NOVELTY - A compound semiconductor layer (101) containing gallium arsenide antimonite is formed on a silicon substrate (100). A compound semiconductor layer (102) is formed on layer (101) at high temperature by simultaneously irradiating gallium raw material, arsenic raw material and antimony raw material on layer (101). A compound semiconductor layer (103) is formed on layer (102). The ratio of molecular beam strength of antimony raw material with respect to the molecular beam strength of gallium raw material is 1 or more. The formation rate of layer (102) is 1 mu m per hour.    USE - Manufacture of compound semiconductor substrate (claimed). Uses include but are not limited to high electron mobility transistor, heterojunction bipolar transistor, LED, laser diode, solar cell, and magnetic sensor.    ADVANTAGE - The method enables manufacture of compound semiconductor substrate having excellent crystallinity and surface flatness.    DETAILED DESCRIPTION - A compound semiconductor layer (101) containing gallium arsenide antimonite of formula: GaAs1-xSbx (where x is 0-0.1) is formed on a silicon substrate. A compound semiconductor layer (102) containing gallium arsenide antimonite of formula: GaAs1-ySby (where y is 0-0.1) is formed on the compound semiconductor layer (101) at high temperature by simultaneously irradiating gallium raw material, arsenic raw material and antimony raw material on compound semiconductor layer (101). A compound semiconductor layer (103) which is a gallium arsenide layer is formed on the compound semiconductor layer (102). The ratio of molecular beam strength of antimony raw material with respect to the molecular beam strength of gallium raw material is 1 or more. The formation rate of compound semiconductor layer (102) is 1 mu m per hour. An INDEPENDENT CLAIM is included for compound semiconductor substrate.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the compound semiconductor substrate.    Compound semiconductor substrate (10)    Silicon substrate (100)    Compound semiconductor layers (101,102,103)