MOSFET (Metal Oxide Semiconductor Field Effect Transistor )is a kind if transistor (a electrical switch) that controls current in an amplifier. Most outboard amplifiers use MOSFET transistors because they are a lot more efficent (produces less heat, in turn makes more output power) and because they are a very fast switch which in turn means a great effective frequency range (ie it sounds better). A lot of head units still use Bi-polar transistors mainly because of cost and packaging reasons but do not have the capabilities of a MOSFET powered head unit. Hope this helps, I'm terrible at explaining this kinda stuff in writing.

Darren

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