10-20A is a lot, but I would think 7 IRF540 in parallel would work, each will need just a small heatsink. I was testing another design earlier using one IRF540, it was a voltage controlled PWM I intend to post under projects soon, and it handled around 3A comfortably with its small heatsink. I was using nichrome wire as a resistance, it gave a good control over it, from cold to red hot or fused depending on its length. The IRF540 datasheet is readily avialable for download, it shows the pinout. As its an N-channel, its connected so the + rail is like the load's ground, and the output drops from +v to -V. Check out when I post my design.

so what is the max amps of the irf540 ?. I wish to control a car fan and either it is a good mechanical design or that is an awsome motor as it is so powerful it might be too much lol. so i want control over it without loosing tons of power on heat in a transistor (BJT) and causing a cooling problem. i suppose i will put the power mos in front of the fan intake so it is cooled ok. the more amps going thru the more air so it will stay balanced with a smaller heatsink

Its rated at 22A, but its rds on is 0.06ohm typically, so it would need a very big heatsink if used singly, I would reccomend at least 3 used. There may be more suitable MOSFETs, but the IRF540 is at least easy to obtain.

hm ok I'll bear that in mind how much are they roughly.

I payed around 80p for each of mine, 0.57euro.

Here is a quick hand drawn diagram of the voltage controlled PWM system I used earlier to control the heating wire at 3A. It was run off of a 14v cordless drill battery, but should work from around 8-34v. I used a TL084 quad op-amp and most resistors were 1% (this is only really important for the half supply 10ks and the triangle wave amplifier to avoid clipping and give the highest level). The MOSFET switches fast enough even with the 15k gate resistor, as the frequency of the circuit is only 150Hz (this is ideal for most PWM applications, and i've tried this with bulbs, heating wire, motors and computer fans). I intend to post this under projects soon as I think quite a few will find it useful.

EDIT: missed a protection diode off of this, it should be included. Also, most motors generate a lot of noise and will need some snubbering to be used sucessfully. 100nF bypass caps on the op-amp are wise too.