Lecture 15Semiconductors: Chemical StructureDoping: p-typeDoping: n-typeP-N Junction: The Basis For ElectronicsDiffusion and DriftPhysics of P-N Junction Case: Open CircuitSlide 8Physics of P-N Junction Case: Short CircuitPhysics of P-N Junction Case: Reverse BiasPhysics of P-N Junction Case: Reverse BreakdownFYI: Zener DiodesPhysics of P-N Junction Case: Forward BiasFYI: Diodes in Forward BiasDiode I-V RelationshipIdeal Diode ModelLarge-Signal Diode ModelSmall-Signal Diode ModelRealistic Diode ModelNotes on Use of ModelsLecture 15Today we willLearn about semiconductors and the P-N junction Chemical properties Physical propertiesSee how a semiconductor can be a voltage-controlled switchIntroduce a new element: the diodeSemiconductors: Chemical StructureStart with a silicon substrate (block of silicon).Silicon has 4 valence electrons, and therefore a “lattice” structure:Each atom bonds with 4 neighbors.No free electrons (poor conductor) unless you heat it up (semi conductor).SiSiSiSiSiSiSiSiSiSiSiOther Group IV elements can be used for substrate, but they are harder to come by (carbon lattice for instance…)Doping: p-typeSiSiSiSiSiGaSiSiSiSiSiMake silicon a better conductor by adding Group III or Group V elements: process called dopingholeAdd Group III elements like gallium or indium to substrate to get p-type material.Material is electrically neutral! Equal number of protons and electrons!Note that the Ga atom has only 3 valence electrons with which to bond; missing bond is called a hole.Doping: n-typeSiSiSiSiSiAsSiSiSiSiSieAdd Group V elements like arsenic or phosphorus to substrate to get n-type material.Note that the As atom has 5 valence electrons; it has an unbonded electron.Material is electrically neutral! Equal number of protons and electrons!P-N Junction: The Basis For ElectronicsPut p-type and n-type material together:Essential Property: Make current flow (or not flow) by applying electric field (voltage) to metal ends.aluminume e e e e e e e e h h h h h h h h h n-typeextra e-p-typeextra holesWe call this device a diode.“Voltage Controlled Switch”+-Diffusion and DriftElectrons can be moved by two types of attraction:DiffusionElectrons will move from an area of greater concentration (extra free electrons, e.g. n-type) to an area of lesser concentration (holes to fill, e.g. p-type)DriftElectrons will move according to an applied electric field, toward a region of higher potential (voltage)Physics of P-N JunctionCase: Open CircuitDiffusion moves free electrons from n-type to holes in p-type.“Uncovered” protons are left in n-type, extra electrons in p-type. e e e e e e h h h h h h h h h n-typep-type+ e e + e e + e e h h - h h - h h -n-typep-typedepletionregionArea near p-n junction now has no charge carriers (free electrons or holes): called depletion regione eePhysics of P-N JunctionCase: Open CircuitThe charged atoms in the depletion region create an electric field, and thus a difference in electric potential.Vdist fromjunctionWhen the potential drop becomes steep, the free electrons no longer cross, since electrons do not want to go to an area of lower potential.Drift takes over, and the electrons stay put.+ e e + e e + e e h h - h h - h h -n-typep-typedepletionregionEAt metal-semiconductor junction, potential changes to balance device. Electrons in metal can redistribute easily to do this.Is there a current when I short a diode? Is KVL violated? NO. h h - h h - h h -n-typep-type+ e e + e e + e e dist from junctionmetal contactmetal contactVPhysics of P-N JunctionCase: Short CircuitElectrons bunch up by positive metal contact, but few cross through wire because of potential drop between contacts. A diode is in reverse bias mode when the + (p-type) terminal is at a (moderately) lower potential than the – (n-type) terminal.ee e h h h –––++++++––––+depletion region(no free e, h)n-typep-typeVS > 0metal contactmetal contactV VSdist from junctionA tiny “leakage current” flows due to these few stray electrons, but basically zero current flow.Physics of P-N JunctionCase: Reverse Biasee eh h h –––++++++––––+depletion region(no free e, h)n-typep-typeWhen the diode + terminal is at a much lower potential than the - terminal, reverse breakdown occurs. The potential rise across the junction becomes so great that electrons from the p-type material travel across the rise.The rise has to be great enough to break the electrons out of their chemical bonds in the p-type material. VS > VZKBreakdown when V > VZK “Zener knee”metal contactmetal contactVVSdist from junctioneePhysics of P-N JunctionCase: Reverse Breakdown+-FYI: Zener DiodesThere are diodes called Zener diodes that are designed to operate in reverse breakdown. The voltage across the diode in the reverse breakdown mode is about constant, as once the voltage gets past VZK, the depletion layer does not really increase—the current increases dramatically (also known as avalanche current).As the reverse voltage is increased, there will be a limit to the current flow: reverse saturation current A Zener diode is used to regulate voltage within a circuit, since it provides about the same voltage (VZK) for a whole range of reverse current conditions. Zener diodes can be obtained for a variety of VZK values, anywhere from 0.5 V to 200 V.ee –––+++–+n-typep-typeVS > VFdist from junctionmetal contactmetal contactVVSh e The voltage source in this orientation makes the drop across the junction less steep.If VS is greater than the diode forward voltage parameter VF, electrons are willing to go over shallow voltage drop to fill holes.h Physics of P-N JunctionCase: Forward BiasElectrons flow across junction and combine with holes.The need to redistribute charge at metal ends ensures continuous supply of electrons and holes. Continuous current flow!FYI: Diodes in Forward BiasWhen the voltage across the forward-biased diode is increased past VF, the current increases dramatically.As the forward voltage is increased, there will be a limit to the current flow: saturation currentWhen operating in forward-bias mode in a circuit, diode voltage is nearly constant (equal to VF). A voltage around VF occurs for a whole range of forward current conditions.Many diodes have a VF of 0.6 to 0.7
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