N-Channel MOSFET Characteristics Lab Manual

Experiment: To Study Transfer and Output Characteristics of an N-Channel MOSFET

1. Aim

To study and plot the transfer and output characteristics of an N-channel MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) and to determine its parameters experimentally.

2. Apparatus Used

  • N-channel MOSFET (e.g., 2N7000, BS170 or similar)
  • DC Power Supply (0-30V, variable) - 2 units
  • Digital Multimeters - 2 units
  • Digital Ammeter (0-200mA range)
  • Breadboard
  • Connecting wires
  • Resistors (220Ω, 1kΩ as needed)
  • Potentiometers (10kΩ, 100kΩ)
  • Graph paper

3. Circuit Diagram

N-Channel MOSFET Circuit Diagram

Fig 1: Circuit for (a) Transfer Characteristics and (b) Output Characteristics

4. Theory

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled device that uses an electric field to control the flow of current. N-channel MOSFETs are widely used in electronic circuits for switching and amplification purposes.

The N-channel MOSFET consists of four terminals:

  • Source (S): The terminal through which the majority carriers (electrons in N-channel) enter the channel
  • Drain (D): The terminal through which the majority carriers exit the channel
  • Gate (G): The control terminal that influences the channel conductivity
  • Body/Substrate (B): Usually connected to the source in most applications

There are two main operating regions for a MOSFET:

  1. Linear (Ohmic) Region: When VDS < (VGS - VTH), the MOSFET behaves like a voltage-controlled resistor, and the drain current increases linearly with VDS.
  2. Saturation Region: When VDS > (VGS - VTH), the drain current becomes nearly independent of VDS and depends mainly on VGS.

The transfer characteristic shows the relationship between the drain current (ID) and the gate-to-source voltage (VGS) at a constant drain-to-source voltage (VDS).

The output characteristic shows the relationship between the drain current (ID) and the drain-to-source voltage (VDS) at various constant gate-to-source voltages (VGS).

An important parameter of a MOSFET is the threshold voltage (VTH), which is the minimum gate-to-source voltage required to create a conducting channel between the source and drain.

5. Formulas

1. In the Linear/Ohmic Region (VDS < (VGS - VTH)):

$I_D = \mu_n C_{ox} \frac{W}{L} \left[ (V_{GS} - V_{TH})V_{DS} - \frac{V_{DS}^2}{2} \right]$

2. In the Saturation Region (VDS > (VGS - VTH)):

$I_D = \frac{1}{2} \mu_n C_{ox} \frac{W}{L} (V_{GS} - V_{TH})^2 (1 + \lambda V_{DS})$

Where:

  • $\mu_n$ = Electron mobility in the channel
  • $C_{ox}$ = Gate oxide capacitance per unit area
  • $W$ = Channel width
  • $L$ = Channel length
  • $\lambda$ = Channel-length modulation parameter

3. Transconductance (gm) - Measure of the device's gain:

$g_m = \frac{\partial I_D}{\partial V_{GS}} = \mu_n C_{ox} \frac{W}{L} (V_{GS} - V_{TH})$

4. Drain Resistance (rd):

$r_d = \frac{\partial V_{DS}}{\partial I_D} = \frac{1}{\lambda I_D}$

6. Procedure

Part A: Transfer Characteristics (ID vs VGS)

  1. Set up the circuit as shown in Figure 1(a).
  2. Connect the drain and gate terminals of the MOSFET to their respective DC power supplies through appropriate resistors.
  3. Set the drain-to-source voltage (VDS) to a constant value (e.g., 5V).
  4. Start with gate-to-source voltage (VGS) at 0V.
  5. Gradually increase VGS in steps of 0.2V and record the corresponding drain current (ID).
  6. Continue until VGS reaches about 5V or as recommended for your specific MOSFET.
  7. Repeat the experiment for different constant values of VDS (e.g., 3V, 5V, 7V).
  8. Plot the transfer characteristics (ID vs VGS) for each VDS value.

Part B: Output Characteristics (ID vs VDS)

  1. Set up the circuit as shown in Figure 1(b).
  2. Set the gate-to-source voltage (VGS) to a constant value (start with 2V).
  3. Start with drain-to-source voltage (VDS) at 0V.
  4. Gradually increase VDS in steps of 0.5V and record the corresponding drain current (ID).
  5. Continue until VDS reaches about 10V or as recommended for your specific MOSFET.
  6. Repeat the experiment for different constant values of VGS (e.g., 2V, 3V, 4V, 5V).
  7. Plot the output characteristics (ID vs VDS) for each VGS value.

7. Observation Tables

Table 1: Transfer Characteristics (VDS = constant)

VGS (V) ID (mA) for VDS = 3V ID (mA) for VDS = 5V ID (mA) for VDS = 7V
0.0
0.2
0.4
0.6
...
5.0

Table 2: Output Characteristics (VGS = constant)

VDS (V) ID (mA) for VGS = 2V ID (mA) for VGS = 3V ID (mA) for VGS = 4V ID (mA) for VGS = 5V
0.0
0.5
1.0
...
10.0

8. Calculations

From Transfer Characteristics:

  1. Threshold Voltage (VTH):

    Plot $\sqrt{I_D}$ vs VGS. The x-intercept of the linear portion of this graph gives the threshold voltage VTH.

  2. Transconductance (gm):

    Calculate the slope of the ID vs VGS curve in the linear region.

    $g_m = \frac{\Delta I_D}{\Delta V_{GS}}$

From Output Characteristics:

  1. Drain Resistance (rd):

    Calculate the reciprocal of the slope of the ID vs VDS curve in the saturation region.

