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Chapter 6: Field-Effect Transistors Fets Vs www.getmyuni.com Chapter 6: Field-Effect Transistors www.getmyuni.com FETs vs . BJTsFETs BJTs Similarities:Similarities:Similarities: • Amplifiers • Switching devices • Impedance matching circuits Differences:Differences: • FETs are voltage controlled devices. BJTs are current controlled devices. • FETs have a higher input impedance. BJTs have higher gains. • FETs are less sensitive to temperature variations and are more easily integrated on ICs. • FETs are generally more static sensitive than BJTs. Electronic Devices and Circuit Theory, 10/e 22 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com FET ypesTFET Types FET ypesT : JFET•• :Junction JFET FET ••MOSFET: Metal–Oxide–Semiconductor FET MOSFETDD--MOSFET: MOSFETDltiMOSFETDepletion MOSFET EE--MOSFET:MOSFET: Enhancement MOSFET Electronic Devices and Circuit Theory, 10/e 33 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET ConstructionJFET Construction There are two types of JFETs channel••nn- -channel channel••pp- -channel The n-chliidldhannel is more widely used. There are three terminals: Drain•• (D)DrainceSour and ceSour(S) are connected to the n-channel Gate•• Gate(G) is connected to the p-type material Electronic Devices and Circuit Theory, 10/e 44 Robert L. Boylestad and Louis Nashelsky www.getmyuni.comJFET Operation: The Basic IdeaJFET Idea JFET operation can be compared to a water spigot. The sourceThe source of water pressure is the accumulation of electrons at the negatilfhdiive pole of the drain-source voltage. The dra iThdin oftithltf water is the electron deficiency (or holes) at the positive pole of the applied voltage. The controlThe control of flow of water is the gate voltage that controls the width of the n -channel and , therefore , the flow of charges from source to drain. Electronic Devices and Circuit Theory, 10/e 55 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Oppgpgerating Characteristics There are three basic operating conditions for a JFET: • VGS = 0, VDS increasing to some positive value • VGS < 0, VDS at some positive value • Voltage-controlled resistor Electronic Devices and Circuit Theory, 10/e 66 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Operating Characteristics: VJFET VGSGS = 0 V= V Three things happen when VGS = 0 and VDS is increased from 0 to a more positive voltage • The depletion region between p-gate and n-channel increases as electrons from n-channel combine with holes from p-gate. • Increasing the depletion region , decreases the size of the n-channel which increases the resistance of the n-channel. • Even though the n-channel resistance is increasing, the current ( ID) from source to drain through the n- channel is increasing. This is because VDS is increasing. Electronic Devices and Circuit Theory, 10/e 77 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Oppgpgerating Characteristics: Pinch Off If VGS = 0 and VDS is further increased to a more positive voltage, then the dlidepletion zone gets so large th hiat it pinches offpinches off the n-channel. This sugges ts tha t the curren t in the n- channel (ID) would drop to 0A, but it does just the opposite–as VDS increases, so does ID. Electronic Devices and Circuit Theory, 10/e 88 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Oppgpgerating Characteristics: Saturation: Saturation At the pinch-off point: • Any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as VVpp. • ID is at saturation or maximum. It is referred to as IIDSSDSS. • The ohmic value of the channel is maximum. Electronic Devices and Circuit Theory, 10/e 99 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com pgJFET Operating Characteristics pg AVAs VGS btithbecomes more negative, the depletion region increases. Electronic Devices and Circuit Theory, 10/e 10 10 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Oppgpgerating Characteristics As VGS becomes more negative: • The JFET experiences pinch-off at a lower voltage (VP). • ID decreases (ID < IDSS) even though h VDS iiis increased . • Eventually ID reaches 0 A. VGS at this point is called V p or VGS(off).. AifAlso note that at high levels of VDS the JFET reaches a breakdown siiituation. ID increases uncontrollably if VDS > VDSmax. Electronic Devices and Circuit Theory, 10/e 1111 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com pgJFET Operating Characteristics: pg oltage-V-Controlled oltageV ResistorResistorControlled The region to the left of the pinch-off point is called the . o g e ocoohmic region. o g e oc The JFET can be used as a variable resistor, where VGS controls the drain-source resistance (rd). As VGS becomes more negative, the resistance (r ) increases. d r r = o d 2 ⎛ VGS ⎞ ⎜1 − ⎟ ⎝ VP ⎠ Electronic Devices and Circuit Theory, 10/e 12 12 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com pp--Channel JFETSChannel JFETS The p-channel JFET behaves the same as the n-channel JFET, except the voltage polarities and current directions are reversed. Electronic