Depletion mode versus enhancement mode

The JFET works by varying the width of the channel. Normally the channel is wide open; as the depletion region gets wider and wider, choking off the channel, the charge carriers are forced to pass through a narrower and narrower path. This is known as the depletion mode of operation for a field-effect transistor.
A MOSFET can also be made to work in the depletion mode. The drawings and schematic symbols of Figs. 1 and 2 show depletion-mode MOSFETs.
However, MOS technology also allows an entirely different means of operation. An enhancement-mode MOSFET normally has a pinched-off channel. It is necessary to apply a bias voltage, E_G, to the gate so that a channel will form. If E_G = 0 in such a MOSFET, that is, if the device is at zero bias, the drain current I_D is zero when there is no signal input.

Fig. 1: At A, simplified cross-sectional drawing of an N-channel MOSFET. At B, the schematic symbol.

Fig. 2: At A, simplified cross-sectional drawing of a P-channel MOSFET. At B, the schematic symbol.

The schematic symbols for N-channel and P-channel enhancement-mode devices are shown in Fig. 3. The vertical line is broken. This is how you can recognize an enhancement- mode device in circuit diagrams.

Fig. 3: Schematic symbols for enhancement-mode MOSFETs. At A, N-channel; at B, P-channel.