Power MOSFETS (Kevin)

Background Information

• A power MOSFET is a specific type of metal–oxide–semiconductor field-effect transistor (MOSFET) designed to handle significant power levels

• The power MOSFET is the most common power semiconductor device in the world

  • In particular, it is the most widely used low-voltage (less than 200 V) switch

• The operating principle of a power MOSFET is similar to that of a regular MOSFET

• The n-channel enhancement-mode, p-channel enhancement-mode, and n-channel depletion-mode are the most common power MOSFETs

Basic Structure

• Several structures had been explored in the 1970s, when the first commercial power MOSFETs were introduced. However, most of them have been abandoned (at least until recently) in favour of the Vertical Diffused MOS (VDMOS) structure

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Vertical Structure of Power MOSFET

• As pictured above, the cross-section of a VDMOS shows the "verticality" of the device: it can be seen that the source electrode is placed over the drain, resulting in a current mainly vertical when the transistor is in the on-state

Capacitance

• Because of their unipolar nature, the power MOSFET can switch at a very high speed. The only intrinsic limitation in commutation speed is due to the internal capacitances of the MOSFET

• These capacitances must be charged or discharged when the transistor switches.

  • This can be a relatively slow process because the current that flows through the gate capacitances is limited by the external driver circuit