Why are there more N type than P type MOSFETs?
I was looking at different transistors, especially MOSFETS and I saw that N type MOSFETs seem to be way more popular to be used in circuit design. Is there a reason for that, or am I mistaken?
2 answers
There are two predominant reasons you see N channel FETs more than P channel:
- N channel is the natural polarity for ground-referenced switches controlling loads with positive voltages. This is convenient because most controlling circuitry is ground-referenced too, with positive supply.
With the FET source grounded, the drain tied to the negative side of the load, the gate voltage swings from 0 to whatever value turns on the FET sufficiently. This is the same voltage range where the controller and other signals already are.
One drawback with FETs, particularly if they have to control high voltage or high current, is that the gate needs to be as high as 12 V to turn on the FET. That can be inconvenient, but is still usually better than what a P channel FET requires. For small enough voltage and current, there are "logic level" FETs that can be driven from normal logic voltages directly. There are also gate driver chips to simplify driving a 0-12 V gate signal from a 0-3.3 V logic signal.
- N channel FETs usually have better switching characteristics for otherwise the same parameters. This is because holes are more sluggish than electrons. N-type material works on conduction of electrons, and P-type material on conduction of holes.
That all said, P channel FETs are not something to shy away from when the right requirements arise. Sometimes the low side of a load must stay grounded, and you need to switch the high side. I usually do use P channel FETs in such cases. The gate voltage only needs to swing from the supply to about 12 V below the supply. An N channel FET would require gate voltage swing from 0 to 12 V above the supply to do the same thing. The extra voltage above the supply is inconvenient, and the larger voltage swing requires higher dV/dt for the same result. That is not only harder to achieve, but will cause more radiation.
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One reason could be that N type MOSFET can switch loads that are in Drain, meaning that whatever the load voltage, you can turn it on/off directly with any microcontroller, provided that the MOSFET is 5V or 3.3V logic level. You could invert it and do the same with P-Channel, but N-Channel just feels more natural, since it shares common ground. Furthermore, going into design details, N-Channel MOSFETS have faster switching due to higher electron-hole mobility. Due to this, P-Channel MOSFETS have a higher resistance when fully on.
Presumably these characteristics lead to N-Channel being more common.
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