unit1 viva
> Hall effect discovered by Edwin Herbert Hall in 1879. 18 years before electron.
>when current is passed in a conductor and then the magnetic field is passed both being perpendicular then a voltage diff is generated due to Lorentz force which is perpendicular to both magnetic field and current this is called hall voltage.
>Gauss meter with Gauss Probe can measure both intensity and direction of the magnetic field.
>Current source is a power generator whose internal resistance is very high compared to load resistance and it supplies constant power current to load resistance even when load resistance changes over a wide range.
>Devices that are used to measure magnetic fields are called Hall probes. It has 4 collinearly placed pins. Current is measured between outer probes and voltage between inner probes.
>In this experiment current and voltage stay below some threshold.
> hall coefficient is positive for P-type and negative for N-type and 0 for intrinsic.
>Metals have a negative hall coefficient because the majority of charge carriers there are electrons.
>Unit of hall coefficient is ohm-cm/G.
>Hall effect sensors are used to detect variables like speed, displacement.
>Used in Magnetometers to measure magnetic field.
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>Four probe method is used to measure the resistivity accurately. this method can measure the resistivity of a wide range from micro ohm to megaohm.
>oven with heater is used to heat the material to study the behavior of sample with the increase in temperature.
>Digital Panel meter is an instrument that displays input signal as a digital output. this signal comes from sensors that are converted into digital form and then displayed for example temperature and pressure.
>At constant temperature
>The difference between the valence band and conduction band is known as bandgap or energy gap
>At 0 k energy gap of semiconductors is approximately equal to 3ev so it behaves as an insulator.
>Accurate resistivity measurement in samples having a variety of shapes is possible by this method which is why it is more efficient than any other method for measuring resistivity.
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>Zener diode is a diode that permits current in both directions. it behaves like a normal diode in forward biasing. it is a heavily doped PN junction diode. at breakdown region, it has very low dynamic resistance. it is used in voltage regulators, rectifiers, inverters, computer systems, etc,
>voltmeter always connected in parallel, and Ammeter is always connected in series.
>there are two types of breakdowns in the Zener diode i.e
Avalanche Breakdown -> The breakdown occurs because of the collision of the electrons inside the PN-junction when voltage is very high.
Zener breakdown -> heavy electric field is applied across the PN- junction.
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>Laser stands for light amplification by stimulated emission of radiation.
>Lasers amplify light and turn it into one powerful concentrated beam.
>Diffraction grating is an optical component which splits the light into different beams travelling in differnet directions. It is slits placed parallel to each other at such a distance which is comparable to wavelenth of light. Distance between two consecutive slits of grating is called grating element.
>When light bends due to edges of the hole it is called diffrection this hole is of size which is comparable to that of wavelength of light.
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>crystalline solids are highly ordered, sharp melting point, Anisotropic(diff physical properties in diff directions) in nature, sharp edges and when they are cut in two halves they have smooth surfaces, they are rigid(mild force can't distort its shape).
> Amorphous solids are not highly ordered like crystalline and all other properties are just opposite of that crystalline. these are also called pseudo solids or supercooled liquids.
>A space lattice is an array of points showing how particles (atoms, ions, or molecules) are arranged at different sites in three-dimensional spaces.
>Each of these basis units is called a unit cell it means the basis is nothing but a unit cell.
>each lattice has 6 parameters which are its edge length in each axis and angle between them.
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>xray crystallography is used to determine the structure of a crystal, size of atoms chemical bonds between them etc.
>limitations of crystallography are when a unit cell becomes complex it becomes hard to resolve.
>diffrection pattern can't be generated if the crystal sample is very small, have an irregular shape or have cracks in it.
>Polycrystalline materials, or polycrystals, are solids that are composed of many crystallites of varying size and orientation. Most inorganic solids are polycrystalline, including all common metals
>atoms in primitive unit cells are only at the corners of the unit cell but in non-primitive there are additional atoms which are at face centre and body centre of the crystal.
>there are 7 crystal systems and 14 Bravias lattices.
>Nacl have Fcc lattice in which Na is at lattice points and cl is at the octahedral void.
>unit cell is the smallest repeating unit in crystal.
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>Miller indices are used to specify direction and planes and a number of miller indices are equal to the dimension of lattice.
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>Simple cubic =52.4%, BCC = 68%, FCC and HCP = 74%
>metals with a high atomic packing fraction will have a higher "workability" (malleability or ductility), similar to how a road is smoother when the stones are closer together, allowing metal atoms to slide past one another more easily.
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> According to the classical theory of free-electron metals containing the free electrons obey the law of classical mechanics.
>some postulates of the above theory are
energy of the electron is given by E = 3/2 KT
force of attraction and repulsion between like and unlike charges are ignored.
