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Today, from ignorance I have fallen head-first into the world of MOSFET transistors. In my scramble to find some information on the MOSFET I will be using as a switch (HEXFET actually), I learned that MOSFETs in general come in two modes, enhancement mode, or depletion mode.

When I tried to find out which mode the IRF3710 was, from the datasheet, I found that it does not say (or maybe I need glasses). At this point I started searching to find how to tell the difference between the two modes. After some time I gathered that the schematic symbols differ:

Enhancement-mode MOSFET:

enter image description here

Depletion-mode MOSFET:

enter image description here

The difference being the highlighted part below.

enter image description here

Three separate lines means enhancement-mode (left) and one solid line means depletion mode (right).

So, my question: Is this the only way to tell which is which, or is there a quicker way to tell (by markings on the device maybe?). Also, are there symbols out there which use a different method to differentiate between them?

I am asking here for my own learning, but also for other people who might have the same experience as me. I did not find that much helpful info in my searching.

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    \$\begingroup\$ Depletion mode silicon MOSFETs are rather rare with only a few vendors making them. In 99% of cases they will be enhancement devices. I have never seen a depletion mode device in 30 years of using them. \$\endgroup\$ – Kevin White Sep 4 '18 at 18:10
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    \$\begingroup\$ If it doesn't say, it is enhancement mode. If you ever come across a depletion mode mosfet, it will be indicated in the datasheet. \$\endgroup\$ – mkeith Sep 4 '18 at 18:18
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Two things I want to add to the answers already given:

  1. Don't trust the schematic symbol. You'll see the depletion-mode symbol used pretty often for an enhancement-mode part because it's easier to draw. (The symbols suggested on the manufacturer datasheets won't make this error, but some random application circuit schematic from the web is not trustworthy at all)

  2. How to tell from the datasheet whether the part is enhancement mode or depletion mode. For an n-channel FET, if the \$V_{gs({\rm th})}\$ is greater than 0, then it's an enhancement mode device. If \$V_{gs({\rm th})} < 0\$ it's a depletion mode device. For p-channel, it's the opposite: \$V_{gs({\rm th})} < 0\$ means enhancement mode, \$V_{gs({\rm th})} > 0\$ means depletion mode.

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  • \$\begingroup\$ Thank you, this was just the type of answer I was looking for. Your note about not trusting the schematic is important, and the method of checking will work in all situations where the datasheet is available. \$\endgroup\$ – hat Sep 5 '18 at 5:44
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You can tell by looking in the datasheet at the numbers. Usually it will say up top, because depletion mode is relatively rare, but that's not always true. For example, small VHF Mosfets are often depletion mode and that's sometimes not mentioned.

For a depletion-mode MOSFET the Idss will be relatively large, and the cutoff current will be relatively small and specified with a negative voltage for N-channel and positive for P-channel. Below is a small N-channel device (as far as I can tell, discrete P-channel depletion mode parts are not manufactured).

enter image description here

For the much more common enhancement mode MOSFET, the Rds(on) and Id(on) will be specified with Vgs positive for N-channel and negative for P-channel, and Idss will just be the leakage current, and relatively small.

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In most supplier's search engines (and in manufacturing selection webpages) the enhanced-mode MOSFETS are considered as "normal" or "standard" and may not have any clear label as "enhanced", while the depletion-mode transistors are explicitly designated as "depletion mode". Here is an example from Digi-Key:

enter image description here

AFAIK, there is no common convention on how the depletion-mode transistors are designated, all is vendor-specific.

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    \$\begingroup\$ It's not a great idea to 100% rely on a distributor search page for part characteristics. I've seen PMOS devices categorized as NMOS and all kinds of other errors in their data. Always double-check the manufacturer datasheet. \$\endgroup\$ – The Photon Sep 4 '18 at 17:46
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Rather than look at symbols you need to examine the device datasheet.

An Enhancement mode FET such as your IRF3710 will have a couple of VGS = 0 characteristics with a Vds current that is very low, usually this is the Vds breakdown voltage or Vds leakage current specification. There will also be a VGs(threshold) specification, which is the beginning of conduction (starting to turn the FET on):

enter image description here

For a depletion mode FET such as the DN2625 the same leakage current specifications will show a non-zero value for VGS (the device must be turned off to measure the breakdown or leakage currents). The inverse of the VGS(threshold) to turn on the enhancement mode FET is the VGS(threshold) required to turn off the depletion mode device (be careful here, in that the value is that required to reduce ID to the leakage current, not the threshold to begin lowering the current):

enter image description here

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Is there another way to tell which is which based on the schematic symbol alone? No, not really.

You can also deduce it from the context:

  • If there's a part number, look up the datsheet.

  • Can the circuit even work with a depletion mode device? Depletion mode MOSFETs are on with zero gate-source voltage, which is unlike enhancement mode MOSFETs (be they N or P channel).

  • Depletion mode MOSFETs are really rare. At the time of writing, digikey lists 242 depletion mode parts out of 47915 discrete MOSFETs. Is it likely that some engineer included one in the design?

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  • \$\begingroup\$ Part of the question says they have a datasheet and don't know how to tell whether the device is enhancement or depletion mode. \$\endgroup\$ – The Photon Sep 4 '18 at 17:44
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Look at the Vgs(th) on the datasheet

It's possible to tell a depletion device from an enhancement one using a single, simple rule: is the Vgs(th) the opposite sign of what you'd expect it to be from the device polarity (PMOS vs NMOS)? If so (negative Vgs(th) on a NMOS, positive Vgs(th) on a PMOS), then you're looking at a depletion mode device. If not (positive Vgs(th) on a NMOS, negative Vgs(th) on a PMOS), then you're looking at an enhancement mode device.

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