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SOLID STATE DEVICESRef: NPTEL videos of Dr.S.KarmalkarLecture 1-IntroductionNecessity - Due to the growth of semiconductor devices ,we cant treat them only as black boxes.

Classification:Small signal devices and Power devicesHigh frequency and low frequency devicesDiscrete devices and Integrated Circuit devices

Some Devices for example Small signal diodes and transistors modify and amplify electrical power

High power insulated gate bipolar transistors handles large amount of powerHas to dissipate large amount of power effecientlyHole to attach heat sinkPackaging is very importantBigger than small signal devices

High electron mobility transistor High frequency amplification

Monolithic accelerometerIntegrated Circuit devices-Number of devices integrated on a single semiconductor substrate

Devices can be classified based on the frequency of the application they are used forAudio- Electromagnetic applicationsAM,FM/TV-Wireless applicationsMicrowave SatelliteVisible light Optoelectronic devices

Others- conversion one form of energy to otherMems Devices micro electro mechanical system electrical energy to mechanical energy and vice versaLaser diodes - Optoelectronic Devices electrical energy to optical energy

Devices can be broadly classified as1. Energy Systems Generation distribution and regulation of large amount of power

2 . Information SystemsStore process and communicate large amount of information with less power and high speed

Devices enhancePerformance ReliabilityCost effectiveness

ObjectiveTo relate the terminal characteristics of devices to the material parameters

Diode characteristics Rectifing current vs voltage characteristicsIn very high reverse bias the device breakdownIn breakdown it generates stable voltage independent of the current

Diode applicationsRectifiersStable voltage sources

BJT characteristicsFor different base current ,different collector current as a function of collector to emitter voltage

BJT applicationAmplifiers

MOSFET characteristicsFor different Gate-source voltage,different drain current as a function of drain to source voltage

MOSFET applicationAmplifiers

Properties of SemiconductorsPolarity of charge carrriers(+ve and -ve)Concentration of charge carrriersTransport(velocity) of charge carrriersInteraction with electromagnetic field

Approximate values of carrier concentrationInsulators1021

Varying concentration byDopingIlluminationTemperature variation

Drift - When you apply electric field in metals ,current flows due to potential gradient.Usually drift velocity to electric field ratio(mobility) is constantPiezo resistivity mobility changes inversely with pressureIn some compound semiconductor like GaAs, drift velocity reduces after reaching a peak value.Negative differential resistivity used to make oscillators. Diffusion In semiconductors, charge carriers flow because of concentration gradient Thermo-electric Current flow- charge carriers flow because of temperature gradient

Resistivity of a 1cc sampleMetals105

Resistivity change due to concentration and mobility

Some special effect with light:Photoconductivity / Photovoltaic - when light falls on semiconductors, electron hole pair generation and so there is a change in conductivity solar cells

Electro luminescence electrical energy to light energy- Laser diodes

Photo luminescence combination of bothLecture 2-Uniqueness and Evolution of semiconductor TechnologyRef Book: Transistors Fundamentals for the integrated circuit engineerAuthor: R.M.Warner and B.L.Grung Chapter : origin and personality of microelectronic devices

TECHNOLOGY processing of material, energy or information to develop a useful productHigh technology - the processing technology is very stringent and product is highly reliability at low cost and better performance

Processing conditions in semiconductor technologyPerfect single crystal materialUltra clean environmentSophisticated equipment

Class10 clean room- no more than 10 particles of size more than 0.3 micron/cubic feetWhereas in Open air- more than a million such particlesresistivity of Deionized water- resistivity should be around 20M ohmWhereas for tap water 20k ohm/cmHigh grade of equipment semiconductor grade and MOS grade Sophisticated equipments like cleaner bench by maintaining high +ve pressure using filtersLimitation High Initial capital investmentAdvantage Price of the products id lessLecture 3 Equilibrium Carrier Concentration(1/9)Carrier concentration as a function of time

At temperature T=300Kni of Ge > Si > GaAs

nn0 electron concentration in an N-type semiconductor at equilibriumNd Doping concentrationpn0 = ni2 / nn0Under equilibrium p0n0 = ni2Two Classifications of semiconductorsSimple or elemental semiconductorCompound semiconductor