>velocity attained by an electron in presence of an applied electric field is called drift velocity.
>merits of the theory
>demerits of theory
it was not able to explain the photoelectric effect, Compton effect and black body radiation.
Wiedemann Franz law is not applicable at lower temperatures.
the electrical conductivity of semiconductor and insulators can't be explained.
>To overcome these demerits Sommerfeld gave quantum free electron model.
he treated electron as a quantum particle and found that electron at Fermi level has maximum velocity.
> Postulates
energy of the electron is quantized
electron at each level of energy obeys Pauli's exclusion principle(no two electrons can have the same set of all four quantum numbers)
electrons are distributed in diff energy levels by the fermi direc function.
attraction or repulsion b/w electrons are ignored.
>merits
can explain temperature dependence of conductivity of metals
can explain the electrical and thermal conductivity of metals.
>Demerits
cannot explain metallic properties exhibited by only certain crystals.
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>Semiconductor is a crystalline solid which has conductivity intermediate between conductor and insulator.
>In single atom electrons are at different energy levels but in solids where atoms are sharing electrons.
electrons in the outermost orbit exhibit different energy levels and the grouping of this diff energy level are called an energy band. when an electric field is applied in solid it forms energy bands.
>Direct bandgap -> those materials which have the highest conduction band and lowest valance band at the same value of k(momentum). example - GaAs, InP, Cds etc.
> Indirect bandgap ->those materials which have highest conduction band and lowest of valance band at different value of k(momentum) .example si, Ge etc.
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>the highest energy level that an electron can occupy at 0k temperature is known as Fermi level.
>for efficient LED's and Lasers we use direct bandgap material.
>band gap : Si = 1.1eV, Ge = 0.7eV
>band is formed when one or more atomic energy levels are split.
>Fermi function tells us the occupancy of Fermi level with fermions.
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>semiconductors are doped because impurities make them more suitable for electronic devices such as diodes and transistors.
>size of dopant is nearly equal to the size of semiconductor atoms.
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>conductivity and mobility helps to differentiate between electron and proton.
>Drift current is due to the effect of the electric field
> Diffusion current is under the concentration difference.
>Semiconductor behaves as an insulator at 0K because the free electrons in the valence band of semiconductors will not carry enough thermal energy to overcome the forbidden energy gap at absolute zero.
>mobility is the drift velocity / electric field.
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>The charges at the junction near the P-type semiconductor are negative and positive for the N-type semiconductor. the flow of electrons and holes across the PN junction is known as diffusion.
>The area around the PN junction now called the Depletion Layer.
>PN semiconductor is neutral so it follows this formula. Dp*Np = Dn*Nn where N is concentration and D is the width of the Depletion region.
>Due to depletion region potential diff is created called Barrier Potential.
>Barrier Potential is from N-P and If the Diode is forwardly biased then voltage should be such that it reduces Barrier Potential.
>Applications of the diode are Rectifiers(Half wave, Full-wave), Clippers, Clampers.
>Barrier potential for Si = 0.5V - 0.7V Ge = 0.3V
>As temperature increases Depletion region decreases because Drift current increases (electrons becomes more energetic).
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>Clarence Melvin Zener found the property of Breakdown in an electrical insulator with the help of which Bell labs made Zener diode.
>Breakdown Voltage also known as Zener knee voltage or Zener voltage.
>Zener Diode has a very thin depletion region.
>Zener Breakdown occurs at low Reverse voltage and it occurs only in the Zener diode. It occurs at Zener voltage <=6V.
>Avlanche breakdown occurs at a high voltage which is >= 6V in Zener diode. In this breakdown, free-electron collides with other electrons and knock them off and make them also free this process repeats and due to this rapid increase in current can be seen.
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>when a semiconductor is in non-equilibrium state electrons gain energy and move from valance band to conduction band leaving holes behind this is known as carrier generation.
>Semiconductors are used to make electronic devices like diodes, transistors integrated circuits, capacitor, and resistance,
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In this condition when current is flowing holes are at a lower energy level than free electrons which are in conduction band at a higher energy level so when these free electrons lose energy and if this energy is in visible range then the photon is released which produces light and the colour of this light is determined by the amount of energy the free-electron loses.
>>Advantages
>Indicators and signals, Lightning and Data communication are the applications LED.
>LED is more efficient than incandescent light bulbs because it emits more lumens(a measure of the amount of brightness) per watt.
>LED's don't use any colour filters to produce light of diff colours and it is smaller in size.
>>Disadvantages
>It is costly, LED can be damaged in high temperature
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>OLED devices available in markets are flat and curved panel TV's and OLED's wallpaper.
>LCD = Liquid Crystal Display
>the wide view angle in the OLED's is up to 160 degrees even in bright light.
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