrons bond with three neighbouring Si atoms. However, there is no electron to bond with the 4th Si atom.
This leads to a hole or a vacancy between the trivalent and the fourth silicon atom. This hole initiates a jump of an electron from the outer orbit of the atom in the neighbourhood to fill the vacancy. This creates a hole at the site from where the electron jumps. In simple words, a hole is now available for conduction.
It is important to remember that the number of holes made available by the dopant atoms is independent of the ambient temperature and primarily depends on the doping level. Also, as the temperature rises, the Si atoms free some electrons and generate some holes. But, the number of these electrons is very small. Hence, at any given point in time, the number of holes is much higher than the number of free electrons. Also, due to recombination, the number of free electrons reduce further.
when a semiconductor is doped with a trivalent atom, holes are the majority charge bearer. On the other hand, the free electrons are the minority charge carriers. Therefore, such extrinsic semiconductors are called p-type semiconductors. In a p-type semiconductor,
Number of holes >> Number of free electrons
Important note: The crystal maintains an overall charge justness. The charge of additional charge carries is equal and opposite to that of the ionized cores in the lattice.
the impurities of trivalent element (Boron, 3 electrons in valence shell), all the electrons combine with surrounding Silicon/Germanium atoms and vacancy for a covalent bond remains. There is a place for one covalent bond which is called Hole. The trivalent element is called accepter because it has a place for an electron. So, the new material form in the procedure is called P-Type Material.
Energy bands of extrinsic semiconductor:
In extrinsic semiconductors, a change in the ambient temperature leads to the production of minority charge carriers. Also, the dopant atoms produce the majority carriers. During recombination, the majority carriers destroy most of these minority carriers. This leads to a decrease in the concentration of the minority carriers.
Therefore, this affects the energy band structure of the semiconductor. In such semiconductors, additional energy states exist:
- Energy state due to donor impurity (ED)
- Energy state due to acceptor impurity (EA)
The over energy band diagram is of n-type Si semiconductor. Here you can see that the energy level of the donor (ED) is lower than that of the conduction band (EC). Hence, electrons can move into the conduction band with minimal energy (~0.01 eV). Also, at room temperature, most donor atoms and very few Si atoms get ionized. Hence, the conduction band has most electrons from the donor impurities.
The over energy band diagram is of p-type Si semiconductor. Here you can see that the energy level of the acceptor (EA) is higher than that of the valence band (EV). Hence, electrons can move from the valence band to the level Ea, with minimal energy. Also, at room temperature, most acceptor atoms are ionized.
This leaves holes in the valence band. Hence, the valence band has most holes from the impurities.The electron and hole concentration in a very semiconductor in equilibrium is:
ne × nh = ni2
Germanium and Silicon may have some impurities which can cause it loses its property and can conduct more current than expected. These semiconductors can be purified by the use of digital technology which is called Intrinsic Semiconductor. The intrinsic semiconductor has very few free current bearer at room temperature. As the temperature increase, electrons gain energy and become free electron and ultimately resistance of the material decreases which is negative temperature coefficient.
1. What Is Semiconductors Used For?
The semiconductor devices are used for all electronic devices nowadays, like a diode, transistor, thyristors. Every electrical machinery has these semiconductor devices in it, either if it is Television, Computer, Refrigerator for home and industrial machinery like CNC machines, Telecommunication equipment etc.
2. Electron Vs Hole Current Flow—
We have been familiar with electron flow but hole flow is something different. A hole is a empty position where a covalent bond could be formed. If an electron from a covalent bond get energy and escape the bond, the electron can fill the hole and form a new bond here. Thus electron will in one direction and hole move in the opposite direction.
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