Ceramic sintered body and substrate for semiconductor device

A substrate for a semiconductor device on which electronic components mounted, the substrate comprising:

• a ceramic sintered body;
• and
• a copper plate or an aluminum plate joined to the ceramic sintered body, wherein
• in the ceramic sintered body,
• a Zr content is 17.5 mass % or more and 23.5 mass % or less in terms of ZrO2,
• an Hf content is 0.3 mass % or more and 0.5 mass % or less in terms of HfO2,
• an Al content is 74.3 mass % or more and 80.9 mass % or less in terms of Al2O3,
• a Y content is 0.8 mass % or more and 1.9 mass % or less in terms of Y2O3,
• an Mg content is 0.1 mass % or more and 0.8 mass % or less in terms of MgO,
• an Si content is 0.1 mass % or more and 1.5 mass % or less in terms of SiO2,
• a Ca content is 0.03 mass % or more and 0.35 mass % or less in terms of CaO,
• a total content of Na and K is 0.01 mass % or more and 0.10 mass % or less when the Na content is converted to Na2O and the K content is converted to K2O,
• a balance content is 0.05 mass % or less in terms of oxide, and
• a sum of the Mg content in terms of MgO, the Si content in terms of SiO2, the Ca content in terms of CaO, the Na content in terms of Na2O, the K content in terms of K2O, and the balance content is 0.3 mass % or more and 2.0 mass % or less.