semiconductor physics and Devices

Week 6: Field Effect Transistors (FETs)

Lecture Topics:

1. Introduction to Field Effect Transistors (FETs)

   • FETs are another type of transistor used for amplifying and switching signals.

   • Unlike BJTs, which rely on current control, FETs are voltage-controlled devices.

   • Key advantages of FETs: High input impedance and low power consumption.

   • Types of FETs:

     • Junction Field Effect Transistor (JFET)

     • Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET)

2. Junction Field Effect Transistors (JFETs)

   • JFET Structure: Consists of three terminals – source (S), drain (D), and gate (G).

     • N-channel JFET: The channel through which current flows is made of n-type material.

     • P-channel JFET: The channel is made of p-type material.

   • Basic operation:

     • The gate-source voltage VGS​ controls the current flow through the channel between the source and drain.

     • As VGS​ becomes more negative (for an n-channel JFET), the channel narrows, reducing the current flow.

     • Eventually, at a certain voltage VGS(off)​, the channel is fully pinched off, and the current flow stops.

3. JFET I-V Characteristics

   • Drain current ID​ in JFETs is controlled by the gate-source voltage VGS​ and the drain-source voltage VDS​.

   • Ohmic region: At low VDS​, the current ID​ increases linearly with VDS​ (like a resistor).

   • Saturation region: As VDS​ increases beyond a certain value, the current reaches a maximum value and becomes almost independent of VDS​.

   • Cutoff region: When VGS​ is below the pinch-off voltage VGS(off)​, the channel is completely pinched off, and ID≈0.

4. Metal-Oxide-Semiconductor FETs (MOSFETs)

   • MOSFET Structure: Similar to JFETs, but with an insulating layer (usually silicon dioxide) between the gate and the channel.

   • Two types of MOSFETs:

     • Enhancement-mode MOSFET: The channel does not exist at zero gate voltage. The channel is created by applying a gate voltage.

     • Depletion-mode MOSFET: A conductive channel exists at zero gate voltage, and applying a gate voltage depletes the channel.

   • N-channel and P-channel MOSFETs: Similar to JFETs, n-channel MOSFETs have electrons as charge carriers and p-channel MOSFETs have holes as carriers.

5. Operation of an N-channel Enhancement-mode MOSFET

   • In an N-channel MOSFET:

     • Applying a positive gate-source voltage VGS creates an inversion layer (n-type) in the p-type substrate, allowing current to flow between the drain and source.

     • The drain current ID​ increases as VGS​ increases, up to a certain saturation point.

     • Threshold voltage VT​: The minimum gate voltage required to form a conductive channel between the source and drain.

   • The MOSFET operates in three regions:

     • Cutoff region: VGS<VT​, no current flows.

     • Linear region: VGS>VT​ and VDS​ is small, and the current increases linearly with VDS​.

     • Saturation region: VDS​ is large, and current saturates and becomes independent of VDS​.

6. MOSFET I-V Characteristics

   • Drain current ID​ in the saturation region for an enhancement-mode MOSFET:

• 1. where μn\mu_nμn​ is the electron mobility, Cox​ is the oxide capacitance, W is the channel width, and L is the channel length.

  2. Switching characteristics: MOSFETs are widely used in digital circuits as switches due to their high input impedance and fast switching times.

• Applications of FETs

  1. JFETs: Used in low-noise, high-input impedance amplifiers.

  2. MOSFETs:

     1. Widely used in both analog and digital circuits.

     2. CMOS technology: Complementary MOSFETs (n-channel and p-channel MOSFETs) are the foundation of most modern integrated circuits (ICs), including microprocessors and memory chips.

     3. MOSFETs are also used in switching power supplies, motor control, and RF amplifiers.

Examples:

• Calculate the drain current ID​ in an N-channel MOSFET for given VGS​, VT​, and VDS​ Values

• Plotting the I-V characteristics of a JFET and identifying the ohmic, saturation, and cutoff regions.

• Designing a simple MOSFET switch circuit for digital logic and explaining its operation.

Homework/Exercises:

1. Calculate the drain current ID​ for a MOSFET with a gate voltage VGS=4 V, threshold voltage VT=1 V, and mobility μn=500 cm^2/V-s. Explain the difference between enhancement-mode and depletion-mode MOSFETs, and provide an example of where each might be used.

2. Plot the I-V characteristics of a JFET and explain how the device operates as a voltage-controlled resistor in the ohmic region.

3. Design a CMOS inverter using an n-channel and p-channel MOSFET and explain its logic function.

Suggested Reading:

• Charles Kittel, Introduction to Solid State Physics, Chapter 8: Semiconductors (continued).

Key Takeaways:

• FETs are voltage-controlled devices with high input impedance, making them ideal for low-power applications and integrated circuits.

• JFETs and MOSFETs are the two main types of FETs, with MOSFETs being more commonly used due to their versatility and efficiency.

• MOSFETs are the foundation of modern digital electronics, especially in CMOS technology, which dominates integrated circuit design.

• Understanding the I-V characteristics and regions of operation of FETs is crucial for designing amplifiers, switches, and digital logic circuits.

This week introduces Field Effect Transistors (FETs), focusing on their operation, I-V characteristics, and applications. Understanding the differences between JFETs and MOSFETs and their use in modern electronic circuits is essential for students working with semiconductor devices.