UNIT-1

CRYSTAL STRUCTURE OF MATERIAL

(A)BONDS IN SOLID

TYPES OF BONDS

1. Metallic Solids

2. Ionic Solids

3. Molecular Solids

4. Covalent network solids

METALLIC SOLIDS

• Metallic solids are solids composed of metal atoms that are held together by metallic bonds.

• These bonds are like huge molecular orbitals that span across the whole solid. This means the electrons in metallic solids are delocalized. In metals, the valence electrons of neighbouring atoms form a sort of ‘electron soup’

• This ‘delocalized electron soup’ can be thought of as a ‘glue’ that holds the

positive nuclei of the metal atoms together.

PROPERTIES OF METALLIC COMPOUND

1. Malleable and ductile – no fixed bond and the ions can roll past each other.

2. Good conductors of heat and electricity because the valence e- are mobile and can transmit energy rapidly.

3. Shiny because when light strikes a metal, the valence electrons absorb energy, oscillate at the same frequency as the incident light (incoming light) and then emit light as a reflection of the original light.

IONIC BONDS

• Occur between an element with low ionization energy (a metal) and an element with high ionization energy (a non-metal).

• An actual transfer of electrons from the metal (becoming a cation) to the non-metal (becoming an anion) occurs.

• This transfer results in the formation of 2 oppositely charged ions.• The electrostatic interaction between the 2 ions holds the compound

together.

PROPERTIES OF IONIC COMPOUNDS

• Brittle – when impact makes anions/cations align – repulsion

• Will conduct electricity as a liquid or aqueous solution because the ions are free to move to oppositely charged electrodes.

• High m.p. and b.p. – the greater the charge on the ions, the higher the m.p./b.p.

• Solids at room temperature because the attraction between the ions is very strong.

• The intramolecular forces are the same as the intermolecular forces (i.e. the electrostatic attraction of oppositely charged ions)

• Mostly soluble in water (the ions will dissociate in water) – can vary from high to very low

COVALENT BOND

• Occur between 2 atoms with high ionization energies (i.e. 2 non-metal atoms).

• sharing of electrons to obtain a full outer energy level (octet rule).

• If the electronegativity difference between the two atoms is less than 0.4, the bond is a true covalent bond and the electrons are shared equally (for example?).

• If the electronegativity difference between the two atoms is between 0.4 and 1.7, the bond is polar and there is an unequal sharing of electrons.

• If a molecule contains polar covalent bonds and is asymmetrical, the molecule will be polar.

• If both electrons being shared come from the same atom, the bond is a coordinate covalent bond.

PROPERTIES OF NON-POLARCOVALENT COMPOUND

• Have low melting and boiling points and are usually gases at room temperature. This is due to the low intermolecular forces that exist in these molecules (London Dispersion Forces only).

• If solid at room temperature, the solid is usually soft and waxy.• Soluble in non-polar solvents such as ethers.

• Will not conduct electricity in any form due to the fact that there

are no ions present.

PROPERTIES OF COVALENT MOLECULA R SOLIDS

• Dipoles - greater intermolecular forces than non-polar covalent compounds (the presence of dipole-dipole forces and possibly H-bonding).

• higher melting and boiling points and are more likely to be liquids or solids at room temperature (may even exhibit a crystal lattice like sugar).

• Will dissolve in polar solvents if H-bonding is present (sugar in water).

• Will not conduct electricity to any appreciable degree (only ionize to a very small degree)

COVALENT NETWORK SOLID

• Consider carbon dioxide (CO2) and silicon dioxide (SiO2).

• What would you expect the physical properties of SiO2 to be?

• CO2 b.p. = -78.5oC

• If London forces are the only intermolecular force, then you might predict the b.p. of SiO2 to be slightly more than CO2.

• A covalent network solid is a solid that consists of atoms held together in large networks or chains by covalent bonds

• .• Every atom is covalently bonded forming a 1,2 or3-dimensional network• Examples include: diamond, graphite, silicon, asbestos

NETWORK SOLIDS Generally strong bonds - very high melting and boiling points, solids at

room temperature, not soluble in polar or nonpolar solvents and they do not conduct electricity.

Can be soft (2-d networks like graphite) or hard (3-d networks like diamonds) or fibres (1-d networks like asbestos)

CRYSTAL STRUCTURE• crystalline solid – the atoms or ions arrange in a pattern that repeats itself in three

dimensions to form a solid which has long-range order

amorphous solid –• materials with only shortrange order space lattice – a network composed

of an infinite three-dimensional array of points

• unit cell – the repeating unit in a space lattice

CRYSTAL STRUCTURE

• FCC (face centered cubic): Atoms are arranged at the corners and center of each cube face of the cell

• Close packed Plane: On each face of the cube Atoms are assumed to touch along face diagonals. 4 atoms in one unit cell.

