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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