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MOSFET enhancement type?

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mneary

New Member
Enhancement type MOSFETs are much more common than depletion type. They are available in both N and P channel, so this is no help. To be sure, look at the data sheet. The gate threshold voltage VGS(th) is positive (for N type) so it is enhancement type.

*Specifically, the gate threshold voltage VGS(th) is the same polarity as the active Drain voltage: V(BR)DSS so it's enhancement mode.
 
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kpatz

New Member
Enhancement-type MOSFETs are "normally off", until a positive voltage (N-channel) or a negative voltage (P-channel) is applied to the gate.

Depletion-type MOSFETs are like JFETs, they're "normally on", and you need to provide an opposite voltage to the gate (negative for N-channel, positive for P-channel) relative to the source to turn the transistor off.
 

Hero999

Banned
Does any one know why they don't make power depletion MOSFETs?

Having a high power, normally on switch would be handy for some applications.
 

Hero999

Banned
I've not seen anything like that before.

Those are all rated to ≥500V, is there anything rated to 50V? Obviously they can be used at lower voltages but 330mΩ channel resistance is too high for low voltage battery applications.
 

crutschow

Well-Known Member
Most Helpful Member
Does any one know why they don't make power depletion MOSFETs?
A depletion-mode MOSFET requires an opposite polarity voltage to turn off, thus they are difficult to use with single supply circuits.

As far as I know all JFETS are inherently depletion mode, so that's the only kind of JFET you can buy.
 

RCinFLA

Well-Known Member
Early FET's, in 60's and 70's were typically depletion mode. They acted very much like a low voltage vacuum tube.

The first DRAM's had negative supply generators for their gate biasing.

Enhancement mode are much easier to work with since they don't require the negative gate bias.

What makes a FET enhancement or depletion is how the drain - source channel is constructed and how the gate is oriented over the channel.
 

Hero999

Banned
A depletion-mode MOSFET requires an opposite polarity voltage to turn off, thus they are difficult to use with single supply circuits.
It could be useful as a source follower in a linear regulator or audio amplifier.

With an enhancement device you have to take the gate above the positive supply to turn it on, with a depletion, you don't.

The other option is to use a p-channel device which is more expense, has poorer characteristics and can be hard to stabilise when used as a common source amplifier driving and unknown and potentially capacitive load.


So far the only depletion power MOSFETs I've seen are those linked by jpanhalt which are high voltage devices. I wonder why one would want a high voltage normally on switch? It sounds like a safety thing but you can't rely on a semiconductor for safety critical switching.

As far as I know all JFETS are inherently depletion mode, so that's the only kind of JFET you can buy.
How many JugFETs have you seen that are rated to ≥10A?
 

jpanhalt

Well-Known Member
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So far the only depletion power MOSFETs I've seen are those linked by jpanhalt which are high voltage devices. I wonder why one would want a high voltage normally on switch?
I don't understand the problem of having a maximum Vds of 500V. They are spec'd to less than 5V Vds and the packages (e.g., TO-252 and TO-268) are common for mosfets rated at 20A. The other producer I found, Supertex, may have smaller packages, as it does not offer products with quite the current capability of the IXYS devices. Perhaps, the high voltage rating is simply a function of the way they are manufactured.

John
 

Hero999

Banned
It's simple: channel resistance, I touched on this above.

The channel resistance increases exponentially with breakdown voltage.

A 330mΩ channel resistance for a 500V MOSFET with an off voltage of 5V is excellent but it's very poor for a 50V MOSFET of a similar calibre.

This might not be important for an audio amplifier but for a LDO regulator or switch it is.
 

crutschow

Well-Known Member
Most Helpful Member
How many JugFETs have you seen that are rated to ≥10A?
None. Where did the ≥10A value come from? JFETs are normally small-signal devices.
 

Hero999

Banned
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