The Field-Effect Transistor Help

By — McGraw-Hill Professional
Updated on Sep 11, 2011


The other major category of transistor, besides the bipolar device, is the field-effect transistor (FET). There are two main types of FETs: the junction FET (JFET) and the metal-oxide FET (MOSFET).

Principle Of The Jfet

In a JFET, the current varies because of the effects of an electrical field within the device. Electrons or holes move along a current path called a channel from the source ( S ) electrode to the drain ( D ) electrode. This results in a drain current I D that is normally the same as the source current I S . The drain current depends on the voltage at the gate ( G ) electrode. As the gate voltage E G changes, the effective width of the channel varies. Thus fluctuations in E G cause changes in the current through the channel. Small fluctuations in E G can cause large variations in the flow of charge carriers through the JFET. This effect makes it possible for the device to act as a voltage amplifier .

N-channel And P-channel

A simplified drawing of an n-channel JFET and its schematic symbol are shown in Fig. 16-10 a and b . The n -type material forms the path for the current. The majority carriers are electrons. The drain is positive with respect to the source. The gate consists of p -type material. Another, larger section of p -type material, the substrate, forms a boundary on the side of the channel opposite the gate. The voltage on the gate produces an electrical field that interferes with the flow of charge carriers through the channel. The more negative E G becomes, the more the electrical field chokes off the current though the channel, and the smaller I D gets.

Semiconductors The Field-Effect Transistor Depletion And Pinchoff

Fig. 16-10 . Pictorial diagram of an n -channel JFET (a), schematic symbol for an n -channel JFET (b), pictorial diagram of a p -channel JFET (c), and schematic symbol for a p -channel JFET (d).

A p-channel JFET (see Fig. 16-10 c and d ) has a channel of p -type semiconductor. The majority of charge carriers are holes. The drain is negative with respect to the source. The gate and substrate are n -type material. The more positive E G becomes, the more the electrical field chokes off the current through the channel, and the smaller I D gets.

In engineering circuit diagrams, the n -channel JFET can be recognized by an arrow pointing inward at the gate and the p -channel JFET by an arrow pointing outward. The power-supply polarity also shows which type of device is used. A positive drain indicates an n -channel JFET, and a negative drain indicates a p -channel type.

An n -channel JFET almost always can be replaced with a p -channel JFET and the power-supply polarity reversed, and the circuit will still work if the new device has the correct specifications.

Depletion And Pinchoff

A JFET works because the voltage at the gate causes an electrical field that interferes, more or less, with the flow of charge carriers along the channel. As the drain voltage E D increases, so does the drain current I D , up to a certain level-off value. This is true as long as the gate voltage E G is constant and is not too large. As E G increases (negatively in an n channel or positively in a p channel), a depletion region develops within the channel. Charge carriers cannot flow in the depletion region, so when there is such a region, they must pass through a narrowed channel. The larger E G becomes, the wider the depletion region gets, and the more constricted the channel becomes. If E G is high enough, the depletion region completely obstructs the flow of charge carriers, and the channel cannot conduct current at all. This condition is known as pinchoff . It is like pressing down on a garden hose until the water can’t flow.

Practice problems of these concepts can be found at: Semiconductors Practice Test

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