The IRF640 (which I haven't used in over a decade) is a standard nMOSFET and doesn't readily switch at logic levels, hence, would require some sort of intermediate driver to produce the required gate voltage, which, IIRC, exceeds 10 volts. If you want a MOSFET to operate directly from the MCU pin, you will need a type that has a lower Vth, and there's a family of devices called logic MOSFETs that do just that. There are logic MOSFETs that you could use, but you may not readily find them where you are. I'd recommend watching eBay for such things, as they, together with postage and even customs fees will probably be cheaper on eBay than at the local parts vendor, unless you're very fortunate. You may find it's less costly to buy a partial reel of several thousand parts costing less than $0.01US rather than buying a half-dozen from your local vendor at $5US each.

There are complementary MOSFET pairs, one pMOSFET, and one nMOSFET, which will allow you to experiment quite a bit. The point is that, unless you can considerably exceed the threshold voltage at the gate, with your MCU output, the nMOSFET, will not be fully ON, hence will not provide the full voltage, as was amply pointed out to me. I made this mistake once, and my tech fixed it without even telling me. A lot depends on how you use your MOSFET. If you switch your current at the high side, as I believe you will want to do, you will want a pMOSFET. A little reading will make this quite clear. I'm in the midst of moving from one location to another, so am unable to produce proper references at the moment, for which I apologize. It's vacation time, however, and many people are not available because they're on holiday, so this is one time of year when I do such things as reconfigure the facility, move, maintain software and hardware, etc.

RE