AN211A: Field Effect Transistors in Theory and Practice, Courtesy of Motorla

There are two types of field-effect transistors, the Junction Field-Effect Transistor (JFET) and the "Metal-Oxide Semiconductor" Field-Effect Transistor (MOSFET), or Insulated-Gate Field-Effect Transistor (IGFET). The principles on which these devices operate (current controlled by an electric field) are very similar - the primary difference being in the methods by which the control elements is made. This difference, however, results in a considerable difference in device characteristics and necessitates variances in circuit design, which are discussed in this note. In its simplest form the junction field-effect transistor starts with nothing more than a bar of doped silicon that behaves as a resistor (Figure 1a). By convention, the terminal into which current is injected is called the source terminal, since, as far at the FET is concerned, current originates from this terminal. The other terminal is called the drain terminal. Current flow between source and drain is related to the drain-source voltage by the resistance of the intervening material. In Figure 1b, p-type regions have been diffused into the n-type substrate of Figure 1a leaving an n-type channel between the source and drain.