JFET transistors general informations – Structure and working principles

The family of unipolar transistors includes JFET transistors, Junction Field Effect Transistor. They control the current flow through the connector using the media. The effect on the flow of electrons is the electric field controlled by the designer.

jfet transistor

The structure of the JFET unipolar transistor is derived from a P or N semiconductor plate from which the channel is built. At the ends of the channel plate, there are connectors S as the source and D as a drain. On both sides of the end, G is a gate made of a reverse semiconductor to which the channel is made.

JFET transistor structure

JFET transistors are present in two types P and N, they differ in the way they are controlled. In P-type transistors, the control voltage applied to the gate must be negative, whereas in the N-type transistors the voltage value must be positive.

jfet structure

In the case when the voltage between the drain and the source is too small and the polarity of the gate G is insufficient to form a channel, the channel structure will penetrate inside.

During the construction and working of jfet is dependent on supply voltage on the drain-source, the Id current will flow, depending on the Uds value. We determine this current as the value of the majority carriers, for the N channels they are electrons, and for the P channel they are holes.

As JFET is referred to as normally switched on, the dissipative positive voltage on the gate G or Ugs affects the channel constriction, the increase in the resistance, and hence the decrease in the Id value. If the value of Ugs is equal to the value given in the JFET transistor model catalog notes, the transistor will go into a clogged state. In this state, the change in voltage between drain D and source S Uds does not affect the current increase.

JFET vs bipolar transistor working principles

If you compare structural parameters between bipolar transistors and JFETs, you can specify the following differences:

  • normally deactivated state occurs in bipolar transistors,

  • normally enabled state occurs in JFET transistors,

  • current control occurs in bipolar transistors,

  • voltage control occurs in JFET (unipolar) transistors.

In a blocking state, the JFET transistor achieves high input resistance and very low Id gate current.

jfet states

If the gate G is polarized in the direction of conduction due to a voltage Ugs of less than 0.7 V due to the voltage of the PN connector, the Id current flows, but the value of the voltage Uds will not affect the current.

The symbols of the JFET transistor show the direction of the fluid current at the gate G for the conductance state of the transistor. It is possible to imagine that at this moment the transistor has a diode between the gate G – source S turned on.

Leave a Reply

Your email address will not be published. Required fields are marked *