The N-channel junction FET (JFET), shown in section view Fig.1a, has an N channel diffused into a P-type substrate and a P-type region diffused or implanted into the N channel to form the P-type gate. Metal deposited directly on the gate, source, and drain regions forms their contacts. Because a JFET has a symmetrical structure, the drain and source are interchangeable. Thus, depending on the location of the ground and the +V power source, the JFET will work in either direction.
Figure 1 Junction field-effect transistors (JFETs): (a) N-channel section view
If a positive voltage is applied at the drain contact and a negative voltage is applied at the source contact with the gate contact open, a drain current flows. If the gate is then biased positive, channel resistance decreases and drain current increases. However, if the gate is biased negative with respect to the source, the PN junction is reverse biased and a depletion region depleted of charge carriers is formed. Because the N-type channel is more lightly doped than the P-type silicon, the depletion region penetrates into the channel, effectively narrowing it and increasing its resistance. If the gate bias voltage is made even more negative, drain current is cut off completely. A gate bias voltage value that will cut off the drain current is called the pinch-off or gate cutoff voltage. The schematic symbol for an N-channel JFET is shown in Fig. 1b. The arrow points from the P-type gate to the N-type channel.
(b) N-channel symbol, (d) P-channel symbol
The P-channel JFET, shown in Fig. 1c, has characteristics similar to those of the N-channel JFET except that the polarities of the voltage and current are reversed. A P-type channel is diffused into an N-type substrate and then an N-type gate region is diffused or implanted into the P-channel to form the N-type gate. If a negative voltage is applied to the drain and a positive voltage is applied to the source, current flows between source and drain. But if the gate is made more negative more current will flow, while if it is made positive with respect to the source, current will be cut off.
(c) P-channel section view
The schematic symbol for a P-channel JFET is shown in Fig. 1d. The arrow points from the P channel to the N gate.