Field Effect Transistor Overview

Field Effect Transistor (FET)

• FET is a semiconductor device used as an amplifier or switch.
• FET operation is voltage-controlled, relying on majority carrier flow (unipolar).
• FETs are less noisy than Bipolar Junction Transistors (BJTs).
• FETs are easier to fabricate and are suitable for integrated circuits (ICs).
• FETs are three-terminal devices (source, drain, and gate).

BJT vs FET

• Similarities: Both can be used as amplifiers and in switching circuits for impedance matching.
• Differences: FETs are voltage-controlled devices, while BJTs are current-controlled. FETs have higher input impedance and are less sensitive to temperature variations, making them more suitable for ICs.

FET Types

• JFET: Junction Field-Effect Transistor
• MOSFET: Metal-Oxide-Semiconductor Field-Effect Transistor
  • D-MOSFET: Depletion MOSFET
  • E-MOSFET: Enhancement MOSFET

JFET Construction

• Two types: n-channel and p-channel.
• The n-channel is more common.
• Three terminals: drain (D), source (S), and gate (G).
• Drain and source are connected to the n-channel.
• Gate is connected to the p-type material.

JFET Operating Characteristics

• VGS = 0 and increasing VDS: The depletion region widens, reducing the n-channel size and thus increasing resistance.
  • Despite increasing resistance, drain current (ID) increases due to the increasing VDS.
• Continued increase in VDS: The depletion region continues to grow until it pinches off the n-channel. This results in the drain saturation current, IDSS, where further increases in VDS don't affect ID. ID remains constant above this pinch-off voltage (|VP|).

JFET Transfer Characteristics (VGS < 0)

• As the gate-source voltage (VGS) becomes more negative, the depletion region increases.
• The drain current (ID) reduces as more electrons are repelled off the n-channel by gate's negative voltage.
• Eventually, when VGS equals the pinch off voltage (VP) (negative), the drain current becomes zero (ID = 0mA).

MOSFETs

• Metal-Oxide Semiconductor Field-Effect Transistors
• Useful due to characteristics similar to JFETs with additional features.
• Two types: Depletion-type and Enhancement-type.

Depletion-Type MOSFET Construction

• Drain (D) and Source (S) connect to n-doped regions, forming the n-channel.
• The gate (G) connects to the p-doped substrate via a SiO2 insulating layer.
• A substrate (SS) terminal is sometimes present.

Depletion-Type MOSFET Basic Operation and Characteristics

• Applying VGS=0 and VDS, free electrons in the n-channel are attracted to the drain, allowing a current flow.
• If VGS is negative, it will increase the depletion region, leading to a reduction in the current flow.
• When VGS reaches the pinch-off voltage (VP), ID becomes 0 mA.

Enhancement-Type MOSFET Construction

• Drain (D) and Source (S) are connected to n-doped regions.
• Gate (G) connects to the p-doped substrate via a SiO2 insulating layer.
• The n-doped material lies on the p-doped substrate, with no inherent channel.

Enhancement-Type MOSFET Construction

• For VGS = 0, no current flows.
• For positive VGS values (greater than the threshold voltage (VT), the gate attracts electrons to the p-substrate, forming a channel causing significant current to flow.
• The increase in drain current for increasing VGS is called the threshold voltage.

Basic MOSFET Operations

• Depletion mode: Operates when VGS < 0.
• Enhancement mode: When VGS > 0.

MOSFET Symbols

• Symbols for n-channel and p-channel enhancement and depletion MOSFETs are presented.

Basic Current Relationships

• For all types of FETs: IG = 0, IS = ID.
• JFETs and Depletion type MOSFETs: ID = IDSS (1 − VGS / VP)^2
• Enhancement type MOSFETs: ID = K(VGS−VT)^2