Abstract: A compound semiconductor device is disclosed which comprises a crystally grown buffer layer formed on a semi-insulating semiconductor substrate provided with an insulating layer. The crystal growth characteristics are such that the insulating layer does not allow crystal growth on its upper surface. During its growth, the buffer layer makes reverse sloped side edges which join to form a void. A gate electrode formed above the void separates a channel from the substrate. In the .delta.-MESFET, side walls are formed at both sides of the gate electrodes and an N.sup.+ -well region is formed by using the side walls as an ion implanting mask. Accordingly, the void separates the channel from the semiconductor substrate, thereby preventing leakage current through the buffer layer, backgating effects in an integrated circuit, and completing crystal growth without an undue effort for lowering the impurity concentration of the buffer layer.