How FETs Work - The Learning Circuit

Summary

In this video, we explore the basics of Field Effect Transistors (FETs), focusing on the differences between Junction Field Effect Transistors (JFETs) and Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs). It covers how JFETs, which function in depletion mode, control current through a channel using reverse bias at the gate. In contrast, MOSFETs can operate in both depletion and enhancement modes, with the enhancement mode allowing current flow by creating a channel via a voltage at the gate. The video also delves into the workings of N-channel and P-channel variants, providing viewers a clear understanding of their roles in electronic circuits.

Takeaways

• 😀 FETs (Field Effect Transistors) have two primary types: Junction Field Effect Transistors (JFETs) and Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs).
• 😀 FETs function with a source, drain, and gate, whereas BJTs (Bipolar Junction Transistors) use a collector, emitter, and base.
• 😀 The gate of a FET operates similarly to the base in a BJT, but the gate controls the flow of current in a different way.
• 😀 JFETs are named based on the type of channel (N-channel or P-channel) and are characterized by the depletion of the channel by reverse biasing the gate.
• 😀 In an N-channel JFET, increasing reverse bias on the gate grows the depletion region, restricting current flow through the channel.
• 😀 The point at which current flow is completely stopped in a JFET due to the depletion region is known as the cutoff voltage.
• 😀 P-channel JFETs work similarly to N-channel JFETs, but with reversed polarity at the gate, meaning a positive gate-source voltage is required.
• 😀 JFETs are depletion-mode devices, meaning they rely on a growing or shrinking depletion region to control current flow.
• 😀 MOSFETs can operate in either depletion mode or enhancement mode, with differences in their internal structure compared to JFETs.
• 😀 Enhancement-mode MOSFETs do not have an initial channel between the source and drain; they require a voltage at the gate to form a conductive channel.
• 😀 Depletion-mode MOSFETs, like JFETs, have a physical channel between source and drain, but the gate controls current flow by expanding or shrinking the depletion region.
• 😀 MOSFETs are distinguished by their metal-oxide-semiconductor construction, where the gate is separated from the semiconductor by an insulating layer, functioning like a capacitor.

Q & A

What are the two basic types of Field Effect Transistors (FETs)?

The two basic types of FETs are the Junction Field Effect Transistor (J-FET) and the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET).

How are Field Effect Transistors (FETs) different from Bipolar Junction Transistors (BJTs)?

FETs have a source, drain, and gate, whereas BJTs have a collector, emitter, and base. FETs control current flow through the channel by varying the gate voltage, whereas BJTs rely on current flowing through the base to control the collector-emitter current.

What is the primary function of the gate in a Field Effect Transistor (FET)?

The gate in a FET controls the flow of current between the source and drain by modulating the width of the channel through which current flows, typically by varying the gate-source voltage.

What is the difference between N-channel and P-channel J-FETs?

In an N-channel J-FET, the channel is made up of N-type material, and the gate is connected to a negative voltage. In a P-channel J-FET, the channel is made up of P-type material, and the gate is connected to a positive voltage. Both types work similarly but with reversed polarity.

What is the cutoff voltage in an N-channel J-FET?

The cutoff voltage is the voltage difference between the gate and source at which the depletion region grows enough to completely cut off the current flow through the channel.

What happens if the gate of a J-FET is forward biased?

If the gate of a J-FET is forward biased, current could flow through the gate, which is not designed to handle significant current, potentially damaging the component.

What are the main modes of operation for MOSFETs, and how do they differ?

MOSFETs can operate in two modes: depletion mode (D-MOSFET) and enhancement mode (E-MOSFET). In depletion mode, a physical channel already exists between the source and drain. In enhancement mode, the channel does not exist initially, and the application of a gate voltage creates a conductive channel.

How does a D-MOSFET operate?

In a D-MOSFET, current can flow between the source and drain without any voltage at the gate. When the gate is reverse biased, the depletion region grows, narrowing the channel and reducing the current flow.

What is the threshold voltage in an E-MOSFET?

The threshold voltage is the minimum gate-source voltage required to create a conductive channel between the source and drain in an enhancement-mode MOSFET.

What is the role of the oxide layer in a MOSFET?

The oxide layer in a MOSFET acts as an insulating barrier between the gate and the semiconductor substrate, allowing the gate to control the flow of charge carriers in the channel without direct electrical contact.