P-channel depletion mode Mosfet

[komalbarun]

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]

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]

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]

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]

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.

 

[komalbarun]

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?

 

[kpatz]

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.

 

[komalbarun]

Thanks for all the help.

 

[jpanhalt]

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