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Chapter 6: Field-Effect Transistors www.getmyuni.com FETs vs . BJTs BJTsFETs
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 Types TypesFETTypes
••JFET: Junction FET
••MOSFET: Metal–Oxide–Semiconductor FET
DD--MOSFETMOSFETMOSFET: DltiMOSFETDepletion 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
••nn--channelchannel ••pp--channelchannel
The n-chliidldhannel is more widely used.
There are three terminals:
••Drain (D) and SourceSource (S) are connected to the n-channel ••GateGate (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 sourcesourceThe of water pressure is the accumulation of electrons at the negatilfhdiive pole of the drain-source voltage.
The d rai iThd n 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, V DS 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: VVJFET GSGS = 0 VV=
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 l arge 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 JFET Oppgpgerating Characteristics
AVAs VGS btithbecomes more negative, the depletion region increases.
Electronic Devices and Circuit Theory, 10/e 1010 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 JFET Oppgpgerating Characteristics: VoltageVoltage--Controlled ResistorResistorControlled
The region to the left of the pinch-off point is called the oocego.ocego.hmic region.o .
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 1212 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 Characteristics CharacteristicsChannelJFET
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 JFETJFET Transfer Characteristics CharacteristicsTransfer
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 DSSD ⎜ ⎟ ⎝ V P ⎠
Electronic Devices and Circuit Theory, 10/e 1616 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Transfer CurveCurveJFET
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 www.getmyuni.comPlotting the JFET Transfer CurveCurvePlotting
Using IDSS and Vp (VGS(off)) values found in a specification sheet, the transfer curve can be plotted according to these three steps:
Step 11Step 2 ⎛ VGS ⎞ = DSSD ⎜1II − ⎟ ⎝ VP ⎠ Solving for VGS = 0V ID = IDSS
Step 22Step 2 ⎛ VGS ⎞ DSSD ⎜1II −= ⎟ ⎝ VP ⎠ Solving for VGS = Vp (VGS(off)) ID = 0A
S3S3Step 3S 3 2 ⎛ VGS ⎞ Solving for VGS = 0V to Vp DSSD ⎜1II −= ⎟ ⎝ VP ⎠
Electronic Devices and Circuit Theory, 10/e 1818 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Specifications SheetJFET Sheet
Electrical CharacteristicsElectrical Characteristics
Electronic Devices and Circuit Theory, 10/e 1919 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com JFET Specifications SheetJFET Sheet
Maximum Ratings
more…more…
Electronic Devices and Circuit Theory, 10/e 2020 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Case and Terminal IdentificationIdentificationCase
Electronic Devices and Circuit Theory, 10/e 2121 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Testinggg JFETs
•• Curve TracerTracerCurve
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 MOSFETsMOSFETs
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 2323 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Depppletionletion--TTypypype MOSFET Construction
The DrainDrain (D) and SourceSource (S) connect to the to n-doppged regions. These n- doped regions are connected via an n- channel. This n-channel is connected to the Gate (G) via a thin insulating layer
of SiO2.
The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SubstrateSubstrate (SS).
