Physics of Solid state devices
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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)