Field Effect Transistor

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Transcript of Field Effect Transistor

  • **Field Effect Transistor

    AEI302.31 TO 33Field effect Transistor is a semiconductor device which depends for its operation on the control of current by an electric Field

    AEI302.31 TO 33

  • **Field Effect TransistorFET has several advantages over BJT

    Current flow is due to majority carriers onlyImmune to radiationHigh input resistanceLess noisy than BJTNo offset voltages at zero drain current High thermal stability

    AEI302.31 TO 33

    AEI302.31 TO 33

  • *JFET Symbol

    EC-302.31 t0 32 N Channel FET P Channel FET Fig 4 . JFET symbolsAEI302.31 TO 33

    EC-302.31 t0 32

  • FETJFETMOSFETN-channelp-channelDepletion typeEnhancement typep-channelN-channelp-channelN-channelClassification of Field Effect TransistorsAEI302.31 TO 33

  • *JFET

    Based on the construction JFETS are of Two types

    N Channel FET

    P Channel FET

    AEI302.31 TO 33

  • **Construction

    DrainSourceGATEFor a N Channel FET an N type silicon Bar is used Heavily doped P type material is deposited on either side of the bar to form GATE The two ends of the bar are known as Source and Drain Fig 3. Construction of N Channel FET EC-302.31 t0 32AEI302.31 TO 33

  • **Construction of FET Source : The source is the terminal through which majority carriers enter the Silicon BarDrain : Terminal through which Majoroty carriers leave the bar Gate: controls Drain current and is always reverse biased

    EC-302.31 t0 32AEI302.31 TO 33

  • **Construction of FET Analogy : The operation of FET can be compared to the water flow through a flexible pipe When One end is pressed the cross sectional area decreases hence water flow decreases In a FET drain is similar to outlet Gate is similar to control in the figure 2

    AEI302.31 TO 33

  • **Principle *

    when the pipe is pressed at one end water flow decreases Fig2. water Pipe analogy Inlet ( Source) Outlet (Drain) Control (Gate) AEI302.31 TO 33

  • **Operation Principle : To control the drain current FET makes use of channel formed in by Space charge region between Gate and the bar By increasing the reverse bias the width of space charge region decreases As a result the channel Resistance increases The Drain current decreases AEI302.31 TO 33

  • **Working

    DrainSourceGATESource is connected to -ve Of the battery 1Drain is connected to +ve of the battery Gate is reverse Biased by battery 2 A channel is established along the length of the bar because of Space charge Region As we increase the Reverse bias on Gate (VGS)The channel width decreases Space charge Region Channel AEI302.31 TO 33

  • **Working

    DrainSourceGATEAs we increase the Reverse bias on Gate (VGS)The channel width decreases It is Possible to adjust the bias such that Drain current becomes zero The Bias voltage at which Drain current becomes Zero is Known as pinch off voltage AEI302.31 TO 33

  • *Working

    N Channel FETP Channel FET Source connected to -VEDrain Connected to +ve Gate connected to ve (Reverse Biased)Source connected to +VEDrain Connected to -ve Gate connected to +ve (Reverse Biased)AEI302.31 TO 33

  • ** Working when Voltage is applied between source and Drain majority carriers move through the channel between depletion region The value of Drain current is maximum when no external voltage is applied between gate and source When gate to source reverse bias increases the depletion region widens and channel width decreases hence Drain current decreases

    *

    AEI302.31 TO 33

  • ** Working Hence Drain current decreasesWhen gate to source voltage is increased further The channel completely closes This is called pinch off region This reduces Drain current to Zero The Gate to source voltage at which the Drain current is zero is called Pinch off Voltage

    *

    AEI302.31 TO 33

  • **P Type and N type FETs

    *

    P Channel FET

    Current carriers are holesMobility of holes is poor More noiseLow Transconductance

    N Channel FET

    Current carriers are ElectronsMobility of electrons is almost twice that of Holes in P channel FETLow input NoiseLarge Transconductance AEI302.31 TO 33

  • **JFET Parameters

    *

    Electrical behavior is described in terms of the parameters of the Device. They are obtained from the characteristics. Important Parameters for FET areDC Drain resistanceAC drain ResistanceTransconductance

    AEI302.31 TO 33

  • **JFET Parameters

    *

    DC Drain resistance : Defined as Ratio of Drain to source Voltage VDS to Drain current ID. Also called static or Ohmic Resistance Mathematically

    RDS= VDS/IDAEI302.31 TO 33

  • **JFET Parameters

    *

    AC Drain resistance : Defined as the resistance between Drain to source when JFET is operating in Pinch off Region or saturation Region Mathematically

