Insulation sheet made from silicon nitride, and semiconductor module structure using the same

An insulation sheet, comprising:

• a sheet-shaped silicon-nitride substrate comprising β-silicon-nitride crystal grains as a main phase;
• and
• a surface layer formed on one face or both front and back faces of Surfaces of the silicon-nitride substrate from at least one metal element selected from the group consisting of In, Sn, Al, Ag, Au, Cu, Ni, Pb, Pd, Sr, Ce, Fe, Nb, Ta, V and Ti;
• wherein:
• the surface layer has a Vickers hardness of 200 or less and a thickness of 100 μm or less,
• the silicon-nitride substrate has a surface roughness Ra of 0.2 to 1.5 μm, and
• the insulation sheet is suitable for a pressure contact structure in which the insulation sheet is pressed to another member through the surface layer.