A method of forming an integrated silicon and III-N semiconductor device, comprising: providing a first substrate (100, 200) of silicon having a first crystal orientation; forming a layer stack of III-N semiconductor material (102, 202) on a top surface of the first substrate (100, 200); defining areas on the first substrate for a silicon device film (110, 210) to be formed; forming isolation regions (104, 204) on the first substrate (100, 200) in the layer stack of III-N material (102, 202) in said areas defined for the silicon device film (110, 210) to be formed; if forming isolation regions (104, 204) comprises removing material, forming dielectric material in the isolation regions and planarising the dielectric material to be coplanar with a top surface of the III-N semiconductor material (102, 202); providing a second substrate of silicon (106, 206) having a second crystal orientation which is different from the first crystal orientation; implanting a cleaving species into said second substrate to form a separation layer (108, 208) in the second substrate which separates the silicon device film (110, 210) from a carrier portion (112, 212) of the second substrate; bonding the silicon device film to the layer stack of III-N semiconductor material, so that the carrier portion remains attached to the silicon device film through the separation layer; after bonding, heating the carrier portion (112, 212) to a temperature above 450 °C to separate the carrier portion from the silicon device film at the separation layer, so that the first substrate remains attached to the layer stack of III-N semiconductor material; removing a portion of the silicon device film (110, 210) so as to expose an area of the layer stack of III-N semiconductor material (102, 202), leaving regions of the silicon device film (110, 210) in place in said areas defined for the silicon device film; forming a first plurality of components (114, 214) in and/or on the silicon device film over the isolation regions (104, 204) in the layer stack of III-N semiconductor material (102, 202); and forming a second plurality of components (116, 216) in and/or on the layer stack of III-N semiconductor material. providing a first substrate (100, 200) of silicon having a first crystal orientation; forming a layer stack of III-N semiconductor material (102, 202) on a top surface of the first substrate (100, 200); defining areas on the first substrate for a silicon device film (110, 210) to be formed; forming isolation regions (104, 204) on the first substrate (100, 200) in the layer stack of III-N material (102, 202) in said areas defined for the silicon device film (110, 210) to be formed; if forming isolation regions (104, 204) comprises removing material, forming dielectric material in the isolation regions and planarising the dielectric material to be coplanar with a top surface of the III-N semiconductor material (102, 202); providing a second substrate of silicon (106, 206) having a second crystal orientation which is different from the first crystal orientation; implanting a cleaving species into said second substrate to form a separation layer (108, 208) in the second substrate which separates the silicon device film (110, 210) from a carrier portion (112, 212) of the second substrate; bonding the silicon device film to the layer stack of III-N semiconductor material, so that the carrier portion remains attached to the silicon device film through the separation layer; after bonding, heating the carrier portion (112, 212) to a temperature above 450 °C to separate the carrier portion from the silicon device film at the separation layer, so that the first substrate remains attached to the layer stack of III-N semiconductor material; removing a portion of the silicon device film (110, 210) so as to expose an area of the layer stack of III-N semiconductor material (102, 202), leaving regions of the silicon device film (110, 210) in place in said areas defined for the silicon device film; forming a first plurality of components (114, 214) in and/or on the silicon device film over the isolation regions (104, 204) in the layer stack of III-N semiconductor material (102, 202); and forming a second plurality of components (116, 216) in and/or on the layer stack of III-N semiconductor material.