It's a #transistor - metal oxide semiconductor field-effect transistor (MOSFET) - designed for high #power and/or fast switching.

Uses

• Price to performance ratio - as of 2010, the most common power semiconductor in the world.
• High-power applications:
  • cheaper than IGBTs
  • good efficiency at low voltages
• Fast switching:
  • on the order of nanonseconds
  • From this power MOSFET tutorial (emphasis mine):
    • In N-channel MOSFETs, only electrons flow during forward conduction– there are no minority carriers.
    • Switching speed is only limited by the rate that charge is supplied to or removed from capacitances in the MOSFET. Therefore switching can be very fast, resulting in low switching losses.
    • This is what makes power MOSFETs so efficient at high switching frequency.

construction

Below is the cross section of a Vertical Depletion-type nMOSFET

• or a Vertical Diffused MOS (VDMOS)
• This was the first commercially successful power MOSFET design.

A single power MOSFET will be constructed on a silicon die of thousands of individual cells with construction like the above image. Vertical structure MOSFETS like this are intended to act like a switch, and so have poor behavioral characteristics in the saturated region

According to some website that I found:

Power MOSFETs have a different structure^[1]. As with most power devices, the structure is vertical and not planar. Using a vertical structure, it is possible for the transistor to sustain both high blocking voltage and high current. The voltage rating of the transistor is a function of the doping and thickness of the N-epitaxial layer (see cross section), while the current rating is a function of the channel width (the wider the channel, the higher the current). In a planar structure, the current and breakdown voltage ratings are both a function of the channel dimensions (respectively width and length of the channel), resulting in inefficient use of the "silicon estate". With the vertical structure, the component area is roughly proportional to the current it can sustain, and the component thickness (actually the N-epitaxial layer thickness) is proportional to the breakdown voltage.^[2]

Power MOSFETs with lateral structure are mainly used in high-end audio amplifiers and high-power PA systems. Their advantage is a better behavior in the saturated region (corresponding to the linear region of a bipolar transistor) than the vertical MOSFETs. Vertical MOSFETs are designed for switching applications.^[3]

comparison table