A method of forming an integrated silicon and III-N semiconductor device, comprising: providing a first substrate of silicon having a first crystal orientation; forming a layer stack of III-N semiconductor material on a top surface of the first substrate ; defining areas on the first substrate for a silicon device film to be formed; forming isolation regions on the first substrate in the layer stack of III-N material in said areas defined for the silicon device film to be formed; if forming isolation regions 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 ; providing a second substrate of silicon 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 in the second substrate which separates the silicon device film from a carrier portion 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 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 so as to expose an area of the layer stack of III-N semiconductor material , leaving regions of the silicon device film in place in said areas defined for the silicon device film; forming a first plurality of components in and/or on the silicon device film over the isolation regions in the layer stack of III-N semiconductor material ; and forming a second plurality of components in and/or on the layer stack of III-N semiconductor material. providing a first substrate of silicon having a first crystal orientation; forming a layer stack of III-N semiconductor material on a top surface of the first substrate ; defining areas on the first substrate for a silicon device film to be formed; forming isolation regions on the first substrate in the layer stack of III-N material in said areas defined for the silicon device film to be formed; if forming isolation regions 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 ; providing a second substrate of silicon 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 in the second substrate which separates the silicon device film from a carrier portion 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 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 so as to expose an area of the layer stack of III-N semiconductor material , leaving regions of the silicon device film in place in said areas defined for the silicon device film; forming a first plurality of components in and/or on the silicon device film over the isolation regions in the layer stack of III-N semiconductor material ; and forming a second plurality of components in and/or on the layer stack of III-N semiconductor material. <EOS>