Physics 160Lecture 12R. JohnsonMOSFET vs JFET• Insulated gate can be at any voltage relative to the source, but body must be reverse biased (or zero) w.r.t. the source!• Diode junction gate must be reverse biased (or zero) relative to source!• Depletion mode only!• Both enhancement-mode and depletion-mode are possible, but most often enhancement-mode.•Zero DC gate current!• Slight DC gate leakage current.• Found both as discrete transistors and in ICs (but not VLSI).Zero DC gate current!• Most widely used as a switch for VLSI digital logic circuits.• Discrete devices are usually only df hi h t i t d– Current sources or input transistors in op-amps, for example.• This is the only type used in your used for high power transistors and for analog switches.• Easily destroyed by static electricity!•4-terminal device:yyp yFET-1 lab.• 3-terminal device:n-channel JFETp-channel JFETGSGBDBMay 8, 2014 Physics 160 2n-channel MOSFETDSp-channel MOSFETN vs P and JFET vs MOSFETNote: IDSSis the drain tith tRemember, you must not forward current with gate connected to source.bias the JFET gate junction!May 8, 2014 Physics 160 3Dependence of Current on VDSSmall VDS: channel looks ~ like a resistor (“linear”region).Large VDS: channel looks ~ like a current source (“saturated”resistor ( linear region).a current source ( saturated region).JFET curvesIDDifferent values of VGSDifferentCareful! “Saturated” has opposite meanings between FETs and BJTsIDIDDifferent values of VGSbetween FETs and BJTs.May 8, 2014 Physics 160 4VDSVDSVN0106IDNMOS FETICurrent Source!Different values of VGSIDSaturated FETIDSatu ated2N3904VDSDifferentVDSBJTDifferent values of VBEICSaturated BJTNote how Zoutof collector (or drain) goes down as IC(or ID) goes up for BJT (or FET).ICSaturated BJTp()Less slopeMay 8, 2014 Physics 160 5VCEVCE2212DSDSTGSDVVVVkI “Linear” region2TGSDVVkI Saturation region. Quadratic dependence on VGS, notexponential as in BJTs!exponential as in BJTs!IDN-channel MOSFETMOSFETNote how VDS needs to be quite large (compared with a BJT), especially for large current, to be in the saturation (normalMay 8, 2014 Physics 160 6VDSsaturation (normal operation) region.JFET Current SourcesYou can buy JFETs with the gate and source tied together internally, making a simple-to-use, 2-terminal current source.LoadJFETJFETUpper FET musthave larger IDSSJFETLoadLoadJFETIDSSJFET(The upper FET has a more negative VGS, but the JFETGSSIVR LoadJFETRStwo FETs here must carry the same current.)RSDIMay 8, 2014 Physics 160 7SimpleCascodeBetter Zout2N5485 N-Channel JFETMin MaxLdJFETLoad2 mA08VRS0.8 VMay 8, 2014 Physics 160 8 400mA2V 8.0DGSSIVRJFET Current SourceJ31 053pA6.869mAV1V1J31 381 A4.254mA15.00VIDSSJ2N5485-1.053pAV115Vdc6.869mAV115Vdc4.254mAR1125J2N5485-1.381pA531.8mV 125mA4V5.0RMay 8, 2014 Physics 160 9DC sweep with no source resistor k3565.679.6712outIVZIDVDSMay 8, 2014 Physics 160 10DC sweep with RS=125 ohm k8316.422.4712outIVZIDVDSMay 8, 2014 Physics 160 11Measuring the FET Threshold Voltage2TGSDVVkI DkTGSIVV1Measure IDas a function of VGSin the saturationkGSVThi i thithe saturation region.Plot VGSvstheTVThis is something that you will do in the lab next week.Plot VGSvsthe square root of ID, and extrapolate to zero current.I0May 8, 2014 Physics 160 12Physics 160 12DIMeasuring the FET Threshold Voltage2TGSDVVkI DkTGSIVV1Measure IDas a function of VGSin the saturationkthe saturation region.Plot VGSvs thePlot VGSvs the square root of ID, and extrapolate to zero current.fMay 8, 2014 Physics 160 13Example from the textbook, but note that the x,y scales are interchangedMOSFET Subthreshold RegionTGSVVDkeIFor very low VGSthe channel doesn’t really exist, and the FET Dsaturationsub-thresholdy,conducts from drain to source via thermally excited minority carriers, giving an exponential Note: all the V and I scales depend on thedependence.MOSFETs are not often operated scales depend on the FET type and geometry (e.g. length and width of the channel)pin this region. (Exception: large, low-noise amplifier input FETs, used in SCIPP labs for silicon-strip dt t hit thi i )channel).May 8, 2014 Physics 160 14detectors, push into this region.)1 nA 1 mA 10 ATransconductance• Amplifiers and current sources operate in the “saturation” region of a FET (don’t confuse with the saturation region of a BJT, which is where youdon’twant to operate a BJT amplifier).which is where you don twant to operate a BJT amplifier).2TGSDVVkI1TBEVVSCeIIFET BJT TGSDVVkI1SCeIIVT =25 mVVT ~1 V DdIVVSCIdITGSGSDmVVkdVdIg 2TBEVVTSBECmeVIdVdIgCI1DmkIg 2eTCmrVIg1The FET transconductance improves only as the square root of May 8, 2014 Physics 160 15pyqcurrent, while the BJT goes linear with current. In general, the BJT will make a more powerful amplifier (voltage or current amp).JFET AmplifiersSource FollowerCommon-Source AmpVDDGenerallytheperformanceisNote thatZdoesnotJFETCiRDVDDGenerallytheperformanceispoor compared with a BJTamp, except for the inputimpedance.ForaJFETitisspectacularlyhigh!Note that Zoutdoes notdepend on the output impedance of the driving voltage source.JFETCinCin JFETspectacularlyhigh!RSRGRSRGCSRG can be a very large resistance!SRZ1outRgGainMay 8, 2014 Physics 160 16SmgoutDmRgGainJFET Amplifiers• Typically, the place you want to use a JFET amplifier is where you need very high input impedance, for example because your signal source has a very high impedance (cannot deliver muchsignal source has a very high impedance (cannot deliver much current).– Probably best used in differential amps or source-followers, not common-source amps.• Another example is where the base current of a bipolar transistor will cause a significant error.g– The LF411 Op-amp that you will soon use in several circuits uses JFETs at its inputs. This is very nice, because the current flowing into the inputs is negligible in all cases.• Otherwise, bipolar transistors will usually give much better performance in terms of gain, predictability, etc.May 8, 2014 Physics 160 17JFET Amplifiers• You need to keep in mind:– The gate-source junction must be reverse biased at all times (or at worst, zero volts).worst, zero volts).– The gate does need some bias current,