The invention discloses a VDMOS
transistor and a preparation method thereof and belongs to the field of semiconductors. The VDMOS
transistor comprises a first
conduction type substrate, a first
conduction type epitaxial layer, a second
conduction type injection region, a first conduction type highly doped source region and a grid structure, the second conduction type injection region and the first conduction type highly doped source region are arranged in the epitaxial layer, and the grid structure comprises a grid insulating layer, a semi-insulating
polycrystalline silicon layer, a
silicon oxynitride layer and a
polycrystalline silicon layer. The grid insulating layer is arranged above a drift region of the epitaxial layer, the semi-insulating
polycrystalline silicon layer is arranged above the grid insulating layer, the
silicon oxynitride layer is arranged above a channel region, and the polycrystalline
silicon layer is covered on the semi-insulating polycrystalline silicon layer and the
silicon oxynitride layer. A heat
nitriding silicon oxynitride layer is introduced into the preparation method, a traditional silica layer which is used as a gate medium layer on a channel is replaced, and the semi-insulating polycrystalline silicon layer is added on an
oxide layer which is arranged above the drift region of the epitaxial layer. The VDMOS
transistor and the preparation method thereof can obviously reduce grid-drain
capacitance and achieve overcoming the defects that insulating performance of the grid insulating layer is poor, current leaking of a grid
electrode is increased, and degeneration of the VDMOS performance is unreliable.