JFET - Junction Field Effect Transistor

JFET or Junction Field Effect Transistor is one of the simplest types of field-effect transistor. Contrary to the Bipolar Junction Transistor, JFETs are voltage-controlled devices. In JFET, the current flow is due to the majority of charge carriers. However, in BJTs, the current flow is due to both minority and majority charge carriers. Since only the majority of charge carriers are responsible for the current flow, JFETs are unidirectional. The first working model of junction field-effect transistors was made in 1953.

JFET Construction

JFET Construction

In an N-channel JFET, the material is of P-type, and the substrate is N-type, while in a P channel JFET the material is of N-type, and the substrate used is p-type. JFET is made of a long channel of semiconductor material. Ohmic contacts are provided at each end of the semiconductor channels to form source and drain connections. A P-type JFET contains many positive charges, and if the JFET contains a large number of electrons, it is called an N-type JFET.

JFET Operation

Let us understand the working of JFET by comparing it to a garden hose pipe. Water flows smoothly through a garden hose pipe if there is no obstruction, but if we squeeze the pipe slightly, the water flow slows down. This is precisely how a JFET works. Here the hose is analogous to JFET, and the water flow is equivalent to a current. By constructing the current carrying-channel according to our needs, we could control the current flow.

When no voltage is applied across the source and gate, the channel is a smooth path for the electrons to flow through. When the polarity that makes the P-N junction reverse biased is applied, the channel narrows by increasing the depletion layer and could put the JFET in the cut-off or pinch-off region.

Junction Field Effect Transistor

The image shows the depletion region becoming wider and narrower during the saturation and the pinch-off mode.

JFET Types

Depending on the source of current flow, JFETs are classified into two types as follows:

  • n-channel JFET
  • p-channel JFET

The classification is based on whether the current flow is due to electrons or holes.

The schematic of an n-channel JFET, along with its circuit symbol, is shown below.

n-channel JFET

The schematic of a p-channel JFET, along with its circuit symbol, is shown below.

p-channel JFET

Junction Field Effect Transistor Applications

Some applications of JFET are listed below:

  • JFET is used as a switch
  • JFET is used as a chopper
  • JFET is used as a buffer
  • JFETs are used in oscillatory circuits
  • JFETs are used in cascade amplifiers

JFET Advantages

Some advantages of JFET are listed below:

  • JFET has a high impedance
  • JFETs are low power consumption devices
  • JFET can be fabricated in a smaller size, and as a result, they occupy less space in circuits due to their smaller size.

JFET Disadvantages

Some disadvantages of JFETs are as follows:

  • It has a low gain-bandwidth product
  • The performance of JFET is affected as frequency increases due to feedback by internal capacitance.

Frequently Asked Questions – FAQs

What is JFET?

JFET or Junction Field Effect Transistor is a unipolar current-controlled semiconductor device with three terminals: source, drain, and gate. JFETs are commonly used as switches and amplifiers.

When was the junction field-effect transistor invented?

The first working model of junction field-effect transistors was made in 1953.

How many diodes do junction field-effect transistors contain?

The Junction Field-effect transistor contains two diodes.

List a difference between JFET and BJT.

In BJT, the output current is controlled by the base current. In JFET, the output current is controlled by the input voltage.

What is a JFET used for?

JFET is used as a switch, buffer and chopper. They are also used in oscillatory circuits.

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