Introduction to Field-Effect Transistors (FETs)

Summary

This lecture introduces Field Effect Transistors (FETs), comparing them to Bipolar Junction Transistors (BJTs). FETs are three-terminal, voltage-controlled devices, unlike BJTs which are current-controlled. FETs can be either n-channel or p-channel, used for amplification and switching. The lecture outlines the history of FETs, from early patents to modern MOSFETs, and discusses their high input impedance, temperature stability, smaller size, and lower sensitivity compared to BJTs.

Takeaways

• 🔬 Field Effect Transistors (FETs) and Bipolar Junction Transistors (BJTs) are both three-terminal devices, but FETs are voltage-controlled while BJTs are current-controlled.
• 🌐 FETs can be used for both amplification and switching, similar to BJTs.
• 🔋 FETs are unipolar devices, meaning they rely solely on either electrons or holes, unlike BJTs which are bipolar.
• 📡 There are two types of FETs: Junction Field Effect Transistors (JFETs) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs).
• 🧬 JFETs can be further divided into n-channel and p-channel types, just like NPN and PNP BJTs.
• 🏗️ MOSFETs are also divided into two types: depletion mode (D-MOSFET) and enhancement mode (E-MOSFET), each with n-channel and p-channel variants.
• 📚 The concept of FETs was patented by Julius Edgar Lilienfeld in 1926 and Oscar Heil in 1934, but the actual devices were developed later.
• 🔎 The first JFET was made in the late 1950s, and the MOSFET, an improvement over JFET, was invented by Dawon Kahng in 1959.
• 🔌 FETs have a high input impedance compared to BJTs, which is an advantage in certain applications.
• 🌡️ FETs are more temperature stable than BJTs, making them suitable for environments with varying temperatures.
• 🏞️ FETs are smaller in size compared to BJTs, which is beneficial for miniaturization in electronic devices.
• 📡 BJTs are more sensitive to the applied signal than FETs, which can be a consideration in signal processing applications.

Q & A

What is the main focus of the new chapter introduced in the lecture?

The main focus of the new chapter is Field Effect Transistors (FETs).

How are Field Effect Transistors (FETs) similar to Bipolar Junction Transistors (BJTs) in terms of terminals?

Both FETs and BJTs are three-terminal devices. In BJTs, the terminals are base, collector, and emitter, whereas in FETs, they are gate, drain, and source.

What is the main difference between BJTs and FETs in terms of control mechanism?

BJTs are current-controlled devices, with the output current (IC) being a function of the input current (IB). In contrast, FETs are voltage-controlled devices, where the output current (ID) is a function of the voltage between the gate and source (V_GS).

Why are Field Effect Transistors referred to as 'unipolar' devices?

FETs are called 'unipolar' because they depend solely on either electrons or holes for conduction, unlike BJTs which are 'bipolar' and involve both types of charge carriers.

What are the two types of Field Effect Transistors discussed in the lecture?

The two types of Field Effect Transistors discussed are Junction Field Effect Transistors (JFETs) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs).

Who is credited with the initial patent of the Field Effect Transistor concept?

The initial patents for concepts similar to FETs were by Julius Edgar Lilienfeld in 1926 and by Oscar Heil in 1934.

How do FETs compare to BJTs in terms of input impedance?

FETs have a higher input impedance compared to BJTs.

What is the significance of the term 'Field Effect' in the name of Field Effect Transistors?

The term 'Field Effect' refers to the electric field developed by charges that controls the conduction path of the output circuit in FETs.

Are FETs more temperature stable than BJTs?

Yes, FETs are more temperature stable than BJTs.

How do the size and area occupied by FETs compare to those of BJTs?

FETs are smaller in size and occupy less area than Bipolar Junction Transistors.

Which type of transistor is more sensitive to the applied signal, BJTs or FETs?

BJTs are more sensitive to the applied signal compared to FETs.