Unit 3: Field Effect Transistor (FET) - Subjective Questions

Structure of N-channel JFET:

• It consists of an N-type semiconductor bar (channel) on which two P-type regions are diffused on opposite sides.
• The two P-regions are joined together and form the Gate (G) terminal.
• The two ends of the N-type bar form the Drain (D) and Source (S) terminals.
• The region between the two P-layers is called the channel through which majority carriers (electrons) flow.

Working:

• The Gate-Source junction is always reverse biased, so gate current is negligible and input resistance is very high.
• When and a positive is applied, electrons flow from source to drain producing drain current .
• As reverse gate voltage is increased (made more negative), the depletion regions widen, reducing the effective channel width.
• This reduces the drain current .
• At a particular negative voltage called the pinch-off/cut-off voltage , the channel is completely closed and .

Thus the JFET is a voltage-controlled device where controls .

Structure of N-channel Enhancement MOSFET:

• Built on a lightly doped P-type substrate.
• Two heavily doped N+ regions are diffused which form the Source and Drain.
• There is no physically diffused channel between source and drain initially.
• A thin layer of silicon dioxide () insulates the metal Gate from the substrate.

Working:

• When , no channel exists, so .
• When a positive voltage is applied to the gate (), it repels holes and attracts electrons toward the surface below the oxide.
• When exceeds the threshold voltage , an N-type inversion layer (channel) is induced connecting source and drain.
• Now drain current flows, and increasing enhances the channel and increases .

The drain current in saturation is given by:

Since the channel must be created (enhanced), it is called an enhancement MOSFET.

Structure of N-channel Depletion MOSFET:

• A P-type substrate with two heavily doped N+ regions for Source and Drain.
• Unlike the enhancement type, a physically diffused N-channel already exists between source and drain.
• The metal gate is insulated from the channel by a thin layer.

Working:

• Since a channel already exists, drain current flows even when (this is ).

Depletion Mode ():

• A negative gate voltage repels electrons from the channel, depleting the carriers.
• This reduces the channel conductivity and hence decreases.

Enhancement Mode ():

• A positive gate voltage attracts more electrons, enhancing the channel and increasing .

Thus the D-MOSFET can operate in both depletion and enhancement modes, making it more versatile.

The output characteristics show the variation of drain current versus drain-source voltage for constant .

Regions of Operation:

• Ohmic (Linear) Region:

  • For small , the channel behaves like a resistor.
  • increases linearly with .
  • The FET acts as a voltage variable resistor in this region.

• Pinch-off / Saturation (Active) Region:

  • Beyond pinch-off voltage , becomes nearly constant even if increases.
  • The FET is used as an amplifier in this region.

• Breakdown Region:

  • At very high , the reverse biased gate junction breaks down and increases sharply.
  • The device may get damaged here.

Each curve corresponds to a fixed ; higher negative gives lower .

Transfer Characteristics:

• It is the plot of drain current versus gate-source voltage at constant .
• The curve is parabolic in nature.
• At , (maximum drain current).
• As becomes more negative, decreases.
• At , .

Shockley's Equation:

The relationship between and is given by:

Where:

• = Drain current at (saturation drain current)
• = Gate-source cut-off voltage
• = Applied gate-source voltage

This equation shows the square-law (parabolic) relationship between and , which is useful for biasing and amplifier design.

1. Drain Resistance ():

• It is the ratio of change in drain-source voltage to change in drain current at constant .

• It is the AC output resistance and is usually very high.

2. Transconductance ():

• It is the ratio of change in drain current to change in gate-source voltage at constant .

• Unit: Siemens (S) or mho. It measures the control of gate voltage over drain current.

3. Amplification Factor ():

• It is the ratio of change in drain-source voltage to change in gate-source voltage at constant .

Relationship:

This is derived as:

Summary: MOSFET offers higher input resistance and can operate in both modes, while JFET is simpler but restricted to depletion mode operation.

Concept:

• In the ohmic region (below pinch-off), the JFET behaves like a resistor whose value can be controlled by the gate voltage .

Working:

• For small values of , the drain-source path acts like a resistance.
• The drain resistance depends on and is approximately given by:

• Where is the resistance at .
• As is made more negative, the channel narrows and increases.

Applications:

• Automatic Gain Control (AGC) circuits
• Voltage controlled attenuators
• Tone/volume control in audio systems
• Electronic gain adjustment

Thus the FET acts as a resistor whose value is electronically adjustable through voltage.

A MOSFET is widely used as an electronic switch because it can be turned fully ON or OFF using the gate voltage.

Circuit Setup:

• An N-channel enhancement MOSFET is connected with a load resistor between and the drain.
• The input control signal is applied at the gate.

OFF State (Cut-off):

• When (threshold voltage), no channel is formed.
• , so the MOSFET acts like an open switch.
• Output voltage .

ON State (Saturation/Triode):

• When (sufficiently high), a channel forms and the MOSFET conducts heavily.
• The MOSFET acts like a closed switch with very low .
• Output voltage V.

Advantages as a switch:

• Very fast switching speed
• Low power consumption (voltage controlled)
• No moving parts

Applications: Digital logic gates, power switching, motor control, and PWM circuits.

Advantages of FET over BJT:

• High Input Impedance: FET has very high input resistance ( to ) because its input junction is reverse biased/insulated, whereas BJT has low input impedance.

• Voltage Controlled Device: FET is controlled by input voltage, while BJT is controlled by input current.

