EE40 Lec 17EE40 Lec 17PN JunctionsPN JunctionsProf Nathan CheungProf. Nathan Cheung10/27/2009Reading: Chapter 10 of HambleyBasic Device Physics Handout (optional)Slide 1EE40 Fall 2009 Prof. CheungBasic Device Physics Handout (optional)PN Junctions–Semiconductor Physics of pnypjunctions (for reference only)– Diode Current and EquationSolar Cells Photo DetectorsZener–Solar Cells, Photo Detectors, ZenerDiodes–Load Line AnalysisSlide 2EE40 Fall 2009 Prof. CheungThe Periodic TableIII IV VSlide 3EE40 Fall 2009 Prof. Cheung•4 nearest neighbors•unit cell length = 5.43ÅThe Si AtomThe Si Crystal•5×1022atoms/cm31s, 2s, 2p orbitals filled by 10 electrons3s, 3p orbitals filled by 4 electronsThe Si AtomSlide 4EE40 Fall 2009 Prof. Cheung“diamond cubic ” structurePure Si is not very conductiveBottom oflt-Bottom of conduction bandelectronEnergy gap=1.12 eV+Top of valence bandholen (electron conc)= p (hole conc) Slide 5EE40 Fall 2009 Prof. Cheung5= niShockley’s Parking Garage Analogy for Conduction in SiTwo-story parking garage on a hill:If the lower floor is full and top one is empty no traffic isIf the lower floor is full and top one is empty, no traffic is possible. Analog of an insulator. All electrons are locked up.Slide 6EE40 Fall 2009 Prof. CheungDopingBy substituting a Si atom with a special impurity atom (Column Vor Column III element), a conduction electron or hole is created.Donors: P, As, SbAcceptors: B, Al, Ga, InSlide 7EE40 Fall 2009 Prof. CheungDopant concentrations typically range from 1014cm-3to 1020cm-3Semiconductor with both acceptors and donors h4kid fh ihas 4 kinds of charge carriersHoleElectronMobile Charge Carriersthey contribute to current flow with electric field is appliedIidImmobile Chargeswith electric field is applied.Ionized DonorImmobile Chargesthey DO NOTcontribute to current flow IonizedAcceptorwith electric field is applied. However, they affect the lllifildSlide 8EE40 Fall 2009 Prof. Cheung8local electric fieldCharge Neutrality ConditionValid for homogeneously doped semiconductor at thermal equilibriumEven NAis not equal to NDEven NAis not equal to ND,microscopic volume surroundingany position x has zero net chargeSi atom (neutral)Ionized DonorIonizedAcceptorHoleElectronElectronSlide 9EE40 Fall 2009 Prof. Cheung9Electrons and holes created by Si atoms with conc niShockley’s Parking Garage Analogy for Conduction in SiTwo-story parking garage on a hill:If one car is moved upstairs it can move AND THE HOLEIf one car is moved upstairs, it can move AND THE HOLE ON THE LOWER FLOOR CAN MOVE. Conduction is possible. Analog to warmed-up semiconductor. Some electrons get free (and leave “holes” behind).Slide 10EE40 Fall 2009 Prof. Cheungg( )Shockley’s Parking Garage Analogy for Conduction in SiTwo-story parking garage on a hill:If an extra car is“donated”to the upper floor it can moveIf an extra car is donated to the upper floor, it can move. Conduction is possible. Analog to N-type semiconductor.(An electron donor is added to the crystal, creating free electrons).Slide 11EE40 Fall 2009 Prof. Cheung)Shockley’s Parking Garage Analogy for Conduction in SiShockley s Parking Garage Analogy for Conduction in SiTwo-story parking garage on a hill:If a car is removed from the lower floor it leaves a HOLEIf a car is removed from the lower floor, it leaves a HOLE which can move. Conduction is possible. Analog to P-type semiconductor. (Acceptors are added to the crystal, “consuming” bonding electrons,creating free holes).Slide 12EE40 Fall 2009 Prof. Cheunggg ,g )Summary of n- and p-type siliconPure silicon is an insulator. At high temperatures it conducts weakly.weakly. If we add an impurity with extra electrons (e.g. arsenic, phosphorus) these extra electrons are set free and we have a pp )pretty good conductor (n-type silicon).If we add an impurity with a deficit of electrons (e.g. boron) then bonding electrons are missing (holes), and the resulting holes can move around … again a pretty good conductor (p-type silicon) Now what is really interesting is when we join n-type and p-type silicon, that is make a pn junction. It has interesting electrical tiSlide 13EE40 Fall 2009 Prof. Cheungproperties.Junctions of n- and p-type RegionsWhat happens to the electrons and holes whenn and p regions are brought into contact :aluminumaluminumnpaluminum aluminum wire ?npSlide 14EE40 Fall 2009 Prof. CheungThe pn Junction DiodeSchematic diagramp-type n-typeIDCircuit symbolptype ntype+ VD–net donorconcentration NDnet acceptorconcentration NAPhysical structure:()lID+DAcross-sectional area AD(an example)p-type SiSiO2SiO2metalD+Vn-type SiVDFor simplicity, assume thatthe doping profile changes abruptly at the junctionSlide 15EE40 Fall 2009 Prof. Cheungmetal–abruptly at the junction.Depletion Region Approximation• When the junction is first formed, mobile carriers diffuseacross the junction (due to the concentration gradients)–Holes diffuse from thepsideto the n sideHoles diffuse from the p sideto the n side, leaving behind negatively charged immobile acceptor ionsEl t diff f thidtth id–Electrons diffuse from the n sideto the p side, leaving behind positively charged immobile donor ionsacceptor ionsdonor ions+++++––––pnÆA region depleted of mobile carriers is formed at the junction.•The space charge due to immobile ions in the depletion region+–Slide 16EE40 Fall 2009 Prof. CheungThe space charge due to immobile ions in the depletion region establishes an electric field that opposes carrier diffusion.Charge Density Distribution and Electric Fieldacceptor ionsdonor ionsUnbalanced Charge is created in the depletion region.+++++–––––pnquasi-neutral p region+depletion region quasi-neutral n regioncharge density (C/cm3)distance+-Slide 17EE40 Fall 2009 Prof. CheungBuild-in electric fieldEffect of Applied Voltagepn++++––––VD++–• The quasi-neutral p and n regions have low resistivity, whereas the depletion region has high resistivity. Thus, when an external voltageVDis applied across the diodewhen an external voltage VDis applied across the diode, almost all of this voltage is dropped across the depletion region. (Think of a voltage divider circuit.)• If VD> 0 (forward bias), depletion charge reduced• If VD< 0 (reverse bias), depletion charge increasedSlide 18EE40 Fall 2009 Prof. CheungSlide 19EE40 Fall 2009 Prof. CheungSlide 20EE40 Fall 2009 Prof. Cheung20Diode Physical
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