Princi Chap 1.docx

Submitted by : Tania Shahid (504) Sidra khan (501) Nudrat sana (503) Uzma qayyum (515) Rabbia allahbakhsh (527) Submitted to: DR. noor-ul-ain Program: Msc physics (4th) Subject: project work Government sadiq college women university Bahawalpur

CHAPTER 1 INTRODUCTION OF MATERIALS Materials science is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering. It includes elements of applied physics and chemistry, as well as chemical, mechanical, civil and electrical engineering. With significant media attention to nanoscience and nanotechnology in the recent years, materials science has been propelled to the forefront at many universities, sometimes controversially. The choice material of a given era is often its defining point: the Stone Age, Bronze Age, and steel age are examples. Materials science is one of the oldest forms of engineering and applied science. Modern materials science evolved directly from metallurgy, which itself evolved from mining. A major breakthrough in the understanding of materials occurred in the late 19th century, when Willard Gibbs demonstrated that thermodynamic properties relating to atomic structure in various phases are related to the physical properties of the material. Important elements of modern materials science are a product of the space race: the understanding and engineering of the metallic alloys and other materials that went into the construction of space vehicles was one of the enablers of space exploration. Materials science has driven, and been driven by, the development of revolutionary technologies such as plastics, semiconductors, and biomaterials. Before the 1960s (and in some cases decades after), many materials science departments were named metallurgy departments, from a 19th and early 20th century emphasis on metals. The field has since broadened to include every class of materials, including: ceramics, polymers, semiconductors, magnetic materials, medical implant materials and biological materials. In materials science, rather than haphazardly looking for and discovering materials and exploiting their properties, one instead aims to understand materials fundamentally so that new materials with the desired properties can be created. The basis of all materials science involves relating the desired properties and relative performance of a material in a certain application to the structure of the atoms and phases in that material through characterization. The major determinants of the structure of a material and thus of its properties are its constituent chemical elements and the way in which it has been processed into its final form. These, taken together and related through the laws of thermodynamics, govern the material’s microstructure, and thus its properties. (1)

Types of materials: 1.1 1.2 1.3

Insulators Conductors semiconductors

1.1

Insulators An electrical insulator is a material whose internal electric charges do not flow freely; very little electric current will flow through it under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors. A perfect insulator does not exist, because even insulators contain small numbers of mobile charges (charge carriers) which can carry current. In addition, all insulators become electrically conductive when a sufficiently large voltage is applied that the electric field tears electrons away from the atoms. This is known as the breakdown voltage of an insulator. Some materials such as glass, paper and Teflon, which have high resistivity, are very good electrical insulators. A much larger class of materials, even though they may have lower bulk resistivity, are still good enough to prevent significant current from flowing at normally used voltages, and thus are employed as insulation for electrical wiring and cables. Examples include rubber-like polymers and most plastics which can be thermo set or thermoplastic in nature. Insulators are used in electrical equipment to support and separate electrical conductors without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is called insulation. The term insulator is also used more specifically to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers. They support the weight of the suspended wires without allowing the current to flow through the tower to ground.

Types of insulators These are the common classes of insulator Pins type insulator - As the name suggests, the pin type insulator is mounted on a pin on the cross-arm on the pole. There is a groove on the upper end of the insulator. The conductor passes through this groove and is tied to the insulator with annealed wire of the same material as the conductor. Pin type insulators are used for transmission and distribution of communications, and electric power at voltages up to 33 kV. Insulators made for operating

voltages between 33kV and 69kV tend to be very bulky and have become uneconomical in recent years. Post insulator - A type of insulator in the 1930s that is more compact than traditional pintype insulators and which has rapidly replaced many pin-type insulators on lines up to 69kV and in some configurations, can be made for operation at up to 115kV. Suspension insulator - For voltages greater than 33 kV, it is a usual practice to use suspension type insulators, consisting of a number of glass or porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the top end is secured to the cross-arm of the tower. The number of disc units used depends on the voltage. Suspension insulator - A dead end or anchor pole or tower is used where a straight section of line ends, or angles off in another direction. These poles must withstand the lateral (horizontal) tension of the long straight section of wire. To support this lateral load, strain insulators are used. For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strings of cap-and-pin (suspension) insulators are used, attached to the cross arm in a horizontal direction. When the tension load in lines is exceedingly high, such as at long river spans, two or more strings are used in parallel. (2)

1.2

Conductors An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Pure elemental silver is the best electrical conductor encountered in everyday life. Copper, steel, gold, aluminum, and brass are also good conductors. In electrical and electronic systems, all conductors comprise solid metals molded into wires or etched onto circuit boards. Some liquids are good electrical conductors. Mercury is an excellent example. A saturated salt-water solution acts as a fair conductor. Gases are normally poor conductors because the atoms are too far apart to allow a free exchange of electrons. However, if a sample of gas contains a significant number of ions, it can act as a fair conductor.

Considering their use in electrical systems, what makes a good conductor is decided on different criteria. Usually good current carrying conductors should have following properties: Low resistivity/ / high conductivity. Low temperature coefficient of resistivity. Good thermal conductivity. Should be easily available. Environmental stability. Malleability .Insulators should be highly ductile Amenable to manufacturing process Easy to get in shapes, today and wires. Economic viability (3)

1.3 semiconductors We can define semiconductor as a substance, usually a solid chemical compound or element that can conduct an electric current under certain conditions, making it a good medium for the control of electricity. Types of Semiconductors Semiconductors are crystalline or amorphous solids with distinct electrical characteristics. Their electrical resistance is high but lower than that of insulators. They are mainly two types of semiconductors: 1.2.1 Intrinsic Semiconductors 1.2.2 Extrinsic Semiconductors 1.3.1 Intrinsic semiconductors The semiconductor material which does not have any impurities is known as intrinsic semiconductor or pure semiconductors.