• BCC: Body Centered Cubic Atoms are arranged at the corners of the cube with another atom at the

cube center. Close Packed Plane cuts the unit cube in half diagonally • 2 atoms in one unit cell

CRYSTAL STRUCTURE

• Hexagonal Close Packed (HCP) Cell of an HCP lattice is visualized as a top and bottom plane of 7 atoms,

forming a regular hexagon around a central atom. In between these planes is a halfhexagon of 3 atoms.

• There are two lattice parameters in HCP, a and c, representing the basal and height parameters respectively. Volume 6 atoms per unit cell

• Coordination number – the number of nearest neighbor atoms or ions surrounding an atom or ion.

FCC STRUCTURE

BCC STRUCTURE

HCP STRUCRURE

ATOMIC PACKINF FACTOR

BCC structure• The primitive unit cell for the body-centered cubic crystal structure

contains several fractions taken from nine atoms: one on each corner of the cube and one atom in the center. Because the volume of each of the eight corner atoms is shared between eight adjacent cells, each BCC cell contains the equivalent volume of two atoms (one central and one on the corner).

• Each corner atom touches the center atom. A line that is drawn from one corner of the cube through the center and to the other corner passes through 4r, where r is the radius of an atom. By geometry, the length of the diagonal is a√3. Therefore, the length of each side of the BCC structure can be related to the radius of the atom by

ATOMIC PACKINF FACTOR(CONTINUED)

• a=4r/√3• APF• =∏√3/8=O.6801

• Hexagonal close-packed• HCP structure• For the hexagonal close-packed structure the derivation is similar. Here

the unit cell (equivalent to 3 primitive unit cells) is a hexagonal prism containing six atoms. Let a be the side length of its base and c be its height.

• The volume of the unit cell of hcp can be taken as 24√2r3. Then:

ATOMIC PACKING FACTOR(CONTINUED)• a=2r• c=4(√2r/3)• ∏/√18=0.7404• MILLER INDICES• There are two equivalent ways to define the meaning of the Miller indices:via

a point in the reciprocal lattice or as the inverse intercepts along the lattice vectors.

• Both definitions are given below. In either case, one needs to choose the three lattice vectors a1, a2, and a3 that define the unit cell (note that the conventional unit cell may be larger than the primitive cell of the Bravais lattice as the examples below illustrate).

• Given these, the three primitive reciprocal lattice vectors are also determined

(denoted b1, b2, and b3).

MILLER INDICES• Then, given the three Miller indices h, k, ℓ, (hkℓ) denotes planes

orthogonal to the reciprocal lattice vector:

• For cubic crystals with lattice constant a, the spacing d between adjacent (hkℓ) lattice planes is (from above)

dhkl =a/√h^2+l^2+k^2• With hexagonal and rhombohedral lattice systems, it is possible to use the

Bravais-Miller system, which uses four indices (h k i ℓ) that obey the constraint

• h + k + i = 0.Here h, k and ℓ are identical to the corresponding Miller indices, and i is a redundant index.

EXAMPLES OF MILLER INDICES

• Planes with different Miller indices in cubic crystals.

• Examples of directions.

• Examples of determining indices for a plane using intercepts with axes; left (111), right (221)

• Miller-Bravais indices.

• Dense crystallographic planes.

DIAGRAM FOR MILLER INDICES

BRAGG’S LAW

• When X-rays are incident on an atom they make the electronci cloud move as does any electromagnetic wave.

• • The movement of these charges re-radiates waves with the

same frequency blurred slightly due to a variety of effects; this phenomenon is known as Rayleigh scattering (or elastic scattering).

• The scattered waves can themselves be scattered but this secondary scattering is assumed to be negligible.

• A similar process occurs upon scattering neutron waves from the nuclei or by a coherent spin interaction with an unpaired electron

BRAGG’S LAW

• These re-emitted wave fields interfere with each other either constructively or destructively (overlapping waves either add up together to produce stronger peaks or are subtracted from each other to some degree), producing a diffraction pattern on a detector or film.

• The resulting wave interference patternin the basis of diffraction analysis. This analysis is called Bragg diffraction.

• Bragg diffraction occurs when radiation, with wavelength comparable to atomic spacings, is scattered in a specular fashion by the atoms of a crystalline system, and undergoes constructive interference

X-RAYS INTERACT WITH ATOMS IN A CRYSTAL.

BRAGG’S DIFFRACTION

X-RAY DIFFRACTION• X-ray crystallography provides an atomistic model of zeolite, an

aluminosilicate. • X-ray crystallography is a technique used for determining the atomic and

molecular structure of a crystal, in which the crystalline atoms cause a beam of incident X-rays to diffract into many specific directions.

• X-ray crystallography is a technique used for determining the atomic and molecular structure of a crystal in which the crystalline atoms cause a beam of incident X-rays to diffract into many specific directions.

• By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their disorder and various other information.

X-RAY DIFFRACTION

STRUCTURAL IMPERPERFECTION

• Materials are often stronger when they have defects. The study of defects is divided according to their dimension:

• 0D (zero dimension) – point defects: vacancies and interstitials. Impurities.• 1D – linear defects: dislocations (edge, screw, mixed)• 2D – grain boundaries, surfaces.• 3D – extended defects: pores, cracks. Point Defects• Vacancies and Self-Interstitials• A vacancy is a lattice position that is vacant because the atom is missing. It is

created when the solid is formed. There are other ways of making a vacancy, but they also occur naturally as a result of thermal vibrations.

