Hi. My first post since finally I figured I might help just a little at least...
It's not very short, but bear with me ok?
MOSFET's are really a can of worms especially when trying to understand them in the beginning. I'm going to be really crude in my explanation since I wish to convey the general idea of why they act the way they act. Please, correct me if I'm using totally wrong explanation to convey the idea and I'll edit my post.
The reason why MOSFET's have the characteristics of capacitors can be understood quite simply: there's no galvanic connection between the gate and rest of the semiconductor layer and they create a capacitor as a result of that. More voltage the MOSFET can handle, the wider the gap between the gate and source/drain parts and bigger the distance between the junctions of the Source <-> Drain. And more current the MOSFET can handle, wider the gate/source interface is. Thus as a rule of thumb: bigger the MOSFET -> bigger the "power" needed to drive it fast.
The why these dimensions matter is because they create stray capacitances and inductances that have to be overcome before the system reacts to a change at the Gate. But what's more, the gate itself has to be proper size and charged to a proper voltage to get the resistance between Source and Drain low enough when switching.
The important thing in MOSFET's datasheet is "Total Gate Charge". That is the sum of all the known factors that affect the amount of energy needed to fully switch on the MOSFET.
As an example:
http://www.pololu.com/file/0J56/irfr3707zpbf.pdfSee the page 2. Total Gate Charge has been given in nano coulombs. a Coulomb is defined as: 1C = 1A * 1s or 1 coulomb = 1 ampere of current in 1 second.
This means in the case of a MOSFET, that if one has a "Total Gate Charge" of 1 coulomb, the current required to charge it in 1 second equals to a 1 amps. Since coulombs are a very large amount of charge, mostly we're using nano or even pico coulombs.
If you wish to charge the Gate faster, let's say in half a second, we can do the arithmetic and we'll arrive to the conclusion that it takes double the current in half the time to charge. The faster you want to switch the MOSFET on, more current you need in one second. After the gate is charged however, the current flow to the gate stops (there's a tiny bit of leakage current, but it's mostly negligible.). When you wish to turn the MOSFET off again, you'll need to discharge the gate and the same rule applies again, faster you discharge it, faster the MOSFET switches to off state.
Thus there's a real need to use external driver circuits when trying to handle large amounts of current and voltage over the MOSFET. Puny micro-controller I/O pins can't handle the current spikes or even give enough current even if they could handle it to drive big MOSFET's fast enough to handle large currents.
Since when the MOSFET is changing it's state, there's a time when the resistance is not infinite or near zero and this is the time the MOSFET is turning all that current going trough it in to a heat. The faster you go over this period (faster you drive the device) less heat you're dissipating in the MOSFET itself. I'm not going into the nitty gritty details of calculating this right now but it's not very hard if need to be.
Whoah that was a long post...
I hope this helps a little and you gain more valuable insight on the why's and how's of the MOSFET's... it wasn't' very intuitive for me either when trying to piece it together but it's valuable information when trying to drive larger currents.
I might edit this post or create a totally different tutorial type post with all the facts and figures and calculations later date, but right now I must sleep and please, ask if you don't' understand something or tell me if I got something totally wrong and i'll fix it.