FET Complete

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  • Field Effect TransistorsField Effect TransistorsCHAPTER 6

  • IntroductionIntroduction Two main types of FET:

    - JFET Junction field effects transistor- MOSFET Metal oxide semiconductor field effect transistor

    - D-MOSFET ~ Depletion MOSFET- E-MOSFET ~ Enhancement MOSFET

    Similarities:-Amplifiers-Switching devices-Impedance matching circuits

    Differences:-FETs are voltage controlled devices whereas BJTs are currentcontrolled devices.-FETs also have a higher input impedance, but BJTs have highergains.-FETs are less sensitive to temperature variations and more easilyintegrated on ICs.- FETs are also generally more static sensitive than BJTs.

  • Construction and characteristics of JFETConstruction and characteristics of JFET

    N-channel device will appear as theprominent device with paragraph andsection devoted to the impact of using ap-channel.

    Major part of structure is n-type material. Top of the n-type channel is connected

    through an ohmic contact to a terminalreferred to as the drain (D)

    The lower end-connected through anohmic contact to a terminal referred assource (S)

    P-type materials are connected togetherand to the gate (G) terminal.

    JFET has two p-n junctions under no-biasconditions.

  • Construction and characteristics of JFETConstruction and characteristics of JFET

    JFET operation can be compared to a water spigot:

    The source of water pressure accumulated electrons at the negative pole ofthe applied voltage from Drain to Source

    The drain of water electron deficiency (or holes) at the positive pole of theapplied voltage from Drain to Source.

    The control of flow of water Gate voltage that controls the width of the n-channel, which in turn controls the flow of electrons in the n-channel fromsource to drain.

  • N-Channel JFET Circuit Layout

    Construction and characteristics of JFETConstruction and characteristics of JFET

  • JFET Operating CharacteristicsJFET Operating Characteristics

    There are three basic operating conditions for a JFET:

    A. VGS = 0, VDS increasing to some positive


    B. VGS < 0, VDS at some positive value

    C. Voltage-Controlled Resistor

  • VVGSGS == 00, V, VDSDS increasing to some positiveincreasing to some positivevaluevalue

    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-channelcombine with holes from p-gate.

    increasing the depletion region, decreasesthe size of the n-channel which increases theresistance of the n-channel.

    But 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.

  • VVGSGS == 00, V, VDSDS increasing to some positiveincreasing to some positivevaluevalue The flow of charge is relatively uninhibited and limited solely by

    the resistance of the n-channel between drain and source.

    The depletion region is wider near the top of both p-typematerials.

    ID will establish the voltage level through the channel.

    The result: upper region of the

    p-type material will be reversed biased by

    about 1.5V with the lower region only

    reversed biased by 0.5V (greater

    applied reverse bias, the wider

    depletion region).

  • VVGSGS == 00, V, VDSDS increasing to some positiveincreasing to some positivevaluevalue IG=0A p-n junction is reverse-biased for the length of the

    channel results in a gate current of zero amperes.

    As the VDS is increased from 0 to a few volts, the current will

    increase as determined by Ohms Law.

    VDS increase and approaches a level referred to as Vp, the

    depletion region will widen, causing reduction in the channel

    width. (p large, n small).

    Reduced part of conduction causes the resistance to increase.

    If VDS is increased to a level where it appears that the 2 depletion

    regions would touch (pinch-off)

  • VVGSGS == 00, V, VDSDS increasing to some positiveincreasing to some positivevaluevalue

    Vp = pinch off voltage.

    ID maintain the saturation level defined as IDSS Once the VDS > VP, the JFET has the

    characteristics of a current source.

    As shown in figure, the current is fixed at ID =

    IDSS, the voltage VDS (for level >Vp) is determined

    by the applied load.

    IDSS is derived from the fact that it is the drain-

    to-source current with short circuit connection

    from gate to source.

    IDSS is the max drain current for a JFET and is

    defined by the conditions VGS=0V and VDS > |


  • VVGSGS == 00, V, VDSDS increasing to some positiveincreasing to some positivevaluevalue

    At the pinch-off point:

    any further increase in VGS does

    not produce any increase in ID.