Silicon and Germanium, which belong to the fourth group element, behave like a semiconductor. Each atom of silicon and germanium share an electron with their neighbors. A Silicon atom and its neighbors share a pair of electrons in covalent bonding. Whenever a covalent bond break, an electron-hole pair is formed. To remove the valence electrons from the outer shells a semiconductor atom needs the energy of the order 1.1 eV. The vacancy in the covalent bond is called a hole. Any other electron can fill this hole. In other words, a hole shifts from one covalent bond to another. We can assume that the hole is a positive charge carrier since the direction of

the hole is opposite to that of the electron. In an intrinsic semiconductor, electrons and holes move in random directions and the number of free electrons (ne) and holes (nh) remain same.

1.3.1Extrinsic semiconductors The introduction of the extrinsic semiconductor is due to the excess holes or excess electrons present in silicon. Pure semiconductors are of no use as there are very few charge carriers which can cause conduction process. By adding some impurities to the pure semiconductor the conductivity can be improved. This process is called doping. Depending on the type of doping material used, extrinsic semiconductors can be classified as: 

N-type semiconductors  P-type semiconductors N-type Semiconductors:

The N-type semiconductor has a large number of electrons in the conduction band and less number of holes in the valence band, so electrons are called majority carriers and holes are called minority carriers. A pentavalent impurity such as phosphorus or arsenic is added to the silicon crystal. Out of five valence electrons, four silicon atoms take part in covalent bonding with one arsenic/phosphorus atom. The fifth electron is loosely bound to the silicon atom. Such a silicon crystal is still electrically neutral as the extra electron does not show up as an additional charge in the atom.

P-type semiconductors The P-type semiconductor has a large number of hole in the conduction band and less number of electrons in the valence band, so holes are called majority carriers and electrons are called minority carriers. A trivalent impurity such as Boron is mixed with the silicon atoms. Boron can share three valence electrons with the silicon atom; the boron atom takes one electron from nearby covalent bonds with the silicon atom in order to complete eight electrons in its valence

shell. As the trivalent impurity atoms accept electrons from the silicon atom, it is known as an acceptor impurity. The p-type silicon crystal so obtained is called p-type extrinsic semiconductor and the holes created are extrinsic carriers. Examples of Semiconductors Semiconductors are very common and are found in almost all electronic devices. Some examples of semiconductor materials are selenium, germanium, and silicon. (4)

Band Gap In solid-state physics, a band gap, also called an energy gap or band gap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the energy required to promote a valence electron bound to an atom to become a conduction electron, which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current. It is closely related to the HOMO/LUMO gap in chemistry. If the valence band is completely full and the conduction band is completely empty, then electrons cannot move in the solid; however, if some electrons transfer from the valence to the conduction band (5)

Direct and Indirect Band Gap Semiconductors The band gap represents the minimum energy difference between the top of the valence band and the bottom of the conduction band. However, the top of the valence band and the bottom of the conduction band are not generally at the same value of the electron momentum. In a direct band gap semiconductor, the top of the valence band and the bottom of the conduction band occur at the same value of momentum, as in the schematic below.

In an indirect band gap semiconductor, the maximum energy of the valence band occurs at a different value of momentum to the minimum in the conduction band energy:

The difference between the two is most important in optical devices. As has been mentioned in the section charge carriers in semiconductors, a photon can provide the energy to produce an electron-hole pair. Each photon of energy E has momentum p = E / c, where c is the velocity of light. An optical photon has an energy of the order of 10–19 J, and, since c = 3 × 108 ms–1, a typical photon has a very small amount of momentum. A photon of energy E.g., where E.g. is the band gap energy, can produce an electron-hole pair in a direct band gap semiconductor quite easily, because the electron does not need to be given very much momentum. However, an electron must also undergo a significant change in its momentum for a photon of energy E.g. to produce an electron-hole pair in an indirect band gap semiconductor. This is possible, but it requires such an electron to interact not only with the photon to gain energy, but also with a lattice vibration called a phonon in order to either gain or lose momentum. The indirect process proceeds at a much slower rate, as it requires three entities to intersect in order to proceed: an electron, a photon and a phonon. This is analogous to chemical reactions, where, in a particular reaction step, a reaction between two molecules will proceed at a much greater rate than a process which involves three molecules. The same principle applies to recombination of electrons and holes to produce photons. The recombination process is much more efficient for a direct band gap semiconductor than for an indirect band gap semiconductor, where the process must be mediated by a phonon. As a result of such considerations, gallium arsenide and other direct band gap semiconductors are used to make optical devices such as LEDs and semiconductor lasers, whereas silicon, which is an indirect band gap semiconductor, is not. The table in the next section lists a number of different semiconducting compounds and their band gaps, and it also specifies whether their band gaps are direct or indirect. (6)

REFERENCES: 1. https://en.wikibooks.org/wiki/Materials_Science/Introduction. 2. https://en.wikipedia.org/wiki/Insulator_(electricity. 3. https://whatis.techtarget.com/definition/conductor. 4. . https://byjus.com/physics/semiconductors-and-insulators/. . 5. https://en.wikipedia.org/wiki/Band_gap. 6. https://www.doitpoms.ac.uk/tlplib/semiconductors/direct.php.