Electronic Devices and Circuit Theory, 10/e 2424 Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 0745 • All rights reserved. www.getmyuni.com Basic MOSFET OperationBasic Operation
AdepletionA depletion-type MOSFET can operate in two modes:
•• Depletion modemodeDepletion •• Enhancement mode modeEnhancementmode
Electronic Devices and Circuit Theory, 10/e 2525 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com DD--Type MOSFET in Depletion ModeType Mode
Depletion ModeModeDepletion
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 ⎞ DSSD ⎜1II −= ⎟ ⎝ VP ⎠
Electronic Devices and Circuit Theory, 10/e 2626 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com DD--Type MOSFET in Enhancement Mode ModeType
Enhancement ModeModeEnhancement
• VGS > 0 V • ID increases above IDSS • The formula used to plot the transfer curve still applies: 2 ⎛ VGS ⎞ DSSD ⎜1II −= ⎟ ⎝ VP ⎠
Note that VGS is now a positive polarity
Electronic Devices and Circuit Theory, 10/e 2727 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com pp--ChannelChannel DD--TTypypype MOSFET
Electronic Devices and Circuit Theory, 10/e 2828 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com DD--TType MOSFET S yypyymbols
Electronic Devices and Circuit Theory, 10/e 2929 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Specification SheetSpecification Sheet
Maximum RatingsRatingsMaximum
more…more…
Electronic Devices and Circuit Theory, 10/e 3030 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Specification SheetSpecification Sheet
Electrical CharacteristicsCharacteristicsElectrical
Electronic Devices and Circuit Theory, 10/e 3131 Robert L. Boylestad and Louis Nashelsky www.getmyuni.comEE--TypeType MOSFET Construction
• The DrainDrain (D) and SourceSource (S) connect tthtto the to n-ddidoped regions. These n- doped regions are connected via an n- channel
• The GateGate (G) connects to the p-doped substrate via a thin insulating layer of
SiO2
• There is no channel
• The n-doped material lies on a p-doped substrate that may have an additional terminal connection called the SubstrateSubstrate (SS)
Electronic Devices and Circuit Theory, 10/e 3232 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Basic Oppperation of the EE--TTypypype MOSFET
The enhancementThe enhancement enhancementTheenhancement--type MOSFET operates only in the enhancement mode modetypeMOSFET .
• VGS is always positive
• As VGS increases, ID increases
• As VGS is kept constant and VDS is increased, then ID satt(Iturates (IDSS) and the saturation level,
VDSsat is reached
Electronic Devices and Circuit Theory, 10/e 3333 Robert L. Boylestad and Louis Nashelsky www.getmyuni.comEE--TypeType MOSFET Transfer Curve
TdtTo determ ine ID giiVven VGS: 2 −= TGSD )VV(kI
Where:
VT = threshold voltage or voltage at which the MOSFET turns on
k, a constant, can bdbe determ ine dbd by us ing VDSsat can blldbbe calculated by: values at a specific point and the formula: = − VVV TGSDsat ID(ON) k = 2 GS(ON) − T)V(V
Electronic Devices and Circuit Theory, 10/e 3434 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com p-Channel E-Type MOSFETs
T he The pp--channelchannel enhancement-enhancement-typetype MOSFET is similar to the nn-- channel, except that the voltage polarities and current directions are reversedreversedare .
Electronic Devices and Circuit Theory, 10/e 3535 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com MOSFET Symbols SymbolsMOSFETSymbols
Electronic Devices and Circuit Theory, 10/e 3636 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Specification SheetSpecification Sheet
Maximum RatingsRatingsMaximum
more…more…
Electronic Devices and Circuit Theory, 10/e 3737 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Specification SheetSpecification Sheet
Electrical CharacteristicsCharacteristicsElectrical
Electronic Devices and Circuit Theory, 10/e 3838 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Handling MOSFETsHandling MOSFETs
MOSFETs are very sensitive to static electricity. Because of the very thin
SiO2 layer between the external terminals and the layers of the device, any small electrical discharge can create an unwanted conduction.
PiPiProtection
• Always transport in a static sensitive bag
• Always wear a static strap when handling MOSFETS • • App lly volt age li miti ng d evi ces b et ween th e gat e and source, such as back-to-back Zeners to limit any transient voltage.
Electronic Devices and Circuit Theory, 10/e 3939 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com VMOS DevicesVMOS Devices
VMOS (vertical MOSFET) increases the surface area of the device.
AdvantagesAdvantages
• VMOS devices handle higher currents by providing more surface area to dissipate the heat.
• VMOS devices also have ftfaster sw ithitiitching times.
Electronic Devices and Circuit Theory, 10/e 4040 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com CMOS DevicesCMOS Devices
CMOS (complementary MOSFET) uses a p-channel and n-cceOS;hannel MOSFET; often on the same substrate as shown here.
AdvantagesAdvantages
• Useful in logic circuit designs • Higher input impedance • Faster switchinggp speeds • Lower operating power levels
Electronic Devices and Circuit Theory, 10/e 4141 Robert L. Boylestad and Louis Nashelsky www.getmyuni.com Summary TableTableSummary
Electronic Devices and Circuit Theory, 10/e 4242 Robert L. Boylestad and Louis Nashelsky