    When VGS is constant AEI302.31 TO 33

  • **JFET Parameters

    *

    Transconductance (gm): It is given by the ratio of small change in drain current to the corresponding change in the Gate to source Voltage VGS. Also known as Forward Transmittance Mathematically

    AEI302.31 TO 33

  • **FET and BJT

    *

    BJT 1. Bipolar deviceCurrent controlled deviceLow input impedanceLow thermal stabiltyLower switching speedsMore noisyDiffuicult to fabricate on IC

    FETUni polar device Voltage controlled DeviceHigh input impedance (in Mega ohms)Better thermal stabilityHigh switching speedsLess NoisyEasy to fabricateAEI302.31 TO 33

  • **Drain Characteristics

    *

    Drain characteristics show the relation between the drain to source voltage and VDS and drain current ID ABAvalanche BreakdownIDVDS- VGSOHMIC Region Pinch Off Region Breakdown Region VGS= 0AEI302.31 TO 33

  • **Drain Characteristics

    *

    Drain characteristics show the relation between the drain to source voltage and VDS and drain current ID ABAvalanche BreakdownIDVDS- VGSOHMIC Region Pinch Off Region Breakdown Region VGS= 0When VDS=0 , ID=0 as VDS increases ID also increases up to Knee Point AThis region is Known as OHMIC region In the region AB Drain current increases as inverse square law rate with Drain to source Voltage At the Drain to source Voltage corresponding to point B Channel width reduces to a minimum value and is known as pinch off AEI302.31 TO 33

  • **Drain Characteristics

    *

    Drain characteristics show the relation between the drain to source voltage and VDS and drain current ID ABAvalanche BreakdownIDVDS- VGSOHMIC Region Pinch Off Region Breakdown Region VGS= 0Pinch Off Region

    This region is Shown by BC known as saturation or constant current region Drain current remains at its maximum value IDSSDrain current ID is given byCAEI302.31 TO 33

  • **Drain Characteristics

    *

    Drain characteristics show the relation between the drain to source voltage and VDS and drain current ID ABAvalanche BreakdownIDVDS- VGSOHMIC Region Pinch Off Region Breakdown Region VGS= 0Breakdown Region

    This region is Shown by CD Drain current increases Rapidly with VDSThe device gets damaged due to avalanche Breakdown mechanism CDEC-302.31 t0 32AEI302.31 TO 33

  • DISADVANTAGES OF FET OVER BJTFETs have a drawback of smaller gain bandwidth product compared to BJT.

    AEI302.31 TO 33

  • The high input impedance, low output impedance and low noise level make FET for superior of the bipolar transistor.

    ApplicationsAs a buffer amplifier which isolates the preceding stage from the following stage.

    Features AEI302.31 TO 33

  • Phase shift oscillators: The high input impedance of FET is especially valuable in phase shift oscillator to minimize the loading effect.

    In voltmeters: The high input impedance of FET is useful in voltmeters to act as an input stage.FET APPLICATIONSAEI302.31 TO 33

  • AEI302.34*JFETN CHANNEL JFETP CHANNEL JFET

    AEI302.34

  • AEI302.34*Classification of MOSFETsMOSFETDepletion typeEnhancement typep-channelN-channelp-channelN-channel

    AEI302.34

  • AEI302.34*Metal oxide semiconductor field effect transistor (MOSFET)MOSFET is an important semiconductor device and is widely used in many circuit application.

    The input impedance of a MOSFET is much more than that if a FET because of very small leakage current.

    MOSFETs has much greater

    commercial Importance than JFET

    AEI302.34

  • AEI302.34*The MOSFET can be used in any of the circuits covered for the FET.

    Therefore all the equations apply equally well to the MOSFET and FET in amplifier connections.

    MOSFET (contd)

    AEI302.34

  • AEI302.34*

    MOSFETs uses a metal

    gate electrode (instead of p-n junction in JFET), separated from the semi conductor by an Insulating thin layer sio2 to modulate the resistance of the conduction channel.

    MOSFET (contd)

    AEI302.34

  • AEI302.34* It is also called as

    insulated gate FET (IGFET)MOSFETs operates both

    in the depletion mode as well as an the enhancement modeMOSFET (contd)

    AEI302.34

  • AEI302.34*N channelP channelCIRCUIT SYMBOLS OF MOSFET

    AEI302.34

  • AEI302.34*There is only a single p-region. This is called substrate.

    A thin layer of metal oxide is deposited over the left side of the channel. A metallic gate is deposited over the oxide layer. As silicon dioxide is an insulator, therefore a gate is insulated from the channel. For this reason MOSFET is some time