I'm looking for common MOSFETs to do some experimenting with. They should be able to switch at least 3A at 15V, have a low on resistance for power efficiency and also be quite cheap.

I require all four types, that is P & N channel, both depletion and enhancement mode.

So far I've found the following enhancement mode MOSFETS:

I think these MOSFETs will do the job. However, I'm struggling to find comparable depletion mode MOSFETs. Can anybody make a recommendation?

In general, is there a preference between using a N channel or P channel for switching purposes? I understand with a P channel the source is connected to positive while with an N channel the source is connected to ground. Practically does this make a difference?

  • \$\begingroup\$ Why do you need depletion mode MOSFETs? They aren't widely used (maybe in chip-level design?), and that's why there aren't many available. \$\endgroup\$ – The Photon May 7 '15 at 17:34
  • \$\begingroup\$ Thanks, I didn't know that. I'm building a battery backup circuit and need to disconnect the battery when mains is available. My idea was to use one enhancement mode MOSFET and another depletion mode MOSFET to use either the battery or mains power. If that is the case I'll try to redesign my circuit to only use enhancement mode MOSFETs \$\endgroup\$ – Johan May 7 '15 at 17:44

There is definitely a preference for switching with N-channel MOSFETs. There are two primary practical reasons:

  1. Electron mobility in silicon is ~2-3x higher than hole mobility. This means that, for a given \$BV_{DSS}\$ and \$R_{DS(ON)}\$, an N-mos will be half to a third of the size of a P-mos, and therefore half to a third of the price (raw silicon cost; this doesn't account for package cost, et al).
  2. N-channel MOSFETs switching high-voltage loads can have their gates driven by a signal that is close to ground, obviating the need for a level shifter. (After all, most control circuits are ground-referenced.) P-mos, on the other hand, require a high-side referenced control signal.

As for discrete depletion devices, those are fairly hard to come by. You'll have much better luck looking for JFETs.

  • \$\begingroup\$ Thanks Zulu. That makes sense. I'll try to design my circuit to use N-Channel depletion mode MOSFETS. \$\endgroup\$ – Johan May 7 '15 at 17:41

In response to your final question & add to @Zulu's reply: Using a N-ch as a "low-side switch" is convenient because you can drive their gates directly from any circuitry that's referenced to the same ground as the N-ch's Source pin, when the Source pin is connected to ground, & that includes driving the gate with a microcontroller (so long as you go for "logic-level gate drive" mosfets with a gate threshold voltage of <~2.5V).

However if you're powering some system/peripheral (load) that's off-board, for example, you may not want to be suppling power out to it, and then pulling its ground connection low with the N-ch mosfet to turn it "on", especially from a safety perspective. If the user thinks a device is "off", when in fact one side of it is held up to your + supply voltage & all that's needed to complete the circuit is their fingers/body between it and Ground, then that's a problem. That's where "high-side" switching with a P-ch comes in. The challenge here is that Vgd-Max is often less than the MOSFET's Vds-Max, so you have to 'level-shift' your P-ch's gate drive to stay within this limit.

And there are many other situations within a circuit where one or the other just makes sense.

  • \$\begingroup\$ That's exactly how I was planning on using it! Thanks for the insightful comment. I think I'll be using the P-channel after all. \$\endgroup\$ – Johan May 8 '15 at 5:48

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