so i've been sitting out of this discussion because i've never taken the time to read up on how transistors work.
as a result i can't tell you anything on why or how they work.
also the language i use may not be correct.
where i can possibly help is how you can identify a MOSFET that will work for you and how to use it.
if you want to use a transistor as a simple on/off switch MOSFETs work well.
MOSFETs expect a certain voltage level to switch them on.
different MOSFETs will require different voltage levels to switch them on. this will be displayed as a graph in the datasheet, typically called "SATURATION CHARICTERISTICS".
if you don't trigger them with enough voltage then they still appear to work but are more wastefull of power, generating more heat and reducing the life of the component.
MOSFETs will always generate some heat. if you are switching a big load then you will need to attach a heatsink.
forget using an op amp to boost the voltage from your micro to the MOSFET.
use a MOSFET driver IC. you will get far better results.
or (better solution) pick a MOSFET that can be driven from TTL voltages. (they are out there, you just have to search.)
that's about it.
o, one more thing,
this might save you a bit of searching,
here's a MOSFET that can handle 30A @ 60V and be driven straight from a microcontroller's 5V logic:http://www.fairchildsemi.com/ds/RF/RFP30N06LE.pdf
take a look at Figure 7. on page 4. of the datasheet.
it shows you what your Gate voltage (Vgs) needs to be for different loads.
you can clearly see that the 5V logic from your microcontroller could power a load of 60A at 4.5V or over.
someone please correct me if i'm wildly off the mark with any of this.
it's only what i have picked up from datasheets.