An indium phosphide crystal substrate has a diameter of 100-205 mm and a thickness of 300-800 μm and includes any of a flat portion and a notch portion. In any of a first flat region and a first notch region, when an atomic concentration of sulfur is from 2.0×10.sup.18 to 8.0×10.sup.18 cm.sup.−3, the indium phosphide crystal substrate has an average dislocation density of 10-500 cm.sup.−2, and when am atomic concentration of tin is from 1.0×10.sup.18 to 4.0×10.sup.18 cm.sup.−3 or an atomic concentration of iron is from 5.0×10.sup.15 to 1.0×10.sup.17 cm.sup.−3, the indium phosphide crystal substrate has an average dislocation density of 500-5000 cm.sup.−2.

An indium phosphide crystal substrate has a diameter of 100-205 mm and a thickness of 300-800 m and includes any of a flat portion and a notch portion. In any of a first flat region and a first notch region, when an atomic concentration of sulfur is from 2.010.sup.18 to 8.010.sup.18 cm.sup.3, the indium phosphide crystal substrate has an average dislocation density of 10-500 cm.sup.2, and when an atomic concentration of tin is from 1.010.sup.15 to 4.010.sup.18 cm.sup.3 or an atomic concentration of iron is from 5.010.sup.15 to 1.010.sup.17 cm.sup.3, the indium phosphide crystal substrate has an average dislocation density of 500-5000 cm.sup.2.