Low Threshold MOSFET

Am 20.03.2017 um 05:34 schrieb rickman:

The 2N7000 (200 mAmps, 60 Volts)

has Gate Threshold Voltage of 0.8V min to 3.0V max

Jorgen

I assume you want N channel and that "low current" means up to a few hundred mA.

I use a lot of BSS138 (VGS(th) 0.5 to 1.5V) when driving from 3.3V logic, and DMG1012T (VGS(th) 0.5 to 1.0V) when driving from 1.8V logic.

Both are cheap and readily available.

Regards, Allan

'logic level mosfet' is a good search term.

Cheers

The DMG1012UW is in a bit larger package, 1.3mm vs 1mm lead spacing. The DMG1013UW is a p-channel equivalent.

Go DigiKey searching on FETs. Choose "logic level" as a category. If I remember, you can also choose the gate drive voltage -- choose 3.3V. You should get some candidates to ponder over.

Alternately, maybe a bipolar makes sense? Zetex invented some pretty awesome little bipolar transistors before they got bought out by Diodes, Inc.

That's the threshold voltage where drain current begins essentially. Looking at the drain current vs. drain-gate voltage typical curves the current at 3.0 volts is well under 100 mA.

I suppose I could put two or three in parallel.

I didn't want to use an NPN because the base current goes through the emitter resistor which I'm using to measure the current, but when I did a simulation that current is pretty small, so I suppose it's not really a problem. The resistors are only 1% accurate.

I've got NPNs on hand, so I'll use one for now.

Fairchild (whoever now) has a nice n channel device speced, if I remember correctly, at 1 volt max turn on. It served as a voltage follower op amp with lots of offset. They came from Mouser.

Hul

rickman wrote:

Assuming you want to switch power to some circuit and thus need P-channel:

Key is that Rdson is guaranteed at your min drive voltage, not just in the graphs but in the tables.

N-channel example:

A beefier one:

Any more info than that? Threshold voltage is not easy to search on because everyone specs it at a different current. Also the threshold voltage can be a *lot* lower than the fully on voltage which is the important one. Or is that what you mean? Still, it takes a lot of data sheet searching to find a part that is optimal for low voltage control and that is why I asked here. I am hoping to benefit from other's experience.

The DMP2100U is a P channel and I am looking for an N channel device. However, the DMN2075U is N channel and looks like a very good device for very low voltage controls. Actually, I might need the P channel device for something else later. Thanks.

Shop for parts with Vds < 30V. That's the biggest determining factor in transconductance per area.

I namedropped RUM001L02T2CL here the other day. It's also very tiny, good for saving PCB area.

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

You are welcome. They also make duals in small SMT 6-pin packages if you are ever fighting real estate on a board.

When it has to be rock-bottom in cost a BJT can be the better choice and those also come "pre-biased" where there is a series bases resistor inside its package and sometimes also a B-E resistor. Some folks call them "digital transistors".

Don't go by any graphs. Those only represent typical values. The processes used for MOSFETs have huge tolerances in threshold voltage, easily +/-30%. The only values to go by are guaranteed max Rdson values for a stated gate drive voltage. Typically two to five different gate drive voltages are spec'd. Must be in the tables.

Try fdv301n. I'll verify that in the morning if I get to the office.

Hul

rickman wrote:

Good number. Thanks.

If you want really low threshold devices Advanced Linear has some zero offset +/-1mV ones.

kevin

We have been very happy with the FDN337N.

I was checking this out and found the prices are >$0.20 even on ebay buying 50 or more. Then I checked aliexpress and found this one...

$2.80 for 100 with free shipping! I would say that have to be fakes, but this store has been online for 3 years with 100%. How can they do that?

Don't know. I just discovered that the FDN337N is unobtanium for the next couple of months or more. That prompted me to find a substitute which I have not yet singled out. We need something in the next week or two. Crap!

Sorry about that.