Slide 1The pn Junction DiodeSlide 3Summary: pn-Junction Diode I-VIdeal Diode Model of pn DiodeLarge-Signal Diode ModelApplication Example: Rectification using the ideal diode modelSlide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Week 11b Lecture MaterialsDiodes and some of their uses: Review of pn-diode structure Diode I-V characteristics: Actual characteristic – exponential Ideal diode characteristic – switches at V = 0 Large-signal diode model – a switch and a voltage source to represent required turn-on voltage Zener diode, varactor diode, light-emitting diode (LED), solar cell PN-diodes used for isolation in integrated circuits Diodes and MOSFET operation – need for a gateThe band picture of semiconductors (if there’s time)Answering one of your questions: A quick picture of superconductivity – EECS Prof. Ted Van DuzerThe pn Junction DiodeSchematic diagramp-type n-typeID+ VD –Circuit symbolPhysical structure:(an example)p-type Sin-type SiSiO2SiO2metalmetalID+VD –net donorconcentration NDnet acceptorconcentration NAFor simplicity, assume thatthe doping profile changes abruptly at the junction.cross-sectional area ADFigure 0.1 Schematic symbol and water model of a pn-diodeWater Model of Diode RectifierSimplistic Simplistic view of why a pn-diode conducts differently in forward and reverse bias: When the p side is made positive with respect to the n side (forward bias), the positively charged holes move toward the negatively charged electrons, and they recombine. Then more carriers flow in from the contacts. In reverse bias, the holes and the electrons move away from each other, leaving no mobile carriers in the middle – hence, the diode has an insulator in its middle region and no current flows through.Summary: pn-Junction Diode I-V•Under forward bias, current increases exponentially with increasing forward bias•Under reverse bias, a potential barrier in the middle of the junction is increased, so that negligible carriers flow across the junctionID(A)VD(V)The net result is an I-V curvethat looks like this, with typically nA currents in thereverse direction (VD < 0), andmA or more in the forwarddirection (VD > 0)0.7 Vfor Si|reverse biasforward bias•An ideal diode passes current only in one direction. •An ideal diode has the following properties:• when ID > 0, VD = 0• when VD < 0, ID = 0Ideal Diode Model of pn DiodeID(A)VD(V)ID+VD –+VD –IDCircuit symbol I-V characteristicDiode behaves like a switch: • closed in forward bias mode • open in reverse bias modeSwitch modelLarge-Signal Diode Modelreverse biasforward biasID(A)VD(V)ID+VD –+VD–IDCircuit symbol I-V characteristicSwitch modelVturn-on+Vturn-onRULE 1: When ID > 0, VD = Vturn-onRULE 2: When VD < Vturn-on, ID = 0Diode behaves like a voltage source in series with a switch: • closed in forward bias mode • open in reverse bias mode For a Si pn diode, Vturn-on 0.7 VApplication Example: Rectification using the ideal diode modelvs(t)++vR(t)–vs(t)tRCvR(t)t20 : 1120VFigure 0.7 Regulated power supplyMC78052500F1FregulatorTo get a really steady voltage out we can add an integrated circuit regulator to the circuit.ba0.7 VFigure 0.1 Battery and resistor circuits with (a) forward- and (b) reverse-biased diodes. PotentialPotential plots for forward- and reverse-biased diodes in series with a voltage source and a resistorFigure 0.1 Diode circuit with alternating supply volt ageVi n-+Time (s)Vi n,Vo utVi nVi n,Vo ut (volts)Vo ut+-43210-1-2-3-4Clipping circuit using a pn-diodeFigure 0.1 The Zener diodeIDVDDiode SymbolIdealized diodecharacteristicDiode turn-onvoltageZener diode I–V characteristicVDIDZenerZener breakdownvoltageOne uses the reverse-bias breakdown voltage as a voltage reference in some circuitsVaractor diodeLight-emitting diode (LED)Screen Printedcontact fingerSingle-layerantireflectioncoatingn+PfingerContactDouble-layerantireflectioncoatingP+P+ Rearn+ dopingOxideSolar cells – two designsShort-circuitcurrentOpen-circuitvoltagelight-currentMaximumPowerIV0 0.5light-currentV o l t a g eP h o t o v o l t a i cju n c t i o n1-0.5-1Dark characteristicg e n e r a t e dgeneratedVISolar cell with load and its I-V characteristic•The basic building block in digital ICs is the MOS transistor, whose structure contains reverse-biased diodes.–pn junctions are important for electrical isolation of transistors located next to each other at the surface of a Si wafer. –The junction capacitance of these diodes can limit the performance (operating speed) of digital circuitsWhy are pn Junctions Important for ICs?p-type Sin nnnnregions of n-type SiNo current flows if voltages are applied between n-type regions, because two pn junctions are “back-to-back”n-regionn-regionp-region=> n-type regions isolated in p-type substrate and vice versaDevice Isolation using pn Junctionsababp-type Sin nTransistor An nTransistor BWe can build large circuits consisting of many transistors without worrying about current flow between devices. The p-n junctions isolate the transistors because there is always at least one reverse-biased p-n junction in every potential current
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