MOSFET vs FET: Understanding the Differences and Applications

When discussing transistors, the terms MOSFET and FET (Field Effect Transistor) often come up. These are both important components in electronic circuits, but they differ in operation and application. In this article, we will explore the differences between MOSFET and FET, their working principles, and their applications.

Introduction to Transistors

Transistors are crucial components in electronic circuits. The term "transistor" is derived from "transfer resistor" and is used to describe devices that control current between two terminals using a small amount of power or energy. Transistors can be used either as simple switches or linear amplifiers. The two main classes of transistors are Bipolar Junction Transistors (BJTs) and FETs (Field Effect Transistors). Most FETs are actually MOSFETs (Metal Oxide Semiconductor FETs), a term that describes the insulated gate construction of these transistors.

Bipolar Junction Transistors (BJTs)

Bipolar Junction Transistors are current-controlled devices. They control a much larger current between the collector and emitter by drawing current from the signal source. The maximum emitter collector current is determined by Beta (β), the current gain times the base-emitter current. BJTs are used in precision and high-speed analog circuits where they can be used as simple switches, linear amplifiers, and as analog multipliers. They can provide very high voltage gain with a single stage, making them suitable for various applications.

Field Effect Transistors (FETs)

Field Effect Transistors are voltage-controlled devices. They can be used to pass current between the drain and source by applying a voltage between the gate and the source. Unlike BJTs, FETs draw little to no current from the signal source, making them the preferred choice when connecting them to devices that can only supply a limited amount of current, such as the output pins of microcontrollers. They are used in almost all modern digital circuits due to their extremely low power draw, allowing the creation of billion-transistor digital ICs.

MOSFETs: Metal Oxide Semiconductor FETs

The most common type of FET is the MOSFET. A Metal Oxide Semiconductor FET is essentially a FET with a metal oxide gate dielectric rather than a metal gate. The most modern MOSFETs use polysilicon for the gate electrode instead of metal, making them more efficient and reliable. They are incredibly important in modern electronics due to their ability to control current with a small voltage, making them ideal for digital circuits.

Other Devices and Applications

In addition to BJTs and MOSFETs, there are other types of transistors and thyristors that are used in various applications:

IGBT (Insulated Gate Bipolar Transistor): Combines the insulated gate with the low "on" voltage of a BJT for switching applications. SCR (Silicon Controlled Rectifier): A BJT-like device that can be turned on with a control terminal. It acts as a high impedance until a threshold current is applied, allowing it to conduct current as long as forward current flows. Triac: A bidirectional triode thyristor used in light dimmers and motor speed controllers. Triacs are similar to SCRs but can flow current in both directions. GTO (Gate Turn Off Thyristor): A device that can be turned off by removing the trigger current while conducting current, similar to a relay but with a more precise control mechanism.

These devices, like SCRs, TRIACs, and GTOs, cannot be used in linear applications. They are similar to relays but with more precise control mechanisms.

Conclusion

Understanding the differences between MOSFET and FET is crucial for selecting the appropriate transistor for your electronics project. Whether you are working on analog or digital circuits, the choice between BJTs, MOSFETs, or other specialized transistors and thyristors will greatly influence the functionality and efficiency of your design.