Devices and Circuit Theory, 10/e 1313 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com pp--Channel JFET CharacteristicsChannelJFETCharacteristics AVAs VGS iitilincreases more positively • The depletion zone increases • ID decreases (ID < IDSS) • Eventually ID = 0 A Also note that at high levels of VDS the JFET reaches a breakdown situation: ID increases uncontrollably if VDS > VDSmax. Electronic Devices and Circuit Theory, 10/e 1414 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com NN--ChannelChannel JFET Syyymbol Electronic Devices and Circuit Theory, 10/e 1515 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com ransferT CharacteristicsJFET JFETTransfer Characteristics ransferT Characteristics The transfer characteristic of input-to-output is not as straightforward in a JFET as it is in a BJT. In a BJT, β indicates the relationship between IB (input) and IC (output). In a JFET , the relationship of VGS (input) and ID (output) is a little more complicated: 2 ⎛ ⎞ ⎜ V ⎟ II= 1− GS D DSS⎜ ⎟ ⎝ V P ⎠ Electronic Devices and Circuit Theory, 10/e 16 16 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Transfer CurveJFET Curve This graph shows the value of ID for a given value of V GS. Electronic Devices and Circuit Theory, 10/e 1717 Robert L. Boylestad and Louis Nashelsky CurveransferT Plotting the JFETwww.getmyuni.com CurveransferT Plotting the JFET Using IDSS and Vp (VGS(off)) values found in a specification sheet, the transfer curve can be plotted according to these three steps: Step 1Step 1 2 ⎛ VGS ⎞ DI= I DSS⎜ − 1 ⎟ ⎝ VP ⎠ Solving for VGS = 0V ID = IDSS Step 2Step 2 2 ⎛ VGS ⎞ DI= I DSS⎜ 1 − ⎟ ⎝ VP ⎠ Solving for VGS = Vp (VGS(off)) ID = 0A S3SStep 3 3 S3 2 ⎛ VGS ⎞ Solving for VGS = 0V to Vp D I= I DSS⎜ 1 − ⎟ ⎝ VP ⎠ Electronic Devices and Circuit Theory, 10/e 18 18 Robert L. Boylestad and Louis Nashelsky Specifications SheetJFETwww.getmyuni.com Specifications SheetJFET Electrical CharacteristicsElectrical Characteristics Electronic Devices and Circuit Theory, 10/e 19 19 Robert L. Boylestad and Louis Nashelsky Specifications SheetJFETwww.getmyuni.com Specifications SheetJFET Maximum Ratings …emor …emor Devices and Circuit Theory, 10/eElectronic 20 20 Nashelskyand Louis L. Boylestad Robert www.getmyuni.com erminal IdentificationTCase and erminal IdentificationTCase and Electronic Devices and Circuit Theory, 10/e 21 21 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Testinggg JFETs •• Curve TracerCurve Tracer A curve tracer displays the ID versus VDS graph for various levels of VGS. •• Specialized FET TestersSpecialized Testers These testers show I DSS for the JFET under test . Electronic Devices and Circuit Theory, 10/e 2222 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com sMOSFET sMOSFET MOSFETs have characteristics similar to JFETs and additional characteristics that make then very useful . There are two types of MOSFETs: •• DepletionDepletion--TypeType •• EnhancementEnhancement--TypeType Electronic Devices and Circuit Theory, 10/e 23 23 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Depppletionletion-yp-TTyp ype MOSFET Construction DrainThe Drain(D)ceSour and ceSour(S) connect to the to n-doppged regions. These n- doped regions are connected via an n- channel. This n-channel is connected to Gatethe (G)Gate via a thin insulating layer of SiO2. The n-doped material lies on a p-doped substrate that may have an additional terminal connectionSubstrate called Substrate (SS). Electronic Devices and Circuit Theory, 10/e 24 24 Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 0745 • All rights reserved. OperationBasic MOSFETwww.getmyuni.com OperationBasic MOSFET AdepletionA depletion-type MOSFET can operate in two modes: •• Depletion modeDepletion mode Enhancement mode•• Enhancement mode Enhancement mode Electronic Devices and Circuit Theory, 10/e 25 25 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com ypeT MOSFET in Depletion ModeDD--Type MOSFET in Depletion Mode ypeT MOSFET in Depletion Mode Depletion ModeDepletion Mode Theec charac ceteris tics are es similar to a JFET. • When VGS = 0V0 V, ID = IDSS • When VGS < 0 V, ID < IDSS • The formula used to plot the transfer curve still applies: 2 ⎛ VGS ⎞ DI= I DSS⎜ 1 − ⎟ ⎝ VP ⎠ Electronic Devices and Circuit Theory, 10/e 26 26 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com ypeT MOSFET in Enhancement ModeDD--Type MOSFET in Enhancement Mode ypeT MOSFET in Enhancement Mode Enhancement ModeEnhancement Mode • VGS > 0 V • ID increases above IDSS • The formula used to plot the transfer curve still applies: 2 ⎛ VGS ⎞ DI= I DSS⎜ 1 − ⎟ ⎝ VP ⎠ Note that VGS is now a positive polarity Electronic Devices and Circuit Theory, 10/e 27 27 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com pp--ChannelChannel DD-yp-TTyp ype MOSFET Electronic Devices and Circuit Theory, 10/e 28 28 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com DD-yp-TTyp ype MOSFET S yymbols y Electronic Devices and Circuit Theory, 10/e 29 29 Robert L.
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