What is a JFET
JFET stands for ‘Junction Field Effect Transistor’. It is a three terminal semiconductor device that consists of an N-type channel whose middle part is sandwiched between two P-type regions or a P-type channel whose middle part is sandwiched between two N-type regions. The JFET is consequently known as N-Channel or P-Channel JFET. Two ohmic contacts are obtained from the two ends of the channel; these are known as Source and Drain. The third ohmic contact known as a Gate is obtained from the diffused regions present near the middle of the channel and it is common to both the regions.
In a JFET, only one type of charge carriers are responsible for conduction; electrons in case of N-channel and holes in case of P-Channel JFET. The terminal through which these carriers enter the channel is known as Source while the terminal through which these carriers leave the channel is known as Drain.
In normal mode of operation of an N-channel JFET, the source is supplied with a negative voltage with respect to the Drain. This causes the electrons to flow through the channel causing a conventional drain current to flow from Drain to source. Now by applying a negative potential on the gate terminal, a reverse bias is obtained at the two p-n junctions in the transistor. This reverse bias causes a wedge shaped depletion region in the N-type channel. The width of this depletion region and as a result the channel width can be controlled by the amount of negative potential applied to the gate. This indeed will control the amount of drain current flowing through the channel. Thus the Drain current flowing in the circuit of low output impedance can be controlled by the amount of negative bias on the gate terminal that provides an input region of high impedance because of the reverse bias conditions. Thus a transistor action is obtained in a JFET. A JFET just like a BJT can operate in three modes – Common Gate, Common Drain and Common Source mode. Among them the common source mode provides the best amplification action.
JFET’s have many advantages over the conventional BJT’s such as high input impedance, higher degree of isolation between the input and output circuitry, absence of junction transition noise, higher power gain, better frequency response etc. These are also very easier to fabricate in IC forms and these days are replacing more and more BJT’s in electronic circuits like amplifiers, power supplies etc.