JFET

 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

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.

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