NMOS vs PMOS and Enhancement vs Depletion Mode MOSFETs
We decided to create a tutorial talking about PMOS versus NMOS and enhanced versus depletion mode MOSFETs because, frankly, these terms were very confusing to me when I was first learning about them. So, hopefully I’ll be able to make things a bit clearer if you’re as confused as I was.
The easiest way to look at this is to consider a MOSFET to be in one of two states - it’s either on or off, we’ll ignore terms like cut-off, saturation, and triode even though those are important. So treating these as a simple switch, let’s address depletion versus enhancement modes first. An enhancement MOSFET is by definition “off” when there is no gate voltage, or when VGS is 0. In contrast, a depletion mode MOSFET is “on” when there is no gate voltage, it is naturally in a conducting state. You can think of it as the threshold voltage needed to turn on the FET is basically 0 for depletion mode devices.
So, if VGS is equal to VTH then an enhancement mode MOSFET will turn on and start conducting and a depletion mode MOSFET will simply remain on. However, if you have the opposite polarity of the gate voltage, it will turn off the depletion mode FETs. To help you remember which one is which, the reason they’re named this way is that if you put a gate voltage on an enhancement mode MOSFET, you are enhancing the number of free carriers, making it so it can conduct. Whereas if you put an opposite gate voltage on a depletion mode MOSFET, you are depleting the number of free carriers, stopping its ability to conduct.
That being said, if you put a gate voltage on a depletion mode MOSFET that would turn on an enhancement mode MOSFET, the depletion mode MOSFET will also be enhanced by attracting even more free carriers. If you’re confused about what’s going on at the semiconductor level of these devices, check out our other tutorials that go into depth into the physical operation of a MOSFET.
I’m also trying to be somewhat vague in the usage of the term threshold voltage because the value can be wildly different depending on the MOSFET and, depending on the type of MOSFET you have, can be either positive or negative, which we’ll discuss now.
With an NMOS, the current carrier is electrons, so as you have a positive charge, it attracts those negatively electrons to the channel and then it can conduct. With a PMOS, the current carrier is holes, so as you have a negative charge, it attracts those positively charged holes to the channel and then it can conduct. Conversely, if you put a negative voltage on an NMOS or a positive voltage on a PMOS, it’s repelling the free carriers so neither an enhancement nor depletion device can conduct. That’s how you turn-off a depletion mode MOSFET.
So, with this, we can create a table showing when each device can or can’t conduct. Again, notice that the enhancement mode MOSFETs can’t conduct when VGS is zero but depletion mode MOSFETs can. And N-channel MOSFETs are able to conduct when VGS is above the positive threshold voltage and P-channel MOSFETs are able to conduct when VGS is more negative than the negative threshold voltage.
What you’re doing will affect how often you see each of these. If you work with integrated circuits, you’ll likely see all four flavors discussed here. Otherwise, it really depends, we use NMOS enhancement mode FETs for low power switching, and many H-bridge configurations include both NMOS and PMOS transistors, but none of us here at CircuitBread have ever used a depletion mode MOSFET. Whatever you end up doing, hopefully now you at least have a better idea of what it all means and won’t get them confused anymore. If you liked this tutorial, give it a like, and also check out all the tools and resources we have here in CircuitBread for all engineers of every level and we will catch you in the next one.
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