Pure no impurityIntrinsic contains impurities but concentration of electrons and holes are same at equilibrium but no defectsExtrinsic electron hole concentration is may not same or may contains defectsThermal Equilibrium or thermodynamic equilibriumIt is an intense dynamic equilibrium It is the convenient idealized condition which is amenable to simple mathematical treatment

No external excitation other than temperatureNo net motion of charge or energy

Principle of detailed balance:For every process thats going on there is an inverse process going on at the same rateAt every level of detail

Quasi Equilibrium modelSlight Deviation from the idealized conditionUsed to model complex real life situation many situations of semiconductor device

Steady stateAll process are constant as a function of timeIts not necessary that every process needs to have an inverse process

Transient analysis-> quasi steady state analysisTransient condition -> quasi steady state condition/quasi static condition->slight deviation from the steady state condition

Lecture 4- Equilibrium carrier concentration (2/9)

Wave particle Duality Energy/matter propagates in the form of stream of particles or travelling waves

E=hP=h/ E(P) represents minimum energy(momentum) quantum that can propagateE,P properties of particle in the stream, properties of travelling wave-variation w.r.t. time -vatiation w.r.t. distance

1eV=hV/E=1.24 eVWhere V is velocity of photon

Travelling wave varies with space and timeElastic wave circular wave disturbance of displaced particleStanding wave Interference of two travelling wave

Bond Model of Intrinsic semiconductorCrystal StructureAtoms at 4 cornersAtoms at 6 face centresAtoms near 4 corners A,B,C,D

At T= 0 KNo thermal agitationNo bond is broken

At T > 0 K4 particles are createdEach atom vibrates with its mean position, so elastic waves (phonons) are generatedEach travelling waves generated interfere with other travelling waves

Positively charged nucleus of an atom and the electron cloud move in different directionsThe displacement of these charges create an oscillating electric dipoleAs per maxwells oscillating dipole will radiate electromagnetic waves( photons)

Waves emenating from different atoms interfere . if the energy of collision due to that is more than the bond energy, bond breaks and gives rise to a free electron

The free electron leave behind a vacancy (holes) in an atom Lecture 5- Equilibrium carrier concentration (3/9)Detailed balanceElectron Hole pair Generation RecombinationPhoto generation / Photo ionization radioactive recombinationImpact ionization / Impact generation Anger recombinationPhonon Ionization Phonon recombination

Recombination when electron and hole come nearer, they combine and annihilation occurs Scattering when electron and hole come nearer, the direction of motion changes

Hole conceptCurrent due to movement of = > free electrons + bound electronFree electrons is faster than bound electrons because of the rate of movementHall effect experiment is a direct demonstation of positively charged particles

Effective mass = constant x m0m0 mass of electron in vacuummp - effective mass of holesmn - effective mass of electronmp > mn Probability of a particle collides, P = intrinsic carrier concentration, ni/ atomic concentration, N

At 300K

Average energy of any particle at thermal equilibrium, E=kT=0.026 eVBond Energy = 1.1eVNo: of particles need to be converged to break a bond in Silicon atom = 1.1 / 0.026 = 42P42 - probability that a free electron is created by collision(hit by a particle)

2. At 400KAverage energy of any particle at thermal equilibrium, E=kT=0.026 x 4 / 3 eVBond Energy = 1.1eVNo: of particles need to be converged to break a bond in Silicon atom = 1.1x3 / 0.026 x4= 31.5P31.5 - probability that a free electron is created by collision(hit by a particle)

P42 0KEf is the energy at which exactly half of the available states are occupied

25Lecture 8 Equilibrium carrier concentration (6/9)

Boltzman approximation is true only if its assumption is trueBy simple reasoning we can conclude that Ef has to be close to the middle of the energy gap to make the electron and hole concentration equal.So half of Eg is greater than 3kT where this approximation is true.

Expression for the intrinsic carrier concentration

Effective mass depends on the situationConductivity effective mass is the mass of the electrons and holes moving in the conduction bandMass of the free electron is lesser than holes since the rate of movement of free electron is faster than the bound electronsLecture 9 Equilibrium carrier concentration (7/9)