EECS130 Integrated Circuit DevicesAnnouncementsEvolution of DevicesWhy “Semiconductors”?What are semiconductorsSilicon Crystal StructureSilicon Wafers and Crystal PlanesBond Model of Electrons and Holes (Intrinsic Si)Dopants in SiliconGaAs, III-V Compound Semiconductors, and Their DopantsFrom Atoms to CrystalsEnergy Band DiagramMeasuring the Band Gap Energy by Light AbsorptionTemperature Effect on Band GapSemiconductors, Insulators, and ConductorsDonor and Acceptor Levels in the Band ModelDopants and Free CarriersElectrons as Moving ParticlesEffective MassEECS130 Integrated Circuit DevicesProfessor Ali Javey8/28/2007Semiconductor FundamentalsLecture 1Announcements• Welcome back and happy Fall semester!!!• No discussion sections this week• No HW assignment for this weekYesterday’s Transistor (1947)Today’s Transistor (2006)Evolution of DevicesWhy “Semiconductors”?• Conductors – e.g Metals• Insulators – e.g. Sand (SiO2 )• Semiconductors– conductivity between conductors and insulators– Generally crystalline in structure• In recent years, non-crystalline semiconductors have become commercially very importantPolycrystalline amorphous crystallineWhat are semiconductorsElements: Si, Ge, CBinary: GaAs, InSb, SiC, CdSe, etc.Ternary+: AlGaAs, InGaAs, etc.Silicon Crystal Structure• Unit cell of silicon crystal is cubic.• Each Si atom has 4 nearest neighbors.5.43 Electrons and Holes in SemiconductorsÅSilicon Wafers and Crystal Planes•Silicon wafers are usually cut along the (100) plane with a flat or notch to help orient the wafer during IC fabrication.•The standard notation for crystal planes is based on the cubic unit cell. (100)(011)(111)xy y yz z zxx (100)plane(011)flatSi (111) planeBond Model of Electrons and Holes (Intrinsic Si)•Silicon crystal in a two-dimensionalrepresentation.Si Si SiSi Si SiSi Si SiSi Si SiSi Si SiSi Si SiSi Si SiSi Si SiSi Si Si•When an electron breaks loose and becomes a conduction electron, a hole is also created.Dopants in SiliconSi Si SiSi SiSi Si SiSi Si SiSi SiSi Si SiAs B•As (Arsenic), a Group V element, introduces conduction electrons and creates N-type silicon,•B (Boron), a Group III element, introduces holes and creates P-type silicon, and is called an acceptor.•Donors and acceptors are known as dopants.and is called a donor.N-type SiP-type SiGaAs, III-V Compound Semiconductors, and Their DopantsGaAsAs AsGaGa•GaAs has the same crystal structure as Si.•GaAs, GaP, GaN are III-V compound semiconductors, important for optoelectronics.•Which group of elements are candidates for donors? acceptors?GaAsAsGa GaFrom Atoms to Crystals•Energy states of Si atom (a) expand into energy bands of Si crystal (b).•The lower bands are filled and higher bands are empty in a semiconductor.•The highest filled band is the valence band.•The lowest empty band is the conduction band .Decreasing atomic separationEnergypsisolated atomslattice spacingvalence bandconduction bandEnergy Band DiagramConduction bandEcEvEgBand gapValence band •Energy band diagram shows the bottom edge of conduction band, Ec , and top edge of valence band, Ev .•Ec and Ev are separated by the band gap energy, Eg .Measuring the Band Gap Energy by Light Absorptionphotonsphoton energy: hv > EgEcEvEgelectronholeBandgap energies of selected semiconductors• Eg can be determined from the minimum energy (hν) of photons that are absorbed by the semiconductor.Material PbTe Ge Si GaAs GaP DiamondEg(eV) 0.31 0.67 1.12 1.42 2.25 6.0Temperature Effect on Band GapDecreasing atomic separationEnergypsisolated atomslattice spacingvalence bandconduction bandHow does the band gap change with temperature?Semiconductors, Insulators, and Conductors•Totally filled bands and totally empty bands do not allow •Metal conduction band is half-filled.EcEvEg=1.1 eVEcEg= 9 eVemptySi (Semiconductor)SiO2(Insulator)ConductorEcfilledTop ofconduction bandEvcurrent flow. (Just as there is no motion of liquid in a totally filled or totally empty bottle.).•Semiconductors have lower Eg's than insulators and can be doped.Donor and Acceptor Levels in the Band ModelConduction BandEcEvValence BandDonor LevelAcceptor LevelEdEaDonor ionization energyAcceptor ionization energyIonization energy of selected donors and acceptors in siliconAcceptorsDopant Sb P As B Al InIonization energy, Ec–Ed or Ea–Ev (meV)3944544557160DonorsHydrogen: Eionm0 q413.6 eV==8ε02h2Dopants and Free CarriersDopant ionizationenergy ~50meV (very low). Donorsn-typeAcceptorsp-typeElectrons as Moving Particles• An electron moves with a certain characteristic mass (from f=ma) in vacuum• In a solid, f=ma changes, so we can model this change via an “effective” massEffective MassIn an electric field, , an electron or a hole accelerates.Electron and hole effective massesSi Ge GaAs GaPmn/m00.26 0.12 0.068 0.82mp/m00.39 0.30 0.50 0.60electronsholesRemember
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