P. Piot, PHYS 375 –Spring 2008Transistors• Bipolar Junction transistors– Principle of operation–Characteristics• Field effect transistors– Principle of operation–CharacteristicsP. Piot, PHYS 375 –Spring 2008Introduction■Fundamental building block of electronics in computers, cellular phone, and more…■Semi-conductor device■Use small voltage or current to control large voltage/current ■Fast response Îtransistor used in many elementary electronic functions including:■Amplification, ■Switch, ■Feedback system, regulation,■Signal modulation, ■Oscillators.■Integrated circuit contains thousands of transistor in very small areas.Radio based on vacuum tubes1stpocket radio based on transistors1956 Nobel price was awarded to William Bradford Shockley , John Bardeen and Walter Houser Brattain for “their researches on semiconductors and their discovery of the transistor effect”P. Piot, PHYS 375 –Spring 2008Transistor types■Two types of transistor (based on two different physical mechanisms:■Field effect transistor■Bipolar Junction transistors.■To 1storder they act as current source■FET ~ voltage-controlled current source■BJT ~ current-controlled current sourceIINCurrent source controlled by a current INOUTIAI⋅=A = current “gain”VINCurrent source controlled by a voltageINOUTVGI⋅=G = transconductance.P. Piot, PHYS 375 –Spring 2008P+PN EBCemittercollectorbaseTransistor PNPECTransistor NPNNNP B+Couplingbetweendiodesdiode « EB »diode « BC »Two coupled PN junctions (or diodes) Ù « transistor effect »diode « EB »diode « BC »TransistorsP. Piot, PHYS 375 –Spring 2008Bipolar Junction Transistor (BJT) Going back to the p-n junctione-“e+”■Electrons moved into the p-type semiconductor■Locally (at the junction interface) there is a recombination hole-electron ■This leave positive ions in the n-type semiconductor and negative ions in the p-type semiconductorP. Piot, PHYS 375 –Spring 2008BJT: N-P-N transistor■In each of the N-layers, conduction can take place (motion of free electrons in conduction band)■In the P-type layer, conduction can take place (movement of free holes invalence band)■In absence of externally applied E-field, depletion zones form at both P-N junction, so no charge move from on layer to the other.P. Piot, PHYS 375 –Spring 2008■Now voltage is applied between collector and base parts of the transistor, with polarity such to increase force pulling N type electron and P-type holes apart■Effect is to widen the depletion zone between Collector and Based■No current flow Îbase-collector diode junction is reversed biased.BJT: N-P-N transistorP. Piot, PHYS 375 –Spring 2008■Now relatively small voltage is across to the emitter-based junction such to forward-biased the junction■Electron from emitter flow toward the based Îcurrent flow across emitter/base junction. ■Elkectron in the experience attractive force from positively biased collector ■Emitter/Collector current with magnitude depending on Emitter-base voltageBJT: N-P-N transistorP. Piot, PHYS 375 –Spring 2008■Kirchoff’ current law imposes■Let’s define the parameter and the current gain■We have■αTis the common base forward short circuit current gain■βFis the forward common emitter current gain (20 to 50)■ An ideal junction would have αT=1, real transistors have 0.95<αΤ<0.99, a value close to unity for thin or weakly doped bases■For a NPN BJT, VC>VEwhile VC<VEfor a PNPCurrent flow in a bipolar junctionP. Piot, PHYS 375 –Spring 2008Saturated mode :VVBE8.0≈VVCE2.0≈BFcIIβ≠ ~0.2VBCE~0.8VSaturated modeCut off mode:0≅BICCCEVV≅0≈CIBCECut off modehFEIBBEC~0.7VIB≅Active modeActive mode :BFcIIβ≈ VVBE7.0≈CCCEVVV<<3.0~BCEVCC= voltage source for C and E. VCEn<Vcc!Operating mode for a NPN transistorP. Piot, PHYS 375 –Spring 2008Active modeBTcIIβ≈ VVBE7.0−≈Cut off mode0≅BI)0(3.0~<<<−CCCEVVVCCCEVV≅0≈CISat, modeVVBE8.0−≈VVCE2.0−≈BTcIIβ≠ BEC~0.7VhFEIBIBBCE≅Active mode~0.2VBCE~0.8VSaturated modeBCECut off modeOperating mode for a PNP transistorP. Piot, PHYS 375 –Spring 2008Characteristics of a bipolar junctionParameters choices● For common base configuration, characteristics : IE(VBE,VBC), IC(VBC,IE)● For common emitter configuration, characteristics : IB (VBE, VCE), IC (VCE, IB)The various operating currents and voltages (IE, IB, VBE,VCE,…) of a transistor are related to each otherSo different equivalent characteristics exist. RERCVEEVCCIEICIBVBEVCBVCEP. Piot, PHYS 375 –Spring 2008Characteristics~ characteristics for a PN junction! Small influence of IC(resp. VCB) −≅ 1expTBEsEVVIIIE (VBE, VCB) :IE(mA)VBE(V)VCB=0 , -150.10.512IC (VCB, IE) :11.52.0IE (mA)VCB(V)0.51.01.5-0.51230Ic (mA)0.5↑BEVECII≈P. Piot, PHYS 375 –Spring 2008Field Effect transistor (FET)● A current (ID) can flow from source S to drain Dvia a “channel” (area located close to the gate): SDchannelGsubstrat (Si)ID VDSVGS● The current flowing though the gate (IG) is small.=> IS= ID!● ID, at constant VDSis controlled by the gate voltage – source (VGS) Ù”electric field effect”● Three terminals : S, D et G, (sometime four: substrat)¾ n-channel FET : current induced by electrons, from S to D¾ p-channel FET: current carried by holes, from S to DP. Piot, PHYS 375 –Spring 2008Field Effect transistor (FET)N-type channelP-type channelP. Piot, PHYS 375 –Spring 2008Typical characteristics()GSVDSDVIPinch-offohmicbreakdownP. Piot, PHYS 375 –Spring 2008()221offoffGSGSGSGSDSSDVVkVVII −=−≅for :satDSDSVV >for :satDSDSVV<Pinch-off regime()DSDSGSGSDVVVVkIoff⋅−−≅22Linear (Ohmic) regime2offGSDSSVIk =PGSsatDSVVV+=ID(mA)VDS(V)24680481216VGS=-1VVGS=0VGS(V)-2 -1.5 -1 -0.5VGS=-1VVGS=0DSSIVGSofftransistor Reverse biasedVPsatDSDSVV >Typical characteristicsP. Piot, PHYS 375 –Spring 2008Differences between FET and BJTz IG<< IBÎ very high input impedance (sometime > 1014Ω)ÎSimpler circuitsz linear regimeÎ slope = f(VGS) Ù variable resistance (nothing equivalent for BJT)Î VDSsat> VCEsat: higher residual voltage in saturated regime. z Saturation regime (active mode)Î IDis controlled by a voltageË transconductance (instead of βF)ÎFrom manufacturing higher dispersion in gmvalue compared to βFgsdmdVdIg =z Different
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