# Fet is named field effect transistor,why?

FET is named Field Effect Transistor while field effect is also producing in bjt. What are the factors/reasons that FET is called field effect while bjt is not? I have read textbook but it didn't explain it.

• The field effect isn't present in BJTs! Try reading a better textbook. Mar 29 '13 at 9:01
• @ Leon Heller Is there not field present at the junctions of transistor? Is that field not affected by applying voltage at the base and Collector/Emitter Mar 29 '13 at 12:09
• @AliKhan, there are fields present in a BJT, but they don't directly control the operation of the device. A BJT is normally considered a current-controlled or charge-controlled device. Mar 29 '13 at 15:52

A electric "field" is produced whenever and wherever there is a voltage difference. Actually the field is always everywhere, but becomes non-zero when voltage differences are non-zero. Since non-zero voltages are fundamental to how every electrical device works, all such devices can be said to work based on or somehow related to the "field".

Field effect transistors (FETs) work on a pricipal that uses the field produced by the gate in a way that makes the channel conduct more or less. In bipolar transistors, the current that can be carried between the collector and emitter is modulated by the base current. Of course there are electric fields inside a bipolar transistor. The FET was named such because the of how the static field, as apposed to current, plays a central role in the operation of the device.

It's a naming scheme, which is not meant to be a detailed description of how the device works.

• Mar 31 '13 at 0:31

The value of drain current from the source can be controlled by the potential applied to the gate i.e. the electric field b/w gate and source. That is why such transistor is known as F.E.T.

• You should elaborate a little more. A BJT transistor is also controlled by a voltage, $V_{BE}$. Why it is a "field effect" in one transistor but not another.
– Kaz
Apr 17 '15 at 17:47

For the FET an electric field is established by the charges present, which control the conducting path of the output circuit without the need for direct contact between the controlling & controlled quantities.

Bipolar transistors are so named because they conduct by using both majority and minority carriers.

The field-effect transistor (FET), sometimes called a unipolar transistor, uses either electrons (in N-channel FET) or holes (in P-channel FET) for conduction.

You are correct. There are fields inside of a BJT. The fields are not what is driving current. Current is actually delivered by the diffusion of minority carriers in the device. The fields in this case just ensure that the carriers go the correct direction across the junctions, and only effect how they cross junctions, nothing else. (But not always and there's some more fuzzy math to go around, taking into account tunneling and other quantum effects. However, that's way out of scope.)

Where as in FETs, the field either creates (MOSFETs) or removes (JFETs) a region with majority carriers that acts as a conducting connector. How conductive this region is, effects the current. So the current is directly dependent on the strength of the field. Thus the electric Field Effects the transistor.