Depletion N-Channel Mosfet

Hello, I'm searching for a depletion (normally-ON) which can work under 9V/5V Vds (3.2V would also be nice but not required).
The Vgs threshold should be around 1-3V for 9V, 1-1.5V for 5V.
Unfortunately I only seem to find components that work with VDS ~25V or more… do you know if anything like what I need exists? It would be best in a packaging suitable for breadboard prototyping, but at the moment I'm open to any packaging.

In case you think this is not possible, do you think there can be another way to arrange this kind of normally-on switch?

Thanks for your time! And forgive my noobness :slight_smile:

Vgs is the voltage of the gate relative to source - note that to turn off an N-channel depletion mode mosfet, you need Vgs to be negative.

Vds is the voltage of the drain relative to source - typically spec'ed as a maximum.

Depletion mode mosfets generally have higher cost and inferior performance to enhancement mode ones, and are available in far fewer designs.

Is there a reason you can't use an enhancement mode MOSFET instead? You always should have a resistor between the gate and source if you want a mosfet to be normally off (otherwise, if the Arduino pin is tristated (like the arduino is in reset or disconnected), the mosfet will randomly turn on and off as the gate voltage changes in response to ambient electrical fields) - if you wanted to make the mosfet normally on, you could just connect that resistor between gate and Vcc and drive it low to turn it off (for n-channel mosfet - for p-channel, between gate and ground, drive high to turn off).

First of all thanks for your reply.
Let me explain better what I need.
I have to switch something on whenever a voltage (my Vg in the mosfet case) goes below a certain threshold (e.g. drops of a couple of volts). The first thing I thought was depletion MOSFET: the drop in Vg, hence in Vgs, makes my MOSFET switch on.
Is there something I don't get?
Thanks

EDIT: I talked about logic level MOSFETS and whatnat because my Vcc is 5V or 9V

So you're not using an Arduino here to control the fet? I assumed you were.

A mosfet (of either kind) does not sharply turn on or off at a certain voltage. There's a threshold voltage where it starts to turn on or off, but it doesn't completely switch at that voltage. That's why an enhancement mode mosfet might have a threshold voltage of 1.2v, but not actually turn on completely until 4.5v. I now think this is even less appropriate for your purpose.

It sounds like what you want is a comparator. If the voltage will be between Vcc and ground, and the trip point isn't too close to either, isn't what you want a comparator, like the ubiquitous LM339 (quad comparator in DIP/SOIC14) or 339 (dual comparatopr in DIP/SOIC8), paired with a voltage reference like a divider between Vcc and ground or zener diode?

I assume you use an Arduino of some sort.
Why can't you just use an analogue input.
You can set the trigger point to any voltage you like.
Leo..

Wawa:
I assume you use an Arduino of some sort.
Why can't you just use an analogue input.
You can set the trigger point to any voltage you like.
Leo..

Part of the circuit can definitely driven by arduino (I don't actually know why I haven't thought about it before), still the problem remains for the 9V array

DrAzzy:
[cut...]

It sounds like what you want is a comparator. If the voltage will be between Vcc and ground, and the trip point isn't too close to either, isn't what you want a comparator, like the ubiquitous LM339 (quad comparator in DIP/SOIC14) or 339 (dual comparatopr in DIP/SOIC8), paired with a voltage reference like a divider between Vcc and ground or zener diode?

I'll look into this one, thanks...

ilfuria:
still the problem remains for the 9V array

? 9 volt array ?

0-5volt can be measured directly with one of the analogue inputs.
0-9volt (or more) needs a simple 2-resistor voltage divider.

If we know exactly what you want to do, we can help you better.
Leo..

Just forget about depletion mode FETs.

They are basically obsolete as they require secondary negative supply voltages.

Explain what the "something" is that you wish to switch, exactly what supply voltage is that you have available, and what the voltage source to be monitored is, exactly what the thresholds are, whether you require hysteresis, what processing you might need on that signal. Everything about it in fact.

If you want to switch depending on comparing two voltages, you need a comparator
with positive feedback controlling the switching device. Standard enhancement mode
power MOSFETs are pretty universal for switching. Any attempt to
switch in the linear domain of a power device will introduce problems of heat generation,
incomplete switching, component variability (threshold voltages vary +/-1V between
devices in the worst case).

A lack of hysteresis in your system could also cause oscillation, as the voltage controlling
the switching may be affected by the act of switching - some thought into the amount
of positive feedback may be needed.

If you explain more fully what you want to achieve you'll get a higher quality answer.
A vague description like "a certain threshold" and so forth doesn't provide the depth
of information needed to judge the best technique for the specific job.

Unless you are doing RF work. Many GaAs and GaN devices are depletion mode devices with schottky gates.