EECS143 Microfabrication TechnologyAnnouncementsEvolution of DevicesWhy “Semiconductors”?What are semiconductorsSilicon Crystal StructureSilicon Wafers and Crystal PlanesBond Model of Electrons and Holes (Intrinsic Si)Dopants in SiliconTypes of charges in semiconductorsDoped Si and ChargeGaAs, III-V Compound Semiconductors, and Their DopantsFrom Atoms to CrystalsEnergy Band DiagramMeasuring the Band Gap Energy by Light AbsorptionSemiconductors, Insulators, and ConductorsDonor and Acceptor Levels in the Band ModelDopants and Free CarriersGeneral Effects of Doping on n and pDensity of StatesThermal EquilibriumThermal Equilibrium An Analogy for Thermal EquilibriumSlide 23Effect of T on f(E)QuestionSlide 26Slide 27Intrinsic SemiconductorEECS143 Microfabrication TechnologyProfessor Ali JaveyIntroduction to MaterialsLecture 1Announcements•The first HW set is due next Tuesday, at the beginning of the class. •Please make sure you are signed up for one of the lab section. The labs started from this week.Yesterday’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 ÅElectrons 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 ( 1 0 0 )p l a n e( 0 1 1 )f l a tSi (111) planeBond Model of Electrons and Holes (Intrinsic Si)-Silicon crystal in a two-dimensionalrepresentation.S i S i S iS i S i S iS i S i S iSi 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 Silicon Si 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 Si P-type SiEE143 – Vivek Subramanian Slide 1-10Ionized DonorIonizedAcceptorImmobile Chargesthey DO NOTcontribute to current flow with electric field is applied. However, they affect the local electric fieldHoleElectronMobile Charge Carriers they contribute to current flow with electric field is applied.Types of charges in semiconductorsDoped Si and Charge•What is the net charge of your Si when it is electron and hole doped?GaAs, III-V Compound Semiconductors, and Their DopantsG aA sAs 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 Diagram Conduction 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: h v > EgEcEvEgelectron hole Bandgap energies of selected semiconductors• Eg can be determined from the minimum energy (h) 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 6.0Semiconductors, Insulators, and Conductors-Totally filled bands and totally empty bands do not allow -Metal conduction band is half-filled. Ec EvEg=1.1 eV EcEg= 9 eVemptySi (Semiconductor) SiO2(Insulator) ConductorEcfilledTop of conduction bandEv current 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 energy Ionization energy of selected donors and acceptors in siliconAcceptorsDopant Sb P As B Al InIonization energy, Ec–Ed or Ea–Ev (meV) 39 44 54 45 57 160DonorsHydrogen: Eionm0 q413.6 eV==802h2Dopants and Free CarriersDopant ionization energy ~50meV (very low). Donorsn-typeAcceptorsp-typeGeneral Effects of Doping on n and pCharge neutrality:daNpNn+_= 0daNpNn= 0Assuming total ionization of acceptors and donors:aN_: number of ionized acceptors /cm3dN+: number of ionized donors /cm3aN: number of ionized acceptors /cm3dN+: number of ionized donors /cm3Density of States EgcgvEcEv g(E)EcEv3cmeV1 volumein states ofnumber )(EEEgc 2)(32**hEEmmEgcnnc 2)(32**hEEmmEgvppvThermal EquilibriumThermal Equilibrium An Analogy for Thermal Equilibrium-There is a certain probability for the electrons in theconduction band to occupy high-energy states under the agitation of thermal energy (vibrating atoms, etc.)DishVibrating Table Sand particlesAt E=EF, f(E)=1/2Effect of T on f(E)T=0KQuestion•If f(E) is the probability of a state being occupied by an electron, what is the probability of a state being occupied by a hole?Nc is called the effective density of states (of the conduction band) .Nv is called the effective density of states of the valence band.Intrinsic Semiconductor• Extremely pure semiconductor sample containing an insignificant amount of impurity atoms.n = p = niMaterial Ge Si GaAsEg (eV) 0.67 1.12 1.42ni (1/cm3) 2 x 10131 x 10102 x 106Ef lies in the middle of the band
View Full Document
Unlocking...