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MOSFET

Lesson Overview

  • Subject: Electrical Engineering / Electronics
  • Topic: Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs)
  • Duration: 2 hours
  • Level: Undergraduate students in Electrical Engineering

Learning Objectives

By the end of this lesson, students should be able to:

  1. Understand the basic structure and operation principles of MOSFETs.
  2. Distinguish between different types of MOSFETs (N-channel and P-channel, Enhancement and Depletion modes).
  3. Analyze the characteristics and parameters of MOSFETs.
  4. Design and simulate simple MOSFET circuits.
  5. Interpret simulation results to understand MOSFET behavior.

Lesson Structure

1. Introduction to MOSFETs

What is a MOSFET?

  • Definition: A MOSFET is a type of Field-Effect Transistor (FET) used to amplify or switch electronic signals.
  • Basic Structure:
    • Source (S): Terminal through which carriers enter the channel.
    • Drain (D): Terminal through which carriers leave the channel.
    • Gate (G): Controls the conductivity of the channel.
    • Body (B): Substrate on which the MOSFET is built.

Types of MOSFETs

  • N-Channel vs. P-Channel:
    • N-Channel: Current is carried by electrons.
    • P-Channel: Current is carried by holes.
  • Enhancement Mode vs. Depletion Mode:
    • Enhancement Mode: Requires a gate voltage to induce a channel.
    • Depletion Mode: Channel exists naturally; gate voltage can deplete it.

2. MOSFET Operation Principles

Physical Operation

  • Threshold Voltage (\(V_TH\)): The minimum gate-to-source voltage required to create a conducting path between the source and drain.
  • Creating the Channel:
    • Applying a voltage to the gate creates an electric field.
    • The field induces a channel in the semiconductor material.

I-V Characteristics:

  • Ohmic Region: The MOSFET operates like a variable resistor.
  • Saturation Region: The current becomes independent of the drain-to-source voltage.

Transconductance (g_m):

  • Definition: Measure of the sensitivity of the drain current to changes in the gate voltage.
  • Importance: Determines the amplification capability of the MOSFET.

3. MOSFET Characteristics and Parameters (30 minutes)

Important Parameters:

  • Drain Current (\(I_D\)): Current flowing from drain to source.
  • Gate-Source Voltage (\(V_GS\)): Voltage between gate and source terminals.
  • Drain-Source Voltage (\(V_DS\)): Voltage between drain and source terminals.

Equations:

  • Cutoff Region:

$$ ID=0I_D = 0ID​=0 when VGS<VTHV_{GS} < V_{TH}VGS​<VTH​. $$ - Ohmic Region (Triode):

\[ ID=μnCoxWL((VGS-VTH)VDS-VDS22)I_D = \mu_n C_{ox} \frac{W}{L} \left( (V_{GS} - V_{TH})V_{DS} - \frac{V_{DS}^2}{2} \right)ID​=μn​Cox​LW​((VGS​-VTH​)VDS​-2VDS2​​) \]
  • Saturation Region:
\[ ID=12μnCoxWL(VGS-VTH)2I_D = \frac{1}{2} \mu_n C_{ox} \frac{W}{L} (V_{GS} - V_{TH})^2ID​=21​μn​Cox​LW​(VGS​-VTH​)2 \]

Capacitance Effects:

  • Gate Capacitance (C_G): Affects the speed of the MOSFET.
  • Miller Effect: Impacts high-frequency performance.

4. Applications of MOSFETs

Switching Applications:

  • Used in digital circuits as logic gates.
  • Key component in power electronics for controlling high voltages and currents.

Amplifiers:

  • Common-source, common-gate, and common-drain configurations.
  • Used in analog circuits for signal amplification.

Digital Circuits:

  • CMOS technology uses complementary N-channel and P-channel MOSFETs.
  • Basis for microprocessors and memory devices.

