A semiconductor structure for a photonic integrated circuit, comprising: a substrate; and a waveguide on the substrate, wherein a passive region of the semiconductor structure comprises: a first waveguide layer as a first portion of a core of the waveguide, for guiding light through the passive region, and a first cladