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Depletion FET is conducting when VGS = 0. The current decreases when VGS < 0 until VGS = VGS(th), when conduction stops.
Depletion MOSFETs, unlike enhancement MOSFETs, are in an on-state even at 0 V of gate-to-source voltage (VGS). This feature makes them suitable for using as a constant current source as well as in other ways. This application note explains how depletion MOSFETs can be used in different applications.
There are 2 types of N-Channel MOSFETs, enhancement-type MOSFETs and depletion-type MOSFETs. A depletion-type MOSFET is normally on (maximum current flows from drain to source) when no difference in voltage exists betweeen the gate and source terminals.
The n-regions are indicated as N + in the figure. The two regions N + act as source and drain for the MOSFET. Further, between the two N + regions, that is, source and drain, an N-channel is implanted as shown in figure (3).
The basic idea behind the operation of n-channel depletion mode MOSFET is to connect the n-type source and n-type drain diffusions by intentionally doped n region as shown in Figure as a n-channel. Hence in n-channel depletion mode MOSFET the n-type channel is already exists without the application of positive gate bias.
Unlike enhancement-mode transistors, which are “normally-off” devices, depletion-mode MOSFETs are “normally-on”. N-channel devices are built with P-type silicon substrates, and P-channel versions are built on N-type substrates. In both cases they include a thin gate oxide situated between the source and drain regions.
22 lis 2022 · The only difference is whether the conductive channel is absent at zero gate voltage (enhancement-mode) or present at zero gate voltage (depletion-mode). You can even build a MOSFET that's somewhere between enhancement-mode and depletion-mode with a threshold voltage of exactly zero volts.
