Observe that the device polarity (n-channel or p-channel) is indicated by the direction of the arrowhead on the gate line. Figures 5.69(b) and (c) show the circuit symbols for JFETs of both polarities. (c) Circuit symbol for the p-channel JFET.Ģ 2 MICROELECTRONIC CIRCUITS SEDRA/SMITH It should be obvious that a p-channel device can be fabricated by simply reversing all the semiconductor types, thus using p-type silicon for the channel and n-type silicon for the gate regions. (b) Circuit symbol for the n-channel JFET. This is a simplified structure utilized to explain device operation. Channel n (a) (b) (c) FIGURE 5.69 (a) Basic structure of n-channel JFET.

The major role that this pn junction plays in the operation of this FET has given rise to its name: Junction Field-Effect Transistor (JFET). Indeed, it is the reverse bias on this junction that is used to control the channel width and hence the current flow from drain to source. The device operation is based on reverse-biasing the pn junction between gate and channel. The n region is the channel, and the p-type regions are electrically connected together and form the gate. It consists of a slab of n-type silicon with p-type regions diffused on its two sides. 5.69(a) shows a simplified structure of the n-channel JFET. Device Structure As with other FET types, the JFET is available in two polarities: n-channel and p-channel. Another important reason for including the JFET in the study of electronics is that it helps in understanding the operation of gallium arsenide devices, the subject of the next section.

In this section, we briefly consider JFET operation and characteristics. Its integrated-circuit applications are limited to the design of the differential input stage of some operational amplifiers, where advantage is taken of its high input resistance (compared to the BJT). Currently, its applications are limited to discrete-circuit design, where it is used both as an amplifier and as a switch. This, together with the many other advantages of MOS transistors, has made the JFET virtually obsolete. Unfortunately, however (for the JFET), the MOSFET has an even higher input resistance. It has some important characteristics, notably a very high input resistance. JFETs, GaAs DEVICES AND CIRCUITS, AND TTL CIRCUITS THE JUNCTION FIELD-EFFECT TRANSISTOR (JFET) The junction field-effect transistor, or JFET, is perhaps the simplest transistor available. These sections provide valuable information, but please note that the references do not correspond to the 6th or 7th edition of the text. 1 This material is from a previous edition of Microelectronic Circuits.

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