Gate oxide

The gate oxide is the third region of the MOSFET between the source and drain. It is a thin layer of pure, defect free, 5 - 200 nm thick thermally grown oxide. It serves as the dielectric layer so that the gate can sustain as high as 1 to 5 MV/cm transverse electric field in order to strongly modulate the conductance of the channel.

Above the gate oxide is a thin electrode layer made of a conductor which can be aluminium, a highly doped silicon, a refractory metal such as tungsten, a silicide (TiSi, MoSi, TaSi or WSi) or a sandwich of these layers. This gate electrode is often called gate metal or gate conductor. The geometrical width of the gate conductor electrode (Z, W) is called the gate width. The gate width or geometrical gate width is not to be confused with the conduction channel width or electrical channel width since they are not equal. The conduction channel may be slightly wider due to fringe electric fields at the two width (Z-direction) edges of the gate electrode.

Below the gate oxide is a thin n-type inversion layer on the surface of the p-type semiconductor substrate (for an n-MOS device). It is induced by the oxide electric field from the applied gate voltage V_G. This is known as the inversion channel. It is the conduction channel that allows the electrons to flow from the source to the drain. ["Fundamentals of Solid-State Electronics", Chih-Tang Sah. World Scientific, first published 1991, reprinted 1992, 1993 (pbk), 1994, 1995, 2001, 2002, 2006, ISBN 9810206372. -- ISBN 9810206380 (pbk).]

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