COEN 180
Field Effect Transistors

Transistor Basics

Field Effect Transistors (FET) use the strength of an electric field to control the conductivity of a connection. There are two types, the Junction FET (JFET) and the Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

Junction Field Effect Transistors

Figure 1: N-Channel JFET

In Figure 1, the connection between source and gate passes through the N-doped portion of the silicon. The width of the channel is a measure for the conductivity between the source and the gate. If the gate is positive, then negative carriers are removed from the channel, which then becomes smaller and less conductive. If the gate becomes more negative, then the channel becomes wider and more conductive. We can increase the efficiency of this design by placing another copy of the gate on the other side of the channel.

See also the Scots guide to Electronics on JFET, the PBS series on transistors, or at Thinkquest.

Metal Oxide Semiconductor Field Effect Transistor

In contrast to the JFET, the gate is electrically isolated from the channel in a MOSFET. The isolation is achieved by a layer of silicon oxide. There is a small channel underneath the gate, which allows conductivity between the source and the drain. However, if the gate is charged negatively, then this channel vanishes.

Figure 2: N-Channel MOSFET


© 2003 Thomas Schwarz, S.J., COEN, SCU                                                                              SCU            COEN            T. Schwarz            COEN 180