what is depletion MOSFET

[preethi19]

Hi could anyone pls tell me the difference between enhanced and depletion MOSFET. Saw some posts. I found this

"Enhancements Mosfet's have a channel already between the Drain and Source, so adding a gate voltage of either positive or negative polarity, controls its conductivity. This allows us to use a signal of both polarities without having to worry about the MOSFET cutting off

While in Depletiong MOSFET the channel needs to be created, and that is done by adding a DC voltage to the Gate, untill the condition of "Inversion" is reached and the channel is formed. This restricts us to use only signals of one polarity to maintain the MOSFET operating and avoid the cut-off condition"

What does it mean by enhanced MOS has already a channel. We form the channel for E-MOS by applying gate voltage right which seems to be same as D-MOS where a channel needs to be created. Also what does it mean in EMOS we can use signals of both polarity? I only worked with EMOS so we first apply the VGS to overcome the threshold voltage VTH and which enables us to have positive current flow from drain to source. How can we use both polarities signal?? Isnt it as straightforward as that. Pls help!!!

 

[Dominik Przyborowski]

I think in text quoted by You is a described devices are swapped.

From user point of view a difference is in threshold voltage value. For simplicity I use only n-channel devices - in enhancement mosfet threshold voltage is positive and greater than zero while in depleted one is negative. It means that for single positive supply, enhancement nmos could work as switch - by providing zero vgs it is cut-off, while to cut-off depelted one You have to provide negative (strongly) vgs to it.
For this reason their are also called normally open or normally closed mosfets.

 

[asdf44]

Right. They have the same basic Vgs -> Current relationship (higher Vgs means more current), but the threshold for depletion is below zero.

The fact that they conduct without any bias voltage makes them useful in a number of applications. They lend themselves to this very minimal current source:

That same basic mechanism can be used with an enhancement but it needs an additional pull-up to the high rail. In higher voltage applications that can be cumbersome.