Principle of operation of a JFET


There are basically two types of transistors
  1. Bipolar junction transistor (BJT) :- a current controlled device in which output characteristics are controlled by base current
  2. Field effect transistor (FET) :- output characteristics controlled by input voltage i.e. electric field

Types of FET

  • Junction Field Effect Transistor (JFET)
  • Metal oxide semiconductor Field Effect Transistor (MOSFET)
The two types of transistor differ in both their operating characteristics and their internal construction.
BJT is so called because both holes and electrons plays part in the conduction process

Principal Disadvantages Of The BJT

  1. It has low input impedance because of the forward biased emitter junction. Although low input impedance problem may be improved by careful design and used of more than one transistor.
  2. it has a considerable noise level.

Difference Between JFET and BJT

  • Only one type of carrier, holes in p – type channel and electron in n- type channel in JFET WHILE electron and holes takes part in conduction in BJT
  • JFET has high input impedance since the input circuit is reversed biased WHILE input circuit for BJT is forward biased hence it has low input impedance
  • No current enters gate of JFET i.e. IG = 0 WHILE BJT base current is of few µA
  • JFET uses voltage applied on gate to control current between drain and source WHILE BJT uses current applied to base to control a large current between the collector and the emitter
  • In JFET there is no junction as in BJT. Conduction through an n –type or p – type semi conductor material for this reason noise level in BJT is very small


A JFET consists of a p-type or n-type silicon bar containing two p-n junctions at the sides as shown in the Figure below. The bar forms the conducting channel for the charge carriers. If the bar is of n-type, it is called n-channel JFET and if the bar is of p-type, it is called a p-channel JFET. The two pn junctions forming diodes are connected internally and a common terminal called gate is taken out. Other terminals are source and drain taken out from the bar. Thus a JFET has essentially three terminals which are gate (G), source (S) and drain (D)

Principle of Operation Of JFET

The JFET operates on the principle that width and hence resistance of the conducting channel can be varied by changing the reverse voltage VGS.
The two p-n junction of the sides from two depletion layers. Current conduction by the charge carriers is through the channel between the two depletion layers and out of the drain. The width and hence resistance of this channel can be controlled by changing the output voltage VGS. The greater the reverse voltage VGS, the wider the depletion layers and narrower will be the conducting channel. The narrower channel means greater resistance and hence source to drain current decreases. Reverse is the case when VGS decrease. The magnitude of current (Is) can be charged by alternating VGS.

Working Of JFET

When a voltage VGS is applied between drain & source terminals and Vas is zero, the two p-n junction at the sides of the bar establish depletion layers. The electrons will flow from source to drain through a channel between the depletion layers. The size of these layers determine the width of the channel and hence the current conduct through the bar.
When a reverse voltage VGS is applied between the gate and source, VGS the width of the depletion layers is increased. This reduce the width of the conducting channel thereby increasing the resistance of n – type bar. Consequently the current from source to drain is decreased.
NOTE:– The resistance of the channel depends on it area of x – section current from source to drain can be controlled by the application of an

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