NOVELTY - A primary compound semiconductor layer (102) is formed on silicon substrate (101), and secondary compound semiconductor layer (103) is formed on primary compound layer at high temperature to obtain compound semiconductor substrate (10). Primary compound semiconductor layer is gallium-arsenic-antimony-containing layer. The secondary compound semiconductor layer is formed using raw materials containing gallium, arsenic and antimony. Ratio of molecular beam intensity of antimony raw material and molecular beam intensity of gallium raw material is 1 or more.    USE - Manufacture of compound semiconductor substrate (claimed) for suprathermal electron device e.g. high electron mobility transistor and heterojunction bipolar transistor, and optical device e.g. light emitted diode, laser diode, high efficient solar cell and magnetic sensor.    ADVANTAGE - The method efficiently provides compound semiconductor substrate having excellent crystallinity and surface flatness.    DETAILED DESCRIPTION - A primary compound semiconductor layer is formed on a silicon substrate, and a secondary compound semiconductor layer is formed on the primary compound semiconductor layer at high temperature more nearly than the time of forming primary compound semiconductor layer to obtain compound semiconductor substrate. The primary compound semiconductor layer is gallium-arsenic-antimony-containing layer (GaAs1-xSbx, where x is 0-0.1). The secondary semiconductor layer is gallium-arsenic-antimony-containing layer (GaAs1-ySby, where y is more than 0 to 0.1). The secondary compound semiconductor layer is formed by irradiating gallium raw material, arsenic raw material and antimony raw material, simultaneously. The ratio of molecular beam intensity of antimony raw material and molecular beam intensity of gallium raw material is 1 or more, when the formation speed of secondary compound semiconductor layer is 1 mu m/hour. An INDEPENDENT CLAIM is included for compound semiconductor substrate.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of compound semiconductor substrate.    Compound semiconductor substrate (10)    Silicon substrate (101)    Primary compound semiconductor layer (102)    Secondary compound semiconductor layer (103)