• An interstitial is an atom that occupies a place outside the normal lattice position. It may be the same type of atom as the others (self interstitial) or an impurity atom.

STRUCTURAL IMPERPERFECTION(CONTINUED)

• An interstitial is an atom that occupies a place outside the normal lattice position. It may be the same type of atom as the others (self interstitial) or

an impurity atom.• Impurities in Solids• All real solids are impure. A very high purity material, say 99.9999% pure

(called 6N – six nines) contains ~ 6 × 1016 impurities per cm3.

• Impurities are often added to materials to improve the properties. For instance, carbon added in small amounts to iron makes steel, which is stronger than iron. Boron impurities added to silicon drastically change its electrical properties.

STRUCTURAL IMPERPERFECTION(CONTINUED)

• Miscellaneous Imperfections• Dislocations—Linear Defects• Dislocations are abrupt changes in the regular ordering of atoms, along a

line (dislocation line) in the solid.

• They occur in high density and are very important in mechanical properties of material.

• They are characterized by the Burgers vector, found by doing a loop around the dislocation line and noticing the extra interatomic spacing needed to close the loop

STRUCTURAL IMPERFECTION(CONTINUED)

• Interfacial Defects• The environment of an atom at a surface differs from that of an atom in

the bulk, in that the number of neighbors (coordination) decreases.

• This introduces unbalanced forces which result in relaxation (the lattice spacing is decreased) or reconstruction (the crystal structure changes).

STRUCTURAL IMPERFECTION

CRYSTAL GROWTH

• A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions.

• Crystal growth is a major stage of a crystallization processa nd consists in the addition of new atoms ions or polymer strings into the characteristic arrangement of a crystalline Bravais lattice.

• The growth typically follows an initial stage of either homogeneous or heterogeneous (surface catalyzed) nucleation, unless a "seed" crystal, purposely added to start the growth, was already present.

CRYSTAL GROWTH

• The action of crystal growth yields a crystalline solid whose atoms or molecules are typically close packed, with fixed positions in space relative to each other

• The crystalline state of matter is characterized by a distinct structural rigidity and virtual resistance to deformation (i.e. changes of shape and/or volume).

• Most crystalline solids have high values both of Young's modulus and of the shear modulus of elasticity. This contrasts with most liquids or fluids, which have a low shear modulus, and typically exhibit the capacity for macroscopic viscous flow.

ENERGY BANDS IN SOLIDS

• A useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having discrete energies as in the case of free atoms, the available energy states form bands.

• Crucial to the conduction process is whether or not there are electrons in the conduction band. In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap

ENERGY BANDS IN SOLIDS

• With such a small gap, the presence of a small percentage of a doping material can increase conductivity dramatically.

• An important parameter in the band theory is the Fermi level, the top of the available electron energy levels at low temperatures.

• The position of the Fermi level with the relation to the conduction band is a crucial factor in determining electrical properties.

ENERGY BANDS IN SOLID(CONTINUED))

CLASSIFICATION OF MATERIAL USING ENERGY BANDS

• Based on the ability of various materials to conduct current, the materials are classified as conductors, insulators and the semiconductors

• A metal which is very good carrier of electricity is called conductor. The copper and aluminium are good examples of a conductor A very poor conductor of electricity is termed as insulator.

• The glass, wood, mica, diamond are the examples of an insulator. A metal having conductivity which is between

• conductor and an insulator is called semiconductor. The silicon and germanium are the examples of a semiconductor. This does not conduct current at low temperatures but as temperature increases these materials behave as good conductors..

CLASSIFICATION OF MATERIAL USING ENERGY BANDS

UNIT-2 CONDUCTIVITY OF METALS

ELECTRON THEORY OF METALS

• The metals form a unique type of bonding known as metallic bonding and form the lattice structure.

• uniqueness in such a type of bonding fashion lies in the fact that unlike ionic bonding and covalent bonding where the sharing of electrons is between two atoms and the electrons remain localized, in metallic bonding the bond is formed among all the atoms in the lattice and the free electrons from each atom is shared by the whole lattice.

• These free electrons move freely throughout the lattice and hence are termed as electron gas.Neglectin the electron-electron interaction and the electron-ion interaction, it appears as if the electrons move in a confined box with periodic collision with ions in the lattice.

ELECTRON THEORY OF METALS

• This idea was given by Drude and he utilized it to explain many properties of metals satisfactorily such as electrical conductivity, thermal conductivity etc.

FACTOR AFFECTING ELECTRICAL RESISTANCE OF MATERIAL• Temperature.• Alloying.• Mechanical stressing.• Age Hardening.• Cold Working.• Temperature The resistivity of materials changes with temperature.

Resistivity of most of the metals increase with temperature.

