Field Effect Transistor

Field Effect Transistor#

Classification of transistor:

Transistor

Type

Sub-Type

Channel

BJT

NPN

PNP

FET

JFET

Depletion

N-Channel

p-Channel

MOSFET

Depletion

N-Channel

p-Channel

Enhancement

N-Channel

p-Channel

Explanation of each type of transistor:

Bipolar Junction Transistor (BJT)

NPN Transistor

An NPN transistor consists of two n-type semiconductors separated by a thin p-type semiconductor.

  • Base Current (I_B): A small current entering the base terminal.

  • Collector Current (I_C): A larger current flowing from the collector to the emitter.

  • Emitter Current (I_E): The sum of the base current and the collector current (I_E = I_C + I_B).

The current relationships are given by:

\[ I_C = \beta I_B \]
\[ I_E = (1 + \beta) I_B \]

Where \(\beta\) is the current gain of the transistor.

PNP Transistor

A PNP transistor is similar to an NPN transistor, but the types of charge carriers and polarities are reversed.

  • Base Current (I_B): A small current leaving the base terminal.

  • Collector Current (I_C): A larger current flowing from the emitter to the collector.

  • Emitter Current (I_E): The sum of the base current and the collector current (I_E = I_C + I_B).

The current relationships are given by:

\[ I_C = \beta I_B \]
\[ I_E = (1 + \beta) I_B \]

Field-Effect Transistor (FET)

Junction Field-Effect Transistor (JFET)

Depletion Mode

  • N-Channel JFET: The current \(I_D\) through the JFET is controlled by the gate-to-source voltage \(V_{GS}\). The drain current \(I_D\) is given by:

\[ I_D = I_{DSS} \left(1 - \frac{V_{GS}}{V_P}\right)^2 \]

Where:

  • \(I_{DSS}\) is the maximum drain current for \(V_{GS} = 0\).

  • \(V_P\) is the pinch-off voltage.

  • P-Channel JFET: Similar to the N-Channel JFET but with reversed polarity.

\[ I_D = I_{DSS} \left(1 - \frac{V_{GS}}{V_P}\right)^2 \]

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

Depletion Mode

  • N-Channel MOSFET: The current \(I_D\) through the MOSFET is controlled by the gate-to-source voltage \(V_{GS}\). The drain current \(I_D\) is given by:

\[ I_D = I_{DSS} \left(1 - \frac{V_{GS}}{V_P}\right)^2 \]

  • P-Channel MOSFET: Similar to the N-Channel MOSFET but with reversed polarity.

\[ I_D = I_{DSS} \left(1 - \frac{V_{GS}}{V_P}\right)^2 \]

Enhancement Mode

  • N-Channel MOSFET: The current \(I_D\) is given by:

\[ I_D = k \left[(V_{GS} - V_{th})V_{DS} - \frac{V_{DS}^2}{2}\right] \]

In the saturation region, where \(V_{DS} \geq V_{GS} - V_{th}\):

\[ I_D = \frac{k}{2} (V_{GS} - V_{th})^2 \]

Where:

  • \(V_{th}\) is the threshold voltage.

  • \(k\) is a process transconductance parameter.

  • P-Channel MOSFET: Similar to the N-Channel MOSFET but with reversed polarity.

\[ I_D = k \left[(V_{SG} - V_{th})V_{SD} - \frac{V_{SD}^2}{2}\right] \]

In the saturation region, where \(V_{SD} \geq V_{SG} - V_{th}\):

\[ I_D = \frac{k}{2} (V_{SG} - V_{th})^2 \]

Overview and Fundamentals of FETs#

Field-effect transistors (FETs) are crucial electronic components in modern electronics, offering low power consumption, high input impedance, and simple biasing requirements. This article explores the history of FETs, from early inventions and patents to modern developments and applications. We delve into FET types, their working principles, and key specifications, providing a comprehensive understanding of these essential devices. FETs can be classified into two main types: Junction Field-Effect Transistor (JFET) and Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET).

Working Principles and Historical Context

FETs operate by controlling the current flow through a semiconductor channel using a voltage applied at the gate terminal. The gate voltage modulates the number of charge carriers in the channel, affecting current flow between the source and drain terminals. The field-effect principle forms the basis of FET operation. The development of FETs began with Julius Edgar Lilienfeld’s patent in 1926 and evolved through contributions from Oskar Heil and the team at Bell Labs, leading to modern FET technology.