    VGS at pinch-off is denoted as Vp.

    ID is at saturation or

    maximum. It is referred to as IDSS.

    The ohmic value of the channelis at maximum.

  • Typical JFET operationTypical JFET operation

  • JFET modeling when ID=IDSS, VGS=0, VDS>VP


  • VGS

  • VGS

  • Characteristic curves for NCharacteristic curves for N--channel JFETchannel JFET

  • VoltageVoltage--Controlled ResistorControlled Resistor

    The region to the left of the pinch-off point is called the ohmic region.

    The JFET can be used as a variable

    resistor, where VGS controls the

    drain-source resistance (rd). As VGSbecomes more negative, the

    resistance (rd) increases.








  • And as summary in practicalAnd as summary in practical

  • pp--Channel JFETSChannel JFETS

    p-Channel JFET acts the same as the n-channel JFET,except the polarities and currents are reversed.

  • PP--Channel JFET CharacteristicsChannel JFET Characteristics

    As VGS increases more positively:

    the depletion zone increases

    ID decreases (ID < IDSS)

    eventually ID = 0A

    Also note that at high levels of

    VDS the JFET reaches a

    breakdown situation. ID

    increases uncontrollably if

    VDS> VDSmax.

  • JFET SymbolsJFET Symbols

  • There is a convenient relationship between IDS and VGS. Beyond pinch-off

    Where IDSS is drain current when VGS= 0 and VGS(off) isdefined as VP, that is gate-source voltage that justpinches off the channel.

    The pinch off voltage VP here is a +ve quantity because itwas introduced through VDS(sat).

    VGS(off) however is negative, -VP.








  • II--V characteristicsV characteristics

  • II--V characteristicsV characteristics

  • The transconductance curveThe transconductance curve

    The process for plotting trans-conductancecurve for a given JFET:

    Plot a point that corresponds to value ofVGS(off).

    Plot a point that corresponds to value ofIDSS.

    Select 3 or more values of VGS between 0 Vand VGS(off). For value of VGS, determine thecorresponding value of ID from

    Plot the point from (3) and connect all theplotted point with a smooth curve.

  • Transfer CharacteristicsTransfer Characteristics

    The transfer characteristic of input-to-output is not asstraight forward in a JFET as it was in a BJT.

    In a BJT, indicated the relationship between IB (input)and IC (output).

    In a JFET, the relationship of VGS (input) and ID (output) isa little more complicated:



    GSDSSD )



  • Transfer CharacteristicsTransfer Characteristics

    Transfer Curve

    From this graph it is easy to determine the value of ID for a given value of VGS.

  • Plotting the Transfer CurvePlotting the Transfer CurveShockleys Equation Methods. Using IDSS and Vp (VGS(off)) values found in a specification sheet, the

    Transfer Curve can be plotted using these 3 steps:

    Step 1:

    Solving for VGS = 0V:

    Step 2:

    Solving for VGS = Vp (VGS(off)):

    Step 3:

    Solving for VGS = 0V to Vp:



    GSDSSD )







    GSDSSD )








    GSDSSD )



  • Plotting the Transfer CurvePlotting the Transfer Curve

    Shorthand method

    VGS ID

    0 IDSS

    0.3VP IDSS/2

    0.5 IDSS/4

    VP 0mA

  • Specification Sheet (JFETs)

  • This information is also available on the specification sheet.

    Case Construction and Terminal Identification


    MOSFETs have characteristics

    similar to JFETs and additional

    characteristics that make them

    very useful.

    There are 2 types:

    1. Depletion-Type MOSFET

    2. Enhancement-Type MOSFET

  • DepletionDepletion--Type MOSFET ConstructionType MOSFET Construction

    The Drain (D) and Source (S) connect to the to n-doped 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 SS.

  • DepletionDepletion--Type MOSFETType MOSFETConstructionConstruction

    VGS is set to 0V by the direct

    connection from one terminal to

    the other.

    VDS is applied across the drain-to-