FACTOR AFFECTING ELECTRICAL RESISTANCE OF MATERIAL

• The change in the resistivity of material with change in temperature is given by formula given below- Where, ρt1 is the resistivity of material at temperature of t1

o C and ρt2 is the resistivity of material at temperature of t2

oC

THERMAL CONDUCTIVITY OF METALS

Heat transfer is the transition of thermal energy from a heated item to a cooler item. When an object or fluid is at a different temperature than its surroundings or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium. Heat transfer always occurs from a hot body to a cold one, a result of the second law of thermodynamics

THERMAL CONDUCTIVITY OF METALS

• Thermal Conductivity is a term analogous to electrical conductivity with a difference that it concerns with the flow of heat unlike current in the case of the latter.

• It points to the ability of a material to transport heat from one point to another without movement of the material as a whole, the more is the thermal conductivity the better it conducts the heat.

• Let us consider a block of material with one end at temperature T1 and other at T2. For T1>T2, heat flows from T1 end to T2 end, and the heat flux(J) flowing across a unit area per unit time is given as-

THERMAL CONDUCTIVITY OF METALS

• Where, K is the thermal conductivity in Joule/meter-sec-K or Watts/meter-K. Generally the heat transfer in solid has two components Lattice conduction.

• Electronic conduction.

• Both types of heat conduction occur in solids but one is dominant over the other depending upon the type of material.

• Thermal conductivity of metals vary from 15 – 450 W/mK at 300K.

HEAT DEVELOPED IN CURRENT CARRYING CONDUCTOR

• According to Joule’s law, the heat developed in a conducting wire is given by I2 R,

• Where I is current flowing through the wire having resistance R.• If p is the electrical resistivity of wire,• L is the length of wire and• A is the area of cross section of wire• Then, Heat developed W=I2R• Or W=v2/R 1)• [because V=IR ( from OHM’s law)]• As V=El (2)• And R=pl/a (3)

HEAT DEVELOPED IN CURRENT CARRYING CONDUCTOR

• Where V is the applied potential,• E is the electric field developed across the wire of length l and resistance R.• By putting equations (2) and (3) in equation(1),we get• W=(El)2/(pl/a)• W=σE2lA ( because

σ=1/p)• Thus ,heat developed per unit volume (lA) per second is• W= σ E2

• Or W=JE (because from point form of Ohm’s law J= σ E)

• Where J is the current density.• If J is in ampere per m2and E is in volts per m then the units of W will be watts

per m3.

THERMOELECTRIC EFFECT

• The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates voltage when there is a different temperature on each side.

• • Conversely, when a voltage is applied to it, it creates a temperature

difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side

• This effect can be used to generate electricity, measure temperature or change the temperature of objects. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices can be used as temperature controllers.

THERMOELECTRIC EFFECT

• This effect can be used to generate electricity, measure temperature or change the temperature of objects.

• Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices can be used as temperature controllers.

• The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck effect, Peltier effect, and Thomson effect.

THE SEEBACK CIRCUIT CONFIGURED AS ATHERMOELECTRIC COOLER

SUPERCONDUCTIVITY

• Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic flux fields occurring in certain

• materials when cooled below a characteristic critical temperature.• The electrical resistance of a metallic conductor decreases gradually as

temperature is lowered. In ordinary conductors, such as copper or silver this decrease is limited by impurities and other defects.

• Even near absolute zero a real sample of a normal conductor shows some resistance.

SUPERCONDUCTIVITY(CONTINUED)

• In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing through a loop of superconducting wire can persist indefinitely with no power source

• Response to a magnetic field: A superconductor can be Type I, meaning it has a single critical field above which all superconductivity is lost; or Type II, meaning it has two critical fields, between which it allows partial penetration of the magnetic field.

• By theory of operation: It is conventional if it can be explained by the BCS theory or its derivatives, or unconventional otherwise.

SUPERCONDUCTIVITY(CONTINUED)

• By critical temperature: A superconductor is generally considered high temperature if it reaches a superconducting state when cooled using liquid nitrogen – that is, at only Tc > 77 K) – or low temperature if more aggressive cooling techniques are required to reach its critical temperature.

• By material: Superconductor material classes include chemical elements (e.g. mercury or lead), alloys (such as niobium-titanium, germanium-niobium, and niobium nitride), ceramics (YBCO and magnesium dibori), superconducting pnictide (like fluorine-doped LaOFeA) or organi superconductors (fullerenes and carbon nanot; though perhaps these examples should be included among the chemical elements, as they are composed entirely of carbon).

SUPERCONDUCTING MATERIAL

• Superconductors and superconducting materials are metals, ceramics, organic materials, or heavily doped semiconductors that conduct electricity without resistance.

• Superconducting materials can transport electrons with no resistance, and hence release no heat, sound, or other energy forms.

• Superconductivity occurs at a specific material's critical temperature (Tc). As temperature decreases, a superconducting material's resistance gradually decreases until it reaches critical temperature.

• At this point resistance drops off, often to zero, as shown in the graph at right.

