FETs Overview and Operation

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Questions and Answers

What is the role of the gate terminal in a FET?

To act as a second source of voltage

To control the current flow between the source and drain (correct)

To inject charge carriers into the channel

To measure current flow between the source and drain

Which of the following correctly describes how an n-channel FET operates?

It operates without any external voltage applied

It repulses electrons to decrease conductivity

A positive voltage attracts free electrons into the channel (correct)

A negative voltage attracts holes into the channel

What distinguishes JFETs from BJTs?

JFETs are voltage-controlled devices, whereas BJTs are current-controlled devices (correct)

JFETs are controlled by current, and BJTs by voltage

JFETs have no gate terminal, unlike BJTs

There's no difference; they function the same way

What are the two modes of operation for FETs?

Enhancement mode and depletion mode Signup and view all the answers

Why are FETs valued in RF applications?

They provide high input impedance and low noise performance Signup and view all the answers

What are the three terminals of a FET called?

Gate, Drain, Source Signup and view all the answers

What happens to the drain current ID when VGS is made more negative?

The drain current decreases Signup and view all the answers

In which region is the FET device completely OFF?

Cutoff Region Signup and view all the answers

What is the relationship between the gate-source voltage VGS and the pinch-off voltage VP in the cutoff region?

VGS is less than VP Signup and view all the answers

Which point in the UJT’s characteristics signifies the transition from off to on state?

Peak Point Voltage (Vp) Signup and view all the answers

What characterizes the negative resistance region in a Unijunction Transistor (UJT)?

Decreasing current despite increasing emitter voltage Signup and view all the answers

What is the function of a FET used as a series switch when the control voltage is negative?

Acts as an open switch Signup and view all the answers

What represents the point at which the drain current remains constant in a JFET?

Pinch-off voltage Signup and view all the answers

Which method is NOT a way to bias a JFET?

Emitter Bias Method Signup and view all the answers

What does the transconductance (gfs) of a JFET measure?

The change in drain current over gate source voltage Signup and view all the answers

In a common source FET configuration, what type of gain is predominantly observed?

Medium voltage gain with medium current gain Signup and view all the answers

What is a unique feature of MOSFETs compared to JFETs?

Insulated gate from the channel Signup and view all the answers

Which parameter represents the ratio of change in drain source voltage to the change in drain current in JFETs?

AC drain resistance Signup and view all the answers

What is the primary role of the source in a JFET?

To act as the entry point for majority carriers Signup and view all the answers

Which of the following is NOT a characteristic of the common gate FET configuration?

High voltage gain Signup and view all the answers

What is the primary insulating material used in MOSFETs between the gate and the channel?

Silicon dioxide Signup and view all the answers

Which type of MOSFET operates with a negative gate voltage?

Depletion MOSFET Signup and view all the answers

What effect does increasing the negative gate voltage have on the n-channel in a Depletion MOSFET?

Decreases current flow Signup and view all the answers

What is a characteristic feature of the common source FET configuration?

High voltage gain and phase change Signup and view all the answers

In which region does the current ID increase due to a rise in VDS while VGS remains constant in a JFET?

Ohmic region Signup and view all the answers

What happens to the channel resistance as the gate-source voltage (VGS) is increased in the ohmic region?

Resistance increases Signup and view all the answers

What is the main function of the gate in a FET?

To control voltage and current Signup and view all the answers

Which region of a FET operates when the drain current ID is independent of VDS?

Saturation region Signup and view all the answers

In which amplifier configuration does the FET provide a high input impedance but the output and input are in phase?

Common drain/source follower amplifier Signup and view all the answers

Which MOSFET type is characterized by a positive gate voltage enhancing conductivity of the channel?

