JFET - Junction Field Effect Transistor

- an overview or tutorial giving the basic facts about the junction field effect transistor (FET) or JFET and the basics of its operation.

The junction field effect transistor or JFET is widely used in electronics circuits. The junction field effect transistor is a reliable and useful circuit component that can be used very easily in a variety of electronic circuits ranging from JFET amplifiers to JFET switch circuits.

The junction field effect transistor is freely available, and JFETs can be bought for very little money. This makes them ideal for use in many circuits where cost and performance are of interest.

JFET history

The idea of the field effect transistor or FET has been in existence for many years. The idea for this type of device first appeared in a proposal made by Lilienfield in 1926. Later in 1935 the idea re-surfaced in a paper presented by a researcher named Heil.

However it was during the 1940s at Bell Laboratories that real progress towards the field effect transistor or FET was made. At this time they set up a semiconductor research group that investigated a number of areas pertaining to semiconductors, one of which was a device that would modulate the current flowing in a semiconductor channel buy placing an electric field close to it.

Unfortunately the idea for the field effect transistor did not work initially and even after repeated attempts to discover the cause of the problem no progress was made. As a result this failure the group investigating this device turned their sights in other directions and ultimately invented the bipolar transistor in 1948.

After the discovery of the bipolar transistor much of the semiconductor research was focussed on improving this device, and the idea for the field effect transistor was not fully investigated for some years.

One of the main reasons why the idea for the field effect transistor did not work initially was that the materials could not be refined sufficiently. However as work progressed on improving semiconductor materials for the bipolar transistor, this also enabled work to proceed with the field effect transistor. This resulted in the successful implementation of the field effect transistor or FET, and gained widespread acceptance during the 1960s when these devices became more widely available.

Basic JFET structure and circuit symbols

Basically a field effect transistor or FET consists of a section of silicon whose conductance is controlled by an electric field. The section of silicon through which the current flows is called the channel, and it consists of one type of silicon, either N-type or P-type.

The connections at either end of the device are known as the source and drain. The electric field to control the current is applied to a third electrode known as a gate.

Junction field effect transistor circuit symbols

Junction field effect transistor circuit symbols

As it is only the electric field that controls the current flowing in the channel, the device is said to be voltage operated and it has a high input impedance, usually many megohms. This can be a distinct advantage over the bipolar transistor that is current operated and has a much lower input impedance.



More Electronic Components:
Resistors     Capacitors     Diodes     Transistor     FET     Thyristor / SCR     Connectors     Valves / Tubes    
    Return to Components menu . . .