SUPERCONDUCTING MATERIAL

• Superconducting materials can transport electrons with no resistance, and hence release no heat, sound, or other energy forms.

• • Superconductivity occurs at a specific material's critical temperature (Tc).

• As temperature decreases, a superconducting material's resistance gradually decreases until it reaches critical temperature. At this point resistance drops off, often to zero, as shown in the graph at right.

GRAPH

SUPERCONDUCTING MATERIAL

• Superconductors exhibit unique features other than their ability to perfectly conduct current. For example, many expel magnetic fields during the transition to the superconducting state.

• This is due to the Meissner effect by which superconducting materials set up electric currents near their surface at Tc, therefore canceling the fields within the material itself.

• A stationary magnet on a superconductor demonstrates this effect: as the superconductor cools through its critical temperature, the expulsion of magnetic flux from the conductor causes the magnet to levitate above the material.

DIAGRAM FOR MISSNER’S EFFECT

ELECTRICAL PROPERTIES OF CONDUCTING MATERIAL

• The Electrical properties of a material are those which determine ability of material to be suitable for a particular Electrical Engineering Application. Some of the typical Electrical properties of engineering materials are listed below-

• Resistivity

• Conductivity

• Temperature coefficient of Resistance

• Permittivity

• Thermoelectricity

APPLICATION OF CONDUCTING MATERIAL

PROPERTIES OF INSULATINGMATERIAL

• The properties of good insulating material can be classified as electrical, mechanical, thermal and chemical.

• Electrically the insulating material should have high resistivity to reduce the leakage current and high dielectric strength to enable it to withstand higher voltage without being punctured or broken down.

• Since the insulator are used on the basis of volume and not weight a low density is preferred. Liquid and gaseous insulator are also used as coolant for e.g transformer oil, hydrogen and helium are used both as insulation and cooling purpose.

PROPERTIES OF INSULATINGMATERIAL(CONTINUED)

• The insulator should also have small thermal expansion to prevent mechanical damage.

• Chemically the insulator should be resistant to oil, liquid, gas flumes, acid and alkalis. Insulating material should have certain mechanical properties depending on the use of which they are put.

• Materials with large electronic and ionic Polaris abilities and therefore large permittivity are used for making dielectrics capacitor.

• • The use of molecules with a permanent dipole moment is not desirable

because of possibility of large dielectric losses at high frequencies.

• I. Mica: Mica sheets are used for the insulating leaves between commutator

APPLICATION OF INSULATING MATERIAL

• SOLID INSULATING MATERIAL:• Mica sheets are used for the insulating leaves between commutator

segments.• The main aim of an insulating material is to separate electrical conductors

without passing current from one to the other and to safeguard individuals from electrically energized wires and parts.

• A complete knowledge of insulating materials and standards for safe working practices is required.

• A material that responds with very high resistance to the flow of electrical current or totally resists electric current is called an insulating material. In insulating materials, the valence electrons are tightly bonded to their atoms.

APPLICATION OF INSULATING MATERIAL(CONTINUED)

• In the electrical field, the purpose of any insulating material is to separate electrical conductors without passing current through it. Material like PVC, glass, asbestos, rigid laminate, varnish, resin, paper, Teflon, and rubber are very good electrical insulators.

• Insulating material is used as a protective coating on electrical wire and cables.

• The most significant insulating material is air. Beside that solid, liquid, and gaseous type of insulators are also used in electrical systems.

MECHANICAL PROPERTIES OF METAL

• Stress and Strain• Tension• Compression• Shear• Torsion• Elastic deformation• Plastic Deformation

MECHANICAL PROPERTIES OF METAL

• Yield Strength • Tensile Strength • Ductility • Toughness • Hardness

UNIT-3 MECHANISM OF CONDUCTION IN SEMICONDUCTOR MATERIAL

TYPES OF SEMICONDUCTOR

• Intrinsic Semiconductor• An intrinsic semiconductor material is chemically very pure and

possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes).

• A silicon crystal is different from an insulator because at any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked loose from its position, leaving behind an electron deficiency called a "hole".

• If a voltage is applied, then both the electron and the hole can contribute to a small current flow.

TYPES OF SEMICONDUCTOR(CONTINUED)

• The conductivity of a semiconductor can be modeled in terms of the band theory of solids.

• The band model of a semiconductor suggests that at ordinary temperatures there is a finite possibility that electrons can reach the conduction band and contribute to electrical conduction.

• The term intrinsic here distinguishes between the properties of pure "intrinsic" silicon and the dramatically different properties of doped n-type or p-type semiconductors.

TYPES OF SEMICONDUCTOR(CONTINUED)

• Extrinsic Semiconductor• Where as an extrinsic semiconductor is an improved intrinsic

semiconductor with a small amount of impurities added by a process, known as doping, which alters the electrical properties of the semiconductor and improves its conductivity. Introducing impurities into the semiconductor materials (doping process) can control their conductivity.

• Doping process produces two groups of semiconductors: the negative charge conductor (n-type) and the positive charge conductor (p-type). Semiconductors are available as either elements or compounds.

