Maker Pro
Maker Pro

P-channel depletion mode Mosfet

komalbarun

Nov 25, 2011
67
Joined
Nov 25, 2011
Messages
67
For P-channel depletion mode Mosfet;

1.Do they require a positve or negative gate voltage to "turn off"? ( i think +ve)
2.Shoud the Drain be +ve and Source -ve?

For N-channel depletion mode Mosfet I think it requires a -ve gate voltage to "turn off"
and Drain +ve and Source -ve, though am not sure about that ^^".
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,700
Joined
Nov 17, 2011
Messages
13,700
Let me answer 2 first: Any P-channel MOSFET has the source positive versus the drain, no matter whether enhancement or depletion type.

1: Any P-channel MOSFET becomes more conductive with negative gate-source voltage. If the transistor is conductive for Vgs=0V (i.e. a depletion type) making Vgs negative will only increase the conductivity. This tells you what Vgs you will need to close the channel.
 

komalbarun

Nov 25, 2011
67
Joined
Nov 25, 2011
Messages
67
Let me answer 2 first: Any P-channel MOSFET has the source positive versus the drain, no matter whether enhancement or depletion type.

1: Any P-channel MOSFET becomes more conductive with negative gate-source voltage. If the transistor is conductive for Vgs=0V (i.e. a depletion type) making Vgs negative will only increase the conductivity. This tells you what Vgs you will need to close the channel.

so this will hold true for n-channels, except polarity reversed?

thnx.
 

kpatz

Feb 24, 2014
334
Joined
Feb 24, 2014
Messages
334
so this will hold true for n-channels, except polarity reversed?

thnx.
Correct. With N-channel FETs the source is negative relative to the drain, and a positive Vgs turns the transistor on. For a depletion-mode N-channel FET Vgs would need to be negative to turn the transistor off.
 

jpanhalt

Nov 12, 2013
426
Joined
Nov 12, 2013
Messages
426
so this will hold true for n-channels, except polarity reversed?

As Harald said, both versions of the depletion-mode mosfet are turned off by a negative Vgs.(That sentence is all wrong. Harald is correct; I mispoke. See edit below.) I used to have a very nice comparison of the two modes, but it is apparently no longer on the Internet.

I found this alternative comparison that has a nice description with graphs in Table 1:
http://www.aldinc.com/pdf/IntroDepletionModeMOSFET.pdf

This more general description by Fairchild is still available and discusses depletion mode:
http://www.fairchildsemi.com/an/AN/AN-9010.pdf

John

@komalbarum

You just beat me to it. My statement is wrong based on a misinterpretation of the graph. I spent much of the morning trying to find a source for a depletion-mode P-channel mosfet. The text in the first link is correct. Vgs needs to be positive to turn off a P-channel, depletion-mode mosfet.
 
Last edited:

komalbarun

Nov 25, 2011
67
Joined
Nov 25, 2011
Messages
67
P-channel depletion mode - turns on with -Ve voltage since --->If the transistor is conductive for Vgs=0V (i.e. a depletion type) making Vgs negative will only increase the conductivity(on)
P-Channel would require a +Ve voltage then to limit conductivity (turn off) .

both versions of the depletion-mode mosfet are turned off by a negative Vgs!?
Did you mean:
both versions of the N-channel mosfet are turned off by a negative Vgs?
 
Last edited:

kpatz

Feb 24, 2014
334
Joined
Feb 24, 2014
Messages
334
N-channel depletion-mode MOSFETS are turned off with a negative Vgs. Positive Vgs turns an N-channel FET on. One way to look at depletion vs. enhancement mode (for N-channel) is: the on-threshold is negative for depletion and positive for enhancement.

P-channel depletion-mode MOSFETs are turned off with a positive Vgs. If you ever come across one. They appear to be rather scarce.
 

jpanhalt

Nov 12, 2013
426
Joined
Nov 12, 2013
Messages
426
I spent a lot of time worrying about my original statement, as it did not seem to make sense, even at the time, but I went ahead and posted. The reason was that I misinterpreted the chart. I have added a correction. (Wish we had line-out as a choice.) BTW, I searched quite awhile and could not find any P-channel version.

Sorry for adding confusion.

John
 
Top