    $r_d = \frac{\Delta V_{DS}}{\Delta I_D}$

  2. Channel-Length Modulation Parameter (λ):

    From the output characteristics, extend the linear portions of the curves in the saturation region to find their x-intercepts (VA). Then:

    $\lambda = \frac{1}{|V_A|}$

Sample Calculation:

(Note: In this section, you would show a step-by-step calculation using the actual data collected during the experiment)

9. Result

Based on the experimental observations and calculations, the following parameters of the N-channel MOSFET were determined:

  1. Threshold Voltage (VTH) = _______ V
  2. Transconductance (gm) = _______ mS
  3. Drain Resistance (rd) = _______ kΩ
  4. Channel-Length Modulation Parameter (λ) = _______ V-1

The experimental curves showing the transfer and output characteristics are attached.

The device behavior was observed to match the theoretical expectations, with:

  • Linear increase in drain current with gate voltage beyond the threshold voltage
  • Clear distinction between linear and saturation regions in the output characteristics
  • Increased drain current with increasing gate voltage at constant drain voltage

10. Precautions

  1. Always handle the MOSFET with care, as it is sensitive to static electricity. Use an anti-static wrist strap if available.
  2. Never exceed the maximum ratings of the MOSFET as specified in its datasheet.
  3. Make sure all connections are proper and tight before applying power.
  4. Start with lower voltage values and gradually increase to avoid damaging the device.
  5. Ensure that the gate voltage is not floating; use appropriate pull-down resistors if necessary.
  6. Switch off the power supply before making any changes to the circuit.
  7. Double-check the pinout of the MOSFET before making connections.
  8. Avoid touching the terminals of the MOSFET with bare hands during operation.
  9. Allow the MOSFET to cool down if it gets heated during the experiment.
  10. Keep the measuring instruments in appropriate ranges to avoid damage and obtain accurate readings.

11. Viva Voice Questions

1. What is a MOSFET and how does it differ from a BJT (Bipolar Junction Transistor)?
A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled device that uses an electric field to control the flow of current. The main differences from a BJT are: MOSFETs are voltage-controlled while BJTs are current-controlled; MOSFETs have higher input impedance; MOSFETs are unipolar devices that operate with one type of charge carrier, while BJTs are bipolar devices using both electrons and holes.
2. What is the significance of threshold voltage in a MOSFET?
Threshold voltage (VTH) is the minimum gate-to-source voltage required to create a conducting channel between the source and drain. It marks the transition from the cutoff region to the active region. Below this voltage, the MOSFET is essentially off, and above it, the device begins to conduct.
3. Explain the different regions of operation in a MOSFET.
The three main regions are: (1) Cutoff/Subthreshold Region: VGS < VTH, where the MOSFET is essentially off with negligible current flow; (2) Linear/Ohmic Region: VGS > VTH and VDS < (VGS - VTH), where the MOSFET behaves like a voltage-controlled resistor; (3) Saturation Region: VGS > VTH and VDS > (VGS - VTH), where the drain current becomes largely independent of VDS.
4. What is channel length modulation in MOSFETs?
Channel length modulation is a phenomenon in which the effective channel length of a MOSFET decreases as the drain-to-source voltage increases. This causes a slight increase in drain current with increasing VDS in the saturation region, resulting in a non-zero slope rather than a perfectly flat line. It's quantified by the parameter λ.
5. How is transconductance related to the MOSFET's amplification capability?
Transconductance (gm) is a measure of the MOSFET's effectiveness in converting input voltage changes to output current changes. It represents the gain of the device and is calculated as the rate of change of drain current with respect to gate-source voltage (∂ID/∂VGS). Higher transconductance indicates better amplification capability.
6. Why are MOSFETs preferred in digital circuits?
MOSFETs are preferred in digital circuits due to their high input impedance, low power consumption, small size allowing for high-density integration, simple fabrication process, compatibility with CMOS technology, fast switching speeds, and excellent scaling properties. These characteristics make them ideal for modern digital electronics.
7. What is the body effect in MOSFETs and why is it important?
The body effect refers to the change in threshold voltage of a MOSFET due to a potential difference between the source and body terminals. When the source and body are at different potentials, it alters the effective threshold voltage. This effect is important in circuit design as it can impact the operating point and performance of the device in complex circuits.
8. How does temperature affect the performance of a MOSFET?
Temperature affects MOSFETs in several ways: (1) Threshold voltage decreases with increasing temperature; (2) Carrier mobility decreases at higher temperatures, reducing current flow; (3) Leakage currents increase with temperature; (4) On-resistance typically increases with temperature. These effects need to be considered in circuit design, especially for power applications.
9. Compare enhancement-mode and depletion-mode MOSFETs.
Enhancement-mode MOSFETs are normally off (no channel exists) when VGS = 0, and require a gate voltage to enhance or create a channel. Depletion-mode MOSFETs are normally on (a channel already exists) when VGS = 0, and require a gate voltage to deplete or reduce the channel. N-channel enhancement-mode MOSFETs are the most commonly used type in modern electronics.
10. What advantages do power MOSFETs have over power BJTs?
Power MOSFETs offer advantages including: (1) Faster switching speeds; (2) Lower drive power requirements due to voltage control; (3) No secondary breakdown phenomenon; (4) Positive temperature coefficient of resistance, allowing for natural current sharing in parallel configurations; (5) Superior thermal stability; (6) Generally higher current densities for the same size; (7) Better suited for high-frequency applications.

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