TYPES OF SEMICONDUCTOR(CONTINUED

• Silicon and Germanium are the most common elemental semiconductors. Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC, GaN. Si and Ge both have a crystalline structure called the diamond lattice. That is, each atom has its four nearest neighbors at the corners of a regular tetrahedron with the atom itself being at the center.

• In addition to the pure element semiconductors, many alloys and compounds are semiconductors. The advantage of compound semiconductor is that they provide the device engineer with a wide range of energy gaps and mobilities, so that materials are available with properties that meet specific requirements

• . Some of these semiconductors are therefore called wide band gap semiconductors

INTRINSIC SEMICONDUCTOR

CURRENT CARRIERS IN SEMICONDUCTOR

• There are two recognized types of charge carriers in semiconductors. One is electrons, which carry a negative electric charge.

• In addition, it is convenient to treat the traveling vacancies in the valence band electron population (holes) as the second type of charge carrier, which carry a positive charge equal in magnitude to that of an electron.

• Carrier generation and recombination• When an electron meets with a hole, they recombine and these free

carriers effectively vanish. The energy released can be either thermal, heating up the semiconductor (thermal recombination, one of the sources of waste heat in semiconductors), or released as photons (optical recombination, used in LEDs and semiconductor lasers).

CURRENT CARRIERS IN SEMICONDUCTOR(CONTINUED)

• Majority and minority carriers:• The more abundant charge carriers are called majority carriers, which are

primarily responsible for current transport in a piece of semiconductor. In n-type semiconductors they are electrons, while in p-type semiconductors they are holes.

• The less abundant charge carriers are called minority carriers; in n-type semiconductors they are holes, while in p-type semiconductors they are electrons.

• Free carrier concentration:

CURRENT CARRIERS IN SEMICONDUCTOR(CONTINUED)

• Free carrier concentration Z the concentrationof free carriers in a doped semiconductor. It is similar to the carrier concentration in a metal and for the purposes of calculating currents or drift velocities can be used in the same way.

• Free carriers are electrons (or holes) which have been introduced directly into the conduction band (or valence band) by doping and are not promoted thermally.

HALL EFFECT

The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current.

The Hall coefficient is defined as the ratio of the induced electric field to the product of the current density and the applied magnetic field. It is a characteristic of the material from which the conductor is made, since its value depends on the type, number, and properties of the charge carriers that constitute the current.

DIAGRAM FOR HALL EFFECT

DRIFT CURRENT

• drift current is the electric currento r movement of charge carriers, which is due to the applied electric field, often stated as the electromotive force over a given distance. When an electric field is applied across a semiconductor material, a current is produced due to the flow of charge carriers.

• The drift velocity is the average velocity of the charge carriers in the drift current. The drift velocity, and resulting current, is characterized by the mobility; for details, see electron mobility (for solids) or electrical mobility

• In current, the positively charged particles called holes move with the electric field, whereas the negatively charged electrons move against the electric field

DRIFT CURRENT(CONTINUED)

• In current, the positively charged particles called holes move with the electric field, whereas the negatively charged electrons move against the electric field But this does not happen in the case of electrons available in good conductors.

• Good conductors have plenty of free electrons moving randomly in between the fixed positive ion cores.

• This random movement of electrons in a straight line is known as drift current. Drift current also depends on the mobility of charge carriers in the respective conducting medium.

DIFFUSION CURRENT

• Diffusion current is a current in a semiconductorcaused by the diffusion of charge carriers (holes and/or electrons).

• The drift current, by contrast, is due to the motion of charge carriers due to the force exerted on them by an electric field. Diffusion current can be in the same or opposite direction of a drift current.

• The diffusion current and drift current together are described by the drift–diffusion equation

DIFFUSION CURRENT(CONTINUED)

• It is necessary to consider the diffusion current when describing many semiconductor devices.

• For example, the current near the depletion region of a p–n junction is dominated by the diffusion current. Inside the depletion region, both diffusion current and drift current are present.

• At equilibrium in a p–n junction, the forward diffusion current in the depletion region is balanced with a reverse drift current, so that the net current is zero.

CONTINUITY EQUATION• A continuity equation in physics is an equation that describes the

transport of some quantity.

• It is particularly simple and particularly powerful when applied to a conserved quantity, but it can be generalized to apply to any extensive quantity.

• Since mass, energy, momentum, electric charge and other natural quantities are conserved under their respective appropriate conditions, a variety of physical phenomena may be described using continuity equations

CONTINUITY EQUATION(CONTINUED)

• Continuity equations more generally can include "source" and "sink" terms, which allow them to describe quantities that are often but not always conserved, such as the density of a molecular species which can be created or destroyed by chemical reactions.

• In an everyday example, there is a continuity equation for the number of people alive; it has a "source term" to account for people being born, and a "sink term" to account for people dying.

ILLUSTRATION OF HOW THE FLUX J OF THE QUANTITY q PASSES THROUGH AN OPEN SURFACE S.

