MOSFET having high and low dielectric materials

A semiconductor device comprising:

• a channel of a first conductivity type formed on a surface layer of a semiconductor substrate;
• a source and a drain of a second conductivity type formed on both sides of the channel;
• a gate insulation film with a first relative permittivity formed at least on said channel directly or through a buffer insulation film;
• a gate electrode formed on said gate insulation film;
• and
• a side insulation film formed at least on a side of said gate insulation film and having a second relative permittivity which is smaller than the first relative permittivity, wherein
• an electric flux density in said gate insulation film on a side towards the channel side is more dense than an electric flux density in said gate insulation film on a side towards the gate electrode,
• an area of a bottom part of said gate electrode is larger than an area of an upper part of said gate insulation film, and
• a length of a bottom part of said gate electrode parallel to a channel length direction is longer than a length of an upper part of said gate insulation film along a channel length direction.

A semiconductor device comprising:

• a plurality of first MOS transistors, each of said first MOS transistors including a first channel of a first conductivity type formed on a surface layer of a semiconductor substrate, a first source and a first drain of a second conductivity type formed to both sides of said first channel, a first gate insulation film with a first relative permittivity formed at least on the first channel directly or through a buffer insulation film, a first gate electrode formed on said first gate insulation film, and a first side insulation film formed at least on a side of said first gate insulation film and having a second relative permittivity which is smaller than the first relative permittivity;
• and
• a plurality of second MOS transistors, each of said second MOS transistors including a second channel of the first conductivity type formed on a surface layer of said substrate, a second source and a second drain of the second conductivity type formed on both sides of said second channel, a second gate insulation film with the first relative permittivity formed at least on said second channel directly or through a buffer insulation film, a second gate electrode formed on said second gate insulation film, and a second side insulation film formed at least on a side of said second gate insulation film and having said second relative permittivity, wherein
• when a cross-section on a side of said first channel is S1, a cross-section on a side of said first gate electrode is S2, a cross-section on a side of said second channel is S3, and a cross-section on a side of said second gate insulation film is S4, a condition of:
• S2/S1>S4/S3
• is satisfied,
• an area of a bottom part of said gate electrode parallel to the channel length direction is larger than an area of an upper part of said gate insulation film, and
• a length of a bottom part of said gate electrode along a channel length direction is longer than a length of an upper part of said gate insulation film along a channel length direction.