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Andrew Carter

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Fundamentals of Field Effect Transistor

In 1947, Shockley, Brattain and Bardeen were investigating the field effect transistor but lead them into inventing the bipolar transistor instead. In 1952, the field effect transistor of Shockley was published. John Atalla produced a working device since the materials processing technology was not mature enough until 1960.
Field Effect Transistor (FET)

Field Effect Transistor (FET)

The FET is a transistor that controls the shape and conductivity of a channel on one type of charge carrier in a semiconductor material by relying on an electric field. It is a 3-terminal unipolar device that conducts current using only one kind of charge carrier. It contrasts their single-carrier-type operation with the dual-carrier-type operation of BJT. FETs are majority-charge-carrier devices that consist of an active channel through which majority charge carriers, electrons or holes, flow from the source to the drain. The conductivity of the channel is a function of potential applied to the gate and through ohmic contacts, the source and drain terminal conductors are connected to semiconductor.

As shown in the image below, the gate is the current control terminal. The gate on the FET passes virtually no current when driven with DC as compared to the base terminal of a bipolar transistor which passes a small amount of current. The current flows along a semiconductor path known as the channel. An electrode called the source is at one end of the channel and an electrode called the drain is at the other end of the channel.

Terminals of a FET

Terminals of a FET

The FET operation is simple at the circuit level. The unipolar region between the gate regions is the channel where its resistance is controlled by applying a voltage to the input element, the gate. The device shown below is an N-channel type that is slightly doped N-type slab of silicon with terminal at the ends. The gate in this N-channel device is a heavy P-type region on both sides of the center of the slab that serves as a control electrode. The emitter and collector of a BJT is similar to the source and drain terminals.

Structure of a FET

Structure of a FET

The flow of electrons is controlled by FET from the source to drain by affecting the size and shape of a conductive channel created and influenced by voltage applied across the gate and source terminals. The stream through which electrons flow from source to drain is the conductive channel.

The FET can be constructed from a number of semiconductors and the most common material is silicon. Using the single crystal semiconductor wafer as the active region or channel, most FETs are made with conventional bulk semiconductor processing techniques. A single IC can contain several thousands of FETs along with other components like resistors, capacitors and diodes. Field Effect Transistors exist in two major classifications known as Junction FET (JFET) and Metal-Oxide-Semiconductor FET (MOSFET). Other types include ISFET (ion-sensitive FET), IGBT (insulated-gate bipolar transistor), OFET (organic FET), NOMFET (nanoparticle organic memory FET), DGMOSFET (dual gates MOSFET), MODFET (modulation-doped FET), and others.

References
http://www.allaboutcircuits.com/vol_3/chpt_2/9.html

Tags: FET

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