P-N JUNCTION DIODE

• P-N junction diode is the most fundamental and the simplest electronics device. When one side of an intrinsic semiconductor is doped with acceptor i.e, one side is made p-type by doping with n-type material, ap-n junction diode is formed. This is a two terminal device.

• p–n junctions are elementary "building blocks" of most semiconductor electronic devices such as diodes, transistors, solar cells, LEDs, and integrated circuits; they are the active sites where the electronic action of the device takes place.

• For example, a common type of transistor, the bipolar junction transistor, consists of two p–n junctions in series, in the form n–p–n or p–n–p.

P-N JUNCTION DIODE

P-N JUNCTION DIODE

JUNCTION TRANSISTOR

• A bipolar junction transistor (bipolar transistor or BJT) is a type of transistor that uses both electron and hole charge carriers. In contrast, unipolartransistors, such as field-effect transistors, only use one kind of charge carrier.

• BJTs are manufactured in two types, NPN and PNP, and are available as individual components, or fabricated in integrated circuits, often in large numbers.

• The basic function of a BJT is to amplify current. This allows BJTs to be used as amplifiers or switches, giving them wide applicability in electronic equipment, including computers, televisions, mobile phones, audio amplifiers, industrial control, and radio transmitters.

CIRCUIT FOR JUNCTION TRANSISTOR

FET &IGFET

• The field-effect transistor (FET) is a transistor that uses an electric field to control the shape and hence the electrical conductivity of a channel of one type of charge carrier in a semiconductor material.

• FETs are also known as unipolar transistors since they involve single-carrier-type operation. The FET has several forms, but all have high input impedance.

• While the conductivity of a non-FET is regulated by the input current (the emitter to base current) and so has a low input impedance, a FET's conductivity is regulated by a voltage applied to a terminal (the gate) which is insulated from the device.

• The applied gate voltage imposes an electric field into the device, which in turn attracts or repels charge carriers to or from the region between a source terminal and a drain terminal.

• The density of charge carriers in turn influences the conductivity between the source and drain.

FET STRUCTURE

SYMBOL FOR IGFET

DIAGRAM FOR IGFET

PROPERTIES OF SEMICONDUCTING MATERIAL

• Semiconductor materials possess electrical, chemical, and physical properties that allow the unique functions of semiconductor devices and circuits.

• Semiconductors are crystalline or amorphous solids with distinct electrical characteristics.[ They are of high electrical resistance — higher than typical resistance materials, but still of much lower resistance than insulators

• Their resistance decreases as their temperature increases, which is behavior opposite to that of a metal.

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PROPERTIES OF SEMICONDUCTING MATERIAL

• Finally, their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure, which lowers its resistance but also permits the creation of semiconductor junctions between differently-doped regions of the extrinsic semiconductor crystal.

• The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics.

UNIT -4

MAGNETIC PROPERTIES OF MATERIAL

ORIGIN OF PERMANENT MAGNET DIPOLE MOMENT

• Magnetic dipole moment • • When an electric current of ‘i’ amperes flows through a circular wire of 1 turn having an area of cross

section ‘a’ m2, then it is said to have a magnetic moment of

• magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the dimensions of the source are reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the electric dipole, but the analogy is not complete. In particular, a magnetic monopole, the magnetic analogue of an electric charge, has never been observed. Moreover, one form of magnetic dipole moment is associated with a fundamental quantum property—the spin of elementary particles.

• The magnetic field around any magnetic source looks increasingly like the field of a magnetic dipole as the distance from the source increases.

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CLASSIFICATION

• The origin of magnetism lies in the orbital and spin motions of electrons and how the electrons interact with one another.

• The best way to introduce the different types of magnetism is to describe how materials respond to magnetic fields.

• This may be surprising to some, but all matter is magnetic. It's just that some materials are much more magnetic than others.

• The main distinction is that in some materials there is no collective interaction of atomic magnetic moments, whereas in other materials there is a very strong interaction between atomic moments.

CLASSIFICATION• The magnetic behavior of materials can be classified into the following five

major groups:• 1. Diamagnetism

• 2. Paramagnetism

• 3. Ferromagnetism

• 4. Ferrimagnetism

• 5. Antiferromagnetism

CLASSIFICATION• 1. Diamagnetism

• Diamagnetism is a fundamental property of all matter, although it is usually very weak. It is due to the non-cooperative behavior of orbiting electrons when exposed to an applied magnetic field. Diamagnetic substances are composed of atoms which have no net magnetic moments (ie., all the orbital shells are filled and there are no unpaired electrons).

• However, when exposed to a field, a negative magnetization is produced and thus the susceptibility is negative. If we plot M vs H .

• Note that when the field is zero the magnetization is zero. The other characteristic behavior of diamagnetic materials is that the susceptibility is temperature independent. Some well known diamagnetic substances, in units of 10-8 m3/kg, include:

• quartz (SiO2) -0.62• Calcite (CaCO3) -0.48• water -0.90

PHASOR DIAGRAM

PARAMAGNETISM

• Paramagnetism• This class of materials, some of the atoms or ions in the material have a

net magnetic moment due to unpaired electrons in partially filled orbitals. One of the most important atoms with unpaired electrons is iron.