• Unipolar Device: FET operation depends on only one type of carrier (majority carriers), so it has less noise compared to BJT (bipolar).

• Low Noise: No recombination noise, hence suitable for input stages of amplifiers.

• Thermal Stability: FET has a negative temperature coefficient, so it does not suffer from thermal runaway.

• Small Size: Occupies less space, ideal for ICs (VLSI).

• Simpler Fabrication: Easier to fabricate in integrated circuits.

• No Offset Voltage: FET acts as an excellent analog switch with zero offset voltage.

• High Power Gain

Disadvantage: FET has a smaller gain-bandwidth product and lower transconductance than BJT.

Transfer Characteristics:

• Enhancement: Curve starts from and increases as increases.

• Depletion: Curve passes through at and extends into both negative and positive regions (follows Shockley's equation).

Given: , ,

Step 1: Drain Current using Shockley's Equation

Step 2: Maximum Transconductance

Step 3: Transconductance at

Results: , ,

Pinch-off Voltage ():

• It is the value of drain-source voltage (at ) at which the drain current becomes almost constant and the depletion regions nearly touch each other.
• Beyond this point, further increase in does not significantly increase .
• It marks the boundary between the ohmic region and the saturation (active) region.

(Drain-Source Saturation Current):

• It is the maximum drain current that flows through the JFET when the gate is shorted to the source, i.e., .
• It is measured in the saturation region.
• It is an important parameter used in Shockley's equation:

Note: Numerically, , but is measured along the axis while is measured along the axis.

JFET Symbols:

• N-channel JFET: Vertical channel line with gate arrow pointing inward (toward the channel). Terminals: Drain (top), Source (bottom), Gate (side).
• P-channel JFET: Same as above but the gate arrow points outward (away from channel).

MOSFET Symbols:

• Depletion MOSFET: Uses a solid (continuous) vertical line for the channel between drain and source, showing that a physical channel already exists.
• Enhancement MOSFET: Uses a broken (dashed) vertical line, showing that the channel does not exist until induced by gate voltage.
• In both, the substrate arrow direction indicates the type:
  • Arrow pointing in → N-channel
  • Arrow pointing out → P-channel

Key Points:

• The gate in MOSFET is shown separated from the channel to represent the insulating layer.
• The arrow direction always identifies the channel type (N or P).

Reason for Very High Input Resistance:

• In a MOSFET, the gate terminal is separated from the channel by a thin layer of silicon dioxide (), which is an insulator.
• Because of this insulation, no current flows from the gate into the channel.
• Hence the input (gate) resistance is extremely high, of the order of to .

Consequence:

• The gate draws almost zero current, so the MOSFET is a purely voltage-controlled device with negligible input power.

Precaution while handling:

• The thin oxide layer can be easily damaged by static electricity (ESD), as even a small static charge can develop a high voltage across the thin insulator and puncture it.
• Therefore:
  • Store MOSFETs with leads shorted (using conductive foam or a ring).
  • Use anti-static wrist straps and grounded work benches.
  • Avoid touching the gate terminal directly.

Modern MOSFETs include built-in Zener protection diodes to guard against ESD.

For an N-channel Enhancement MOSFET, there are three regions of operation:

1. Cut-off Region:

• Condition:
• No channel is formed, so .
• The MOSFET acts as an OFF switch.

2. Triode / Ohmic (Linear) Region:

• Condition: and
• The device behaves like a voltage-controlled resistor.
• Drain current:

3. Saturation (Active) Region:

• Condition: and
• Drain current becomes nearly independent of .

• Used for amplification.

Where is the conduction parameter. For switching applications, the MOSFET operates between cut-off and triode regions.

Note: A Depletion MOSFET can operate in both modes, JFET works only in depletion mode, and Enhancement MOSFET works only in enhancement mode.

CMOS (Complementary MOS) uses both a P-channel and an N-channel enhancement MOSFET connected in series.

Circuit:

• The PMOS is connected to (top).
• The NMOS is connected to ground (bottom).
• Both gates are joined as the input, and the common drain node is the output.

Working:

• When Input = LOW (0):

  • PMOS is ON, NMOS is OFF.
  • Output is connected to → Output = HIGH (1).

• When Input = HIGH (1):

  • PMOS is OFF, NMOS is ON.
  • Output is connected to ground → Output = LOW (0).

Thus it performs the NOT (inverter) logic function.

Main Advantage:

• In both stable states, one transistor is always OFF, so there is almost no static power consumption.
• CMOS consumes power only during switching, making it ideal for low-power digital ICs and VLSI.

FET as a Unipolar Device:

• The operation of a FET depends on the flow of only one type of charge carrier — the majority carriers.
  • In an N-channel FET, only electrons carry the current.
  • In a P-channel FET, only holes carry the current.
• Since a single (uni) polarity of carrier is involved, it is called a unipolar device.

Difference from Bipolar Transistor (BJT):

• In a BJT, current conduction involves both majority and minority carriers (electrons and holes), hence it is bipolar.
• BJT depends on minority carrier injection across junctions, which introduces recombination and more noise.
• FET has no minority carrier storage, giving:
  • Faster switching in some applications
  • Lower noise
  • No minority carrier storage delay

Summary: FET → majority carriers only (unipolar); BJT → both carriers (bipolar).

Given: , ,

Step 1: Overdrive Voltage

Step 2: Drain Current in Saturation

Using the saturation equation:

Step 3: Saturation Voltage

The MOSFET enters saturation when:

Results:

• Drain current

For , the device stays in saturation and remains approximately constant at .