The elements of one type of FET, the junction type (JFET), are compared with the bipolar transistor
and the vacuum tube in figure 3-44. As the figure shows, the JFET is a three-element device comparable
to the other two. The "gate" element of the JFET corresponds very closely in operation to the base of the
transistor and the grid of the vacuum tube. The "source" and "drain" elements of the JFET correspond to
the emitter and collector of the transistor and to the CATHODE and plate of the vacuum tube.
Figure 3-44.Comparison of JFET, transistor, and vacuum tube symbols.
The construction of a JFET is shown in figure 3-45. A solid bar, made either of N-type or P-type
material, forms the main body of the device. Diffused into each side of this bar are two deposits of
material of the opposite type from the bar material, which form the "gate." The portion of the bar between
the deposits of gate material is of a smaller cross section than the rest of the bar and forms a "channel"
connecting the source and the drain. Figure 3-45 shows a bar of N-type material and a gate of P-type
material. Because the material in the channel is N-type, the device is called an N-channel JFET.
Figure 3-45.JFET structure.
In a P-channel JFET, the channel is made of P-type material and the gate of N-type material. In
figure 3-46, schematic symbols for the two types of JFET are compared with those of the NPN and PNP
bipolar transistors. Like the bipolar transistor types, the two types of JFET differ only in the configuration
of bias voltages required and in the direction of the arrow within the symbol. Just as it does in transistor
symbols, the arrow in a JFET symbol always points towards the N-type material. Thus the symbol of the
N-channel JFET shows the arrow pointing toward the drain/source channel, whereas the P-channel
symbol shows the arrow pointing away from the drain/source channel toward the gate.