• However, the individual magnetic moments do not interact magnetically, and like diamagnetism, the magnetization is zero when the field is removed.

• In the presence of a field, there is now a partial alignment of the atomic magnetic moments in the direction of the field, resulting in a net positive magnetization and positive susceptibility.

PHASOR DIAGRAM

FERROMAGNETISM

• 3. Ferromagnetism• When you think of magnetic materials, you probably think of iron, nickel or

magnetite. Unlike paramagnetic materials, the atomic moments in these materials exhibit very strong interactions.

• These interactions are produced by electronic exchange forces and result in a parallel or antiparallel alignment of atomic moments. Exchange forces are very large, equivalent to a field on the order of 1000 Tesla, or approximately a 100 million times the strength of the earth's field.

• The exchange force is a quantum mechanical phenomenon due to the relative orientation of the spins of two electron.

• Ferromagnetic materials exhibit parallel alignment of moments resulting in large net magnetization even in the absence of a magnetic field.

FERROMAGNETISM

• The elements Fe, Ni, and Co and many of their alloys are typical ferromagnetic materials.

• Two distinct characteristics of ferromagnetic materials are their• (1) spontaneous magnetization and the existence of• (2) magnetic ordering temperature

DIAGRAM FOR FERROMAGNETISM

CURIE TEMPERATURE

• Curie Temperature• Even though electronic exchange forces in ferromagnets are very large,

thermal energy eventually overcomes the exchange and produces a randomizing effect.

• This occurs at a particular temperature called the Curie temperature (TC). Below the Curie temperature, the ferromagnet is ordered and above it, disordered.

• The saturation magnetization goes to zero at the Curie temperature. A typical plot of magnetization vs temperature for magnetite is shown below.

GRAPH

FERRIMAGNETISM

• Ferrimagnetism:

• In ionic compounds, such as oxides, more complex forms of magnetic ordering can occur as a result of the crystal structure. One type of magnetic ordering is call ferrimagnetism. A simple representation of the magnetic spins in a ferrimagnetic oxide is shown here.

• The magnetic structure is composed of two magnetic sublattices (called A and B) separated by oxygens. The exchange interactions are mediated by the oxygen anions. When this happens, the interactions are called indirect or superexchange interactions. The strongest superexchange interactions result in an antiparallel alignment of spins between the A and B sublattice.

• In ferrimagnets, the magnetic moments of the A and B sublattices are not equal and result in a net magnetic moment. Ferrimagnetism is therefore similar to ferromagnetism

FERRIMAGNETISM

• It exhibits all the hallmarks of ferromagnetic behavior- spontaneous magnetization, Curie temperatures, hysteresis, and remanence. However, ferro- and ferrimagnets have very different magnetic ordering.

• Magnetite is a well known ferrimagnetic material. Indeed, magnetite was considered a ferromagnet until Néel in the 1940's, provided the theoretical framework for understanding ferrimagnetism.

FERRIMAGNETISM

ANTI FERROMAGNETISM

• In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions.

• This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism. Generally, antiferromagnetic order may exist at sufficiently low temperatures, vanishing at and above a certain temperature, the Néel temperature (named after Louis Néel, who had first identified this type of magnetic ordering)

• . Above the Néel temperature, the material is typically paramagnetic.

DIAGRAM FOR ANTI FERRO MAGNETISM

MAGNETOSTRICTION

• Magnetostriction (cf. electrostriction) is a property of ferromagneticmaterials that causes them to change their shape or dimensions during the process of magnetization.

• The variation of materials' magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ.

GRAPH

HARD MAGNETIC MATERIAL• Materials which retain their magnetism and are difficult to demagnetize

are called hard magnetic materials.

• These materials retain their magnetism even after the removal of the applied magnetic field. Hence these materials are used for making permanent magnets.

• In permanent magnets the movement of the domain wall is prevented. They are prepared by heating the magnetic materials to the required temperature and then quenching them. Impurities increase the strength of hard magnetic materials.

HARD MAGNETIC MATERIAL(CONTINUED)

• They have large hysteresis loss due to large hysteresis loop area.

• Susceptibility and permeability are low.

• Coercivity and retentivity values are large.

• Impurities increase the strength of hard magnetic materials.

SOFT MAGNETIC MATERIAL

SOFT MAGNETIC MATERIAL

• Soft magnetic materials are easy to magnetize and demagnetize.

• These materials are used for making temporary magnets. The domain wall movement is easy.

• Hence they are easy to magnetize. By annealing the cold worked material, the dislocation density is reduced and the domain wall movement is made easier.

• Soft magnetic materials should not possess any void and its structure should be homogeneous so that the materials are not affected by impurities.

PERMANENT MAGNET MATERIAL

• A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnetused to hold notes on a refrigerator door.

• Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone.

• Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.

PERMANENT MAGNETIC MATERIAL