DEPLETION VERSUS ENHANCEMENT

In a JFET, the channel conducts with zero bias, that is, when the potential difference between the gate and the source is zero. As the depletion region grows, charge carriers pass through a narrowed channel. This is known as depletion mode. A MOSFET can work in the depletion mode, too. Metal-oxide-semiconductor technology allows a second mode of operation.
An enhancement-mode MOSFET has a pinched-off channel at zero bias. It is necessary to apply a gate bias voltage E_G to create a channel. If E_G = 0, the drain current I_D is zero when there is no signal input. The schematic symbols for n-channel and p-channel enhancement-mode devices are shown in Fig. 1. In schematic diagrams, they can be differentiated from depletion-mode devices by looking at the vertical lines inside the circles. Depletion-mode MOSFETs have solid vertical lines; enhancement-mode devices have broken vertical lines.

Fig. 1. (a) The symbol for an n-channel enhancementmode MOSFET. (b) The symbol for a p-channel enhancement-mode MOSFET