Junction Field Effect Transistor | JFET VI Characteristic

By Mohit Uniyal|Updated : May 27th, 2022

A junction field effect transistor is a three-terminal device in which the voltage is applied at one terminal to control the output current. The conduction current is by one type of carrier, either by electrons or by holes. Only the majority of charge carriers are responsible for the current flow, therefore JFET are unidirectional. 

In this article, you will get an overview of what is a Field effect transistor(FET) and its brief description, and its type which is JFET. You will get to know about the detailed description of JFET’s construction, its types which are N-type and P-type, its modes of operation, its VI characteristics curve, its applications, and some relative advantages and disadvantages.  

Table of Content

What is a Field Effect Transistor? 

The field-effect transistor (FET) is a transistor that uses an electric field to control the flow of current. These are devices with three terminals: source, gate, and drain. The flow of current can be controlled by applying a voltage to the gate, which in turn alters the conductivity between the drain and source.

They are also known as unipolar transistors as they involve single carrier type operation, which means they use either electrons or holes as charge carriers in their operation, but not both. There are two types of FETs, one is Junction Field-Effect Transistor (JFET) and the other is a “Metal-Oxide-Semiconductor” Field-Effect Transistor (MOSFET), or Insulated-Gate Field-Effect Transistor (IGFET).

Construction of Junction Field Effect Transistor 

In an N-channel JFET, the body is of P-type, and the substrate is N-type, it is doped with donor impurities meaning that the flow of current through the channel is negative in the form of electrons, on the other hand in P-channel JFET the body is of N-type and the substrate used is p-type, it is doped with acceptor impurities meaning that the flow of current through the channel is positive in the form of holes. 

Operation of Junction Field Effect Transistor

When no voltage is applied across the source and gate, the channel is like an open path for the carriers to flow through it. When a reverse bias is applied, the channel narrows by increasing the depletion layer and puts the JFET in the cut-off or pinch-off region.

When zero Gate voltage is applied and a small voltage is applied between the Drain and the Source, maximum saturation current (IDSS) will flow through the channel and it will be restricted by the small depletion region around the junctions.

When a small negative voltage is applied to the Gate, the size of the depletion region increases, reducing the overall effective area of the channel and so the current flowing through the channel decreases. When a reverse bias voltage is applied, there is an increase in the width of the depletion region due to which conduction current reduces through the channel.

When the PN-junction is reverse biased, little current flows into the gate connection. When the Gate voltage is made more negative, channel width decreases until no more current flows through it and the FET is said to be “pinched-off”. The voltage at which the channel almost closes is called the “pinch-off voltage”. 

VI Characteristic Curve of Junction Field Effect Transistor

The VI characteristic curve of Junction field effect transistors is given below and in that the voltage VGS represents the voltage applied between the gate and the source and the voltage VDS represents the voltage applied between the drain and source.

The JFET has different stages of operation depending on the input voltages and the characteristics of JFET in different regions are explained below. It operates in ohmic, saturation, cut-off, and break-down regions.

  • Ohmic Region: If the voltage across Gate and Source is zero then the depletion region of the channel is very small and in this region, it acts as a voltage-controlled resistor.
  • Pinched-off Region: It is basically the cut-off region. JFET enters this region when the gate voltage is negative, then the channel closes hence no current flows through the channel.
  • Saturation or Active Region: In this region, the channel acts as a good conductor which is controlled by the gate voltage.
  • Breakdown Region: If the drain to source voltage is very high, then the channel of the JFET breaks down and uncontrolled maximum current passes through the device.

Applications of JFET

JFET has a high impedance. These are low power consumption devices and they are smaller in size, have more gate current, and low modulation distortion,  so with regards to these advantages, there are many applications of JFET. Some applications of JFET are given below:- 

  • JFET is used as a switch, amplifier, and buffer.
  • JFET is used as a chopper.
  • JFET is used in oscillatory circuits because of its low-frequency drift, used in cascade amplifiers and in RF amplifiers.
  • Used in digital circuits because of their small size.
  • Used in communication equipment because of their low modulation distortion.
  • Used as voltage-controlled resistors in operational amplifiers.

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FAQs on JFET

  • Junction field effect transistor is a 3 terminal device in which the voltage is applied at 1 terminal to control the output current. These are devices with three terminals which are source, gate, and drain. Only the majority of charge carriers are responsible for the current flow, therefore JFET are unidirectional.

  • When no voltage is applied across the source and gate, the channel is a smooth path for the electrons to flow through, JFET is in the ohmic region, when the Gate voltage is negative it is in Pinch off region and when the Gate voltage is more than threshold voltage it is in Saturation region when the Gate voltage is very high JFET breaks down.

  • There are basically two types of field effect transistors, one is the Junction Field-Effect Transistor (JFET) and the other is the “Metal-Oxide-Semiconductor” Field-Effect Transistor (MOSFET). The principles of operation are very similar. The primary difference is which element is used to make these transistors.



  • Characteristics of JFET for which it is mostly used are fast switching, and for low frequency operation, source and drain can be interchanged. In JFET, gate voltage controls the current. They have one majority carrier which is responsible for conduction current. JFET are small in size and have high input impedance.

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