Depletion mode versus enhancement mode
المؤلف:
Stan Gibilisco
المصدر:
Teach Yourself Electricity and Electronics
الجزء والصفحة:
425
12-5-2021
2493
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, EG, to the gate so that a channel will form. If EG = 0 in such a MOSFET, that is, if the device is at zero bias, the drain current ID 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.
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