war_spigot:
Maybe something like a IGBT. They're made for things with high voltage/current
IGBT's outperform MOSFETs at 100V or more, but they have 2 to 3 volts of Vsat so aren't viable at lower voltages / higher currents.
300A motor controller from scratch is a major project, if you haven't a history of building motor controllers this is not a recommended project, you'll get really demoralized when the Nth incarnation explodes all its MOSFETs yet again...
Check out the woes people have on the OSMC.org mailing list for example:
http://tech.groups.yahoo.com/group/osmc/