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Depletion MOSFETs?

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boxer4

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Disclaimer: I see Enhancement MOSFETs all day at work, millions and millions of them, so I'm a bit biased...

Based on that argument in another thread, and working with enhancement devices so often, I'm curious if depletion mode MOSFETs still being used/created often today?

Well, I mean circuitry design, specifically for analog circuitry whether it's integrated or discrete high-frequency use. As far as I can tell they've pretty much disappeared - the last few devices I've torn apart are basically full of enhancement devices and BJTs.

But I'd just like a reality check here :)
 
Umm, what's the difference ? :x

Aren't depletion the ones with a solid line next to the gate, instead of a dotted line ? As far as I've seen, I've never found an enhanced one anywere before.
 
I think the basic difference is one is always on while the other is always off.
 
Hi,

The 'Depletion type FET's are the type which are very much like a
triode in their operation.
In that the gate has to go below the 'drain' voltage to reduce the
current flow through the device, rather like the grid has to go below
the cathode voltage to reduce the current through the triode.

Of course triodes have a positive supply rail, but such FETs come as
NPN and also PNP so they could have a positive or a negative supply
rail.

There are a wide range of Oxide FETs, even some with more than one
gate. I don't know much about these newer fancy ones, they may just
be for specialist purposes.

Dunno if the depletion type are still being made, i still have a few
in my little box of FETs.

John :)
 
john1 said:
The 'Depletion type FET's are the type which are very much like a
triode in their operation.
In that the gate has to go below the 'drain' voltage to reduce the
current flow through the device, rather like the grid has to go below
the cathode voltage to reduce the current through the triode.
John :)

Hi John.

I think you meant "has to go below the SOURCE voltage", right? ;-)

Yes, I looked, and depletion mode JFETs are still in production. Just one example:

https://www.fairchildsemi.com/ds/2N/2N5484.pdf

They make a handy simple constant current source by just connecting the gate to the source terminal.

I don't know if enhancement mode JFETs are still available though. Those are very similar in operation to MOSFETs, requiring a positive voltage on the gate in order to conduct. This applies to N channel devices. Of course all the voltages are of opposite polarity with the less popular P channel devices.
 
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Hi Bob,

Yes, i think you're right there.
When i wrote that i put 'source' but when i looked at it, it didn't
look right ....
so i changed it to 'drain' and i still wasn't sure ....
Still, i reckoned that someone would soon say so if i got it backwards.

I meant of course, the one that would correspond to the emitter in
a conventional transistor.
Mind, these Metal Oxide types are probably more prevalent now than
all the other types put together, their power consumption can be so
low that they are likely to be first choice for most applications.

(i wont be editing my previous post, cos then later posts wont make
sense)

Cheers, John :)
 
Thanks, it seems to be the case.

I've never had a depletion MOSFET in my junk pile. I don't think I've ever had a JFET and it looks like the typical JFET behaves as a depletion-type device so any JFET circuit is a depletion mode circuit...

But I still wonder, are there still modern circuits that use depletion type devices, or are the ones made today are merely for replacement in old circuitry?
 
An enhancement mode JFET seems to make no sense, because the gate, being a junction, would be forward-biased, limiting Vgs to 0.7V and causing the gate to draw current.
If you Google "depletion-mode MOSFET", you will find that they are available.
 
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I would think the D MOSFET is still in use; it can work in D mode or E mode, and can be biased easily, with Vgs = 0 volts. On an N channel device, a - gate voltage narrows the channel, and a + gate voltage widens the channel.

The E MOSFET schematic symbol has the broken line, symbolizing the fact that there is no channel in an E MOSFET until you put one there by raising Vgs to the threshold. The D MOSFET has a solid line with the arrow connected to the source, and the gate separate, while in a JFET the arrow is on the gate connection.
 
Roff(LMAO)

Roff said:
An enhancement mode JFET seems to make no sense, because the gate, being a junction, would be forward-biased, limiting Vgs to 0.7V and causing the gate to draw current.
If you Google "depletion-mode MOSFET", you will find that they are available.

See here, oh wise one:

**broken link removed**

It says they are the basis for CMOS ICs, and it looks like the gate is insulated from both junctions?

Regards,
Bob
 
Bob Scott said:
Roff(LMAO)



See here, oh wise one:

**broken link removed**

It says they are the basis for CMOS ICs, and it looks like the gate is insulated from both junctions?

Regards,
Bob
Bob, I'm not getting your point. I was talking about the (im)possibility of enhancement mode JFETs. The article you referenced was about enhancement mode MOSFETs, which indeed are used in CMOS ICs. I spent the last 10 years designing CMOS ICs.
 
There are both N and P channel gated JFET's but they are only depletion mode. Due to their higher transconductance it usually makes more sense to use a JFET rather than a depletion mode MOSFET unless you need the very low current draw.
 
Roff said:
Bob, I'm not getting your point.

I just got your point. For years I have been under the mistaken impression that JFETs existed in both enhancement and depletion modes and that MOSFETs are only available in enhancement mode.:D

Thanks for teaching me something new today. I was staring directly at the truth on that web page but did not realise it until you pointed it out.

It's funny how I could have designed so many circuits with FETs and not pick up the right knowledge about the existence or characteristics of such similar parts that I had no need to use.

Bob
 
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I don't know why you can't get depletion power MOSFETs.

I can accept that if you need a small signal amplifier then JFETs make more sense but depletion power MOSFETs would come in handy for high side switches. Yes I know that we normally either use a higer voltage supply to drive the gate or a p-channel (which have crap charicteristics) but why we can't just use depletaion p-MOSFETS is a mystery to me.
 
Hero999 said:
I don't know why you can't get depletion power MOSFETs.

I can accept that if you need a small signal amplifier then JFETs make more sense but depletion power MOSFETs would come in handy for high side switches. Yes I know that we normally either use a higer voltage supply to drive the gate or a p-channel (which have crap charicteristics) but why we can't just use depletaion p-MOSFETS is a mystery to me.
Do you mean "depletion n-MOSFETs"?
If so- with a grounded load connected to the source, you would have to have a negative supply to turn the transistor off. Not always inconvenient, but often.
 
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Yes, I meant depletion n-MOSFETs and for the situation you've described I would use an enhancement n-MOSFET. I was only advocating depletion n-MOSFETs for high side drivers as opposed to p-channel enhancement MOSFETs which are crap.
 
Hero999 said:
Yes, I meant depletion n-MOSFETs and for the situation you've described I would use an enhancement n-MOSFET. I was only advocating depletion n-MOSFETs for high side drivers as opposed to p-channel enhancement MOSFETs which are crap.
But, as I said, it would necessitate that you also have a (low current) negative supply.
 
No it wouldn't.

The N-MOSFET would be wired as a source follower, when the gate is at +V it would conduct and when it's at 0V it would cut off.
 
Hero999 said:
No it wouldn't.

The N-MOSFET would be wired as a source follower, when the gate is at +V it would conduct and when it's at 0V it would cut off.
Depletion mode FETs are not off when Vgs=0. Think of any Jfet, and what the quiescent conditions would be if you used it as a source follower, with the load returned to zero volts, and the gate at zero volts.
 
You're right I've just simulated it in LTSpice.

The voltage across R2 never goes to 0V unless V3 is about -2V.
 

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