5. Sample MOSFET Circuit Experiment

Designing a Common-Source Amplifier:

  • Objective: Build and analyze a simple N-channel MOSFET common-source amplifier.
  • Circuit Diagram:
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\begin{circuitikz}[american]
\draw
  (0,0) node[nmos, anchor=S] (M1) {}
  (M1.D) to [R, l=$R_D$, *-*, v^=$V_{out}$] ++(0,2) -- ++(2,0) to [V, l=$V_{DD}$] ++(0,0)
  (M1.G) -- ++(-2,0) to [R, l=$R_G$] ++(0,-2) -- (0,-2)
  (M1.S) -- (0,-2) node[ground]{}
  (-4,-2) node[ground]{} to [V, l=$V_{in}$] (-4,0) -- (M1.G)
;
\end{circuitikz}

Components:

  • MOSFET (M1): N-channel enhancement-mode MOSFET.
  • Resistors:
    • RDR_DRD​: Drain resistor.
    • RGR_GRG​: Gate resistor.
  • Voltage Sources:
    • VDDV_{DD}VDD​: Supply voltage.
    • VinV_{in}Vin​: Input signal.

Experiment Steps:

  1. Assemble the Circuit:
    • Connect the components as per the circuit diagram.
  2. Apply Input Signal:
    • Use a function generator to apply a small AC signal VinV_{in}Vin​.
  3. Measure Output Voltage (VoutV_{out}Vout​):
    • Use an oscilloscope to observe the amplified signal.
  4. Analyze Amplification:
    • Calculate the voltage gain Av=VoutVinA_v = \frac{V_{out}}{V_{in}}Av​=Vin​Vout​​.

6. Simulation to Understand MOSFET Parameters

Objective:

  • Simulate the I-V characteristics of an N-channel MOSFET to understand the effects of threshold voltage and transconductance.

Simulation Software:

  • Use a SPICE-based simulator (e.g., LTspice, PSpice).

Simulation Steps:

  1. Set Up the MOSFET Model:

    • Use a predefined NMOS model or specify parameters like VTHV_{TH}VTH​, μnCox\mu_n C_{ox}μn​Cox​, WWW, and LLL.

    • Create the Test Circuit:

    • Circuit Diagram:

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Drain (D) --- [ Ammeter ] --- [ V_DS ] --- Source (S)
                |
                Gate (G) --- [ V_GS ]

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-   **Description:**

    -   Sweep VGSV_{GS}VGS​ from 0 V to a value above VTHV_{TH}VTH​.
    -   For each VGSV_{GS}VGS​, sweep VDSV_{DS}VDS​ from 0 V to VDDV_{DD}VDD​.

  1. Run DC Sweeps:

    • Perform nested sweeps of VGSV_{GS}VGS​ and VDSV_{DS}VDS​.

    • Observe I-V Curves:

    • Plot IDI_DID​ versus VDSV_{DS}VDS​ for different VGSV_{GS}VGS​ values.

    • Analyze Results:

    • Threshold Voltage (VTHV_{TH}VTH​):

      • Identify the VGSV_{GS}VGS​ at which IDI_DID​ begins to increase.
    • Transconductance (gmg_mgm​):
      • Calculate gm=∂ID∂VGSg_m = \frac{\partial I_D}{\partial V_{GS}}gm​=∂VGS​∂ID​​ in the saturation region.
    • Channel Length Modulation:
      • Observe the slight increase in IDI_DID​ with increasing VDSV_{DS}VDS​ in saturation.

Discussion Points:

  • How does varying VGSV_{GS}VGS​ affect IDI_DID​?
  • The impact of threshold voltage on the MOSFET's switching behavior.
  • The role of transconductance in amplification applications.

Assessment

  • Quiz:
    • Short questions on MOSFET operation and characteristics.
  • Lab Report:
    • Document the experiment and simulation results.
    • Analyze the data and provide conclusions.

References

Notes for Students

  • Safety First: Always double-check connections before powering the circuit.
  • Simulation Tip: Pay attention to the MOSFET model parameters; they significantly affect the simulation results.
  • Experiment Variation: Try changing RDR_DRD​ or RGR_GRG​ to see how the amplifier's gain is affected.

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References

How MOSFET Works - Ultimate guide, understand like a PRO The Engineering Mindset

  • Textbook: "Microelectronic Circuits" by Sedra and Smith.
  • Datasheets: Refer to specific MOSFET datasheets for real-world parameters.