Enhancement MOSFET Signup and view all the answers

Study Notes

FETs

FET Operation: A semiconductor device with a channel connecting two electrodes (drain & source) controlled by a third electrode (gate)
Types of FETs:
- JFET (Junction Field-Effect Transistor): Operates using the movement of majority carriers (electrons/holes) through a silicon bar with PN junctions.
- MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor): Uses an insulated gate to control channel conductivity.
FET Modes of Operation:
- Enhancement Mode: More voltage at the gate increases channel conductivity.
- Depletion Mode: More voltage at the gate decreases channel conductivity.
JFET Parameters:
- AC Drain Resistance (Rd): Measures the change in drain-source voltage (VDS) per unit change in drain current (ID) at a constant gate-source voltage (VGS).
- Transconductance (gfs): Measures the change in drain current (ID) per unit change in gate-source voltage (VGS) at a constant drain-source voltage (VDS).
- Amplification Factor (µ): Determines the change in drain-source voltage (VDS) per unit change in gate-source voltage (VGS) at a constant drain current (ID)
JFET Biasing Techniques:
- Self-Bias: Uses a resistor (Rs) to provide the bias voltage.
- Voltage Divider Bias: Employs resistors R1 and R2 to create a voltage divider, supplying the bias voltage.
MOSFET Types:
- Depletion MOSFET: Has a built-in channel and can operate with both positive and negative gate voltages.
- Enhancement MOSFET: Requires a voltage at the gate to create a channel.
FET Amplifier Configurations:
- Common Source: Medium input and output impedance, provides voltage gain with 180° phase shift.
- Common Drain (Source Follower): High input impedance, low output impedance, unity voltage gain, and no phase shift.
- Common Gate: Low input impedance, high output impedance, high voltage gain, but low current gain.
FET Characteristics:
- Ohmic Region: Drain current increases with increasing drain-source voltage.
- Saturation Region: Drain current becomes constant and independent of drain-source voltage.
- Cutoff Region: Drain current reaches zero, the device is off.
FET as a Switch:
- ON State: Operating in the saturation region with a low resistance.
- OFF State: Operating in the cutoff region with a high resistance.
FET Applications:
- Amplifiers: Preamplifiers, buffers.
- Power Control: Power MOSFETs for switching applications.
- Sensors: High input impedance makes them suitable for connecting to sensors.
- Oscillators: Dynamic components in oscillators.
- RF Circuits: Amplifiers and mixers due to their high input impedance, low noise, and capability for high-frequency operations.
- Logic Circuits: Replacing BJTs due to lower power consumption, higher speeds, and ability to operate at lower voltages.
- Analog Circuits: Used in operational amplifiers, filters, and regulators.

UJT (Unijunction Transistor)

UJT Structure: A semiconductor device with a single PN junction and three terminals: Emitter (E), Base 1 (B1), and Base 2 (B2).
UJT Characteristics:
- Negative Resistance Region: Emitter current (IE) decreases with increasing emitter voltage (VE) after a peak point.
- Peak Point Voltage (Vp): The voltage at which the UJT switches from OFF to ON.
- Valley Point Voltage (Vv): The minimum voltage needed to keep the UJT conducting once it has been turned on.
- Intrinsic Stand-off Ratio (η): The ratio of resistance between the base 1 (B1) and emitter (E) to the total resistance between base 1 (B1) and base 2 (B2).
UJT Operation:
- OFF State: No voltage applied to emitter.
- ON State: Triggering the UJT with a voltage exceeding the peak point voltage (Vp) initiates conduction.
- UJT Applications:
- Switching Circuits: For timing and oscillation control.
- Relaxation Oscillators: Utilizes the negative resistance region for generating timing signals.

Unijunction Transistor (UJT) Operation

The UJT is a three-terminal semiconductor device with a single p-n junction.
It exhibits a negative resistance characteristic in its emitter-base1 (EB1) junction.
The UJT is initially in its "off" state with the emitter junction reverse-biased.
When the emitter voltage (VE) reaches the peak point voltage (VP), the emitter junction forward-biases, and the device turns "on."
Current flows from the emitter to base B1, causing a significant drop in VE.
Further increase in emitter current decreases VE, resulting in a negative resistance region.
The device remains in this region until VE drops below the valley point voltage (VV).
Once VE falls below VV, the UJT turns off and returns to its initial state.

UJT as a Relaxation Oscillator

The UJT can be used to generate a periodic waveform, such as a sawtooth or square wave.
A simple relaxation oscillator circuit comprises a capacitor (C), a resistor (R), and the UJT.

Relaxation Oscillator Operation

The capacitor (C) charges through the resistor (R) when power is applied.
As the capacitor voltage rises, it eventually reaches the peak point voltage (VP) of the UJT.
At VP, the UJT turns on, causing the capacitor to discharge rapidly through the UJT.
This rapid discharge produces a voltage drop across the capacitor, forming a sawtooth waveform.
Once the capacitor voltage reaches the valley point voltage (VV), the UJT turns off, and the charging cycle repeats.

Waveform and Frequency Calculation

The output waveform across the capacitor is a sawtooth wave.
The voltage across the resistor is a series of pulses.
The frequency of oscillation (f) is calculated using the formula: f = 1/(R * C * ln(1/(1 - η))), where η is the intrinsic stand-off ratio.

Applications of UJT Relaxation Oscillator

Timing Circuits: Used in applications requiring precise timing intervals.
Pulse Generation: Generates trigger pulses for thyristors or other switching devices.
Sawtooth Waveform Generation: Used in function generators and waveform synthesizers.

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Description

This quiz covers the fundamentals of Field-Effect Transistors (FETs), including their operation, types, and modes. You'll learn about JFETs and MOSFETs and their respective parameters such as AC Drain Resistance and Transconductance. Test your understanding of these essential semiconductor devices!