Abstract: A method of fabricating a field effect transistor comprising the steps of forming an active layer of semiconductor material (GaAs) over a surface of a first substrate of semiconductor material (GaAs), applying a second substrate of insulating material, e.g. glass, over the surface of the active layer, removing the first substrate so that the active layer is now supported on the insulating second substrate, and forming source, drain and gate electrodes over the free surface of the active layer. To facilitate removal of the GaAs first substrate by selective etching, a buffer layer of GaAlAs resistant to the GaAs etchant, may be formed between the active layer and the first substrate, which buffer layer is removed, following removal of first substrate, using a selective etchant to which the GaAs active layer is resistant. The technique is particularly applicable to high frequency FETs requiring a very thin active channel region interfaced to a substrate having good insulating properties.