what is FET ? what are the different types of FET ?

 FET stands for Field-Effect Transistor. It is a type of transistor that uses an electric field to control the flow of current through a semiconductor channel. FETs are widely used in electronic circuits and serve as key components in various applications, including amplifiers, switches, oscillators, and digital logic circuits.

The following is the symbol of a NJFET & PJFET,

 

Gate − By using diffusion or alloying technique, both sides of N type bar are heavily doped to create PN junction. These doped regions are called gate (G).

Source − It is the entry point for majority carriers through which they enter into the semiconductor bar.

Drain − It is the exit point for majority carriers through which they leave the semiconductor bar.

Output Characteristics of JFET

 the drain current (ID) rises rapidly with drain source voltage (VDS) however suddenly becomes constant at a voltage known as pinch-off voltage (VP). Above pinch-off voltage, the channel width becomes so narrow that it allows very small drain current to pass through it. Therefore, drain current (ID) remains constant above pinch-off voltage.

Parameters of JFET

1. AC drain resistance (Rd)
2. Transconductance
3. Amplification factor

This three are the main parameters of JFET.

AC drain resistance (Rd) 

It is the ratio of change in the drain source voltage (ΔVDS) to the change in drain current (ΔID) at constant gate-source voltage. 

Rd = (ΔVDS)/(ΔID) at Constant VGS

Transconductance (gfs) 

It is the ratio of change in drain current (ΔID) to the change in gate source voltage (ΔVGS) at constant drain-source voltage.

gfs = (ΔID)/(ΔVGS) at constant VDS

Amplification Factor (u) 

 It is the ratio of change in drain-source voltage (ΔVDS) to the change in gate source voltage (ΔVGS) constant drain current (ΔID). 

u = (ΔVDS)/(ΔVGS) at constant ID

There are several different types of FETs available on the market that details will discuss in upcoming sections ,

Junction Field-Effect Transistor (JFET):

 JFETs are the simplest type of FETs. They have a channel made of either n-type or p-type semiconductor material. JFETs have three terminals: the source, drain, and gate. The voltage applied to the gate controls the width of the channel, thereby regulating the current flow. JFETs are mainly used in low-power applications and as voltage-controlled resistors.

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

MOSFETs are the most widely used type of FETs in modern electronic devices. They have a metal oxide insulator layer between the gate and the semiconductor channel, allowing for better control of the current flow. MOSFETs can be further classified into two main categories:

a. Enhancement-Mode MOSFET (E-MOSFET): 

E-MOSFETs require a positive voltage at the gate to create a conducting channel between the source and drain terminals. They operate as normally-off devices and are commonly used in digital circuits and power amplifiers.

b. Depletion-Mode MOSFET (D-MOSFET): 

D-MOSFETs are normally-on devices that have a conducting channel in the absence of any applied voltage to the gate. A negative voltage at the gate depletes the channel, reducing the current flow. D-MOSFETs are used in applications where a normally-closed switch is required.

Insulated-Gate Bipolar Transistor (IGBT): 

IGBTs combine the characteristics of both MOSFETs and bipolar junction transistors (BJTs). They have a MOSFET-like gate structure and a BJT-like current conduction mechanism. IGBTs are commonly used in high-power applications, such as motor drives, power inverters, and electric vehicle control systems.

Gallium Nitride (GaN) FET: 

GaN FETs are a relatively newer type of FETs that offer advantages in terms of high switching speeds, low on-resistance, and high breakdown voltage. GaN FETs are suitable for high-frequency and high-power applications, including wireless charging, RF amplifiers, and power supplies.

These are some of the main types of FETs available on the market. Each type has its own characteristics, advantages, and applications. The choice of FET depends on the specific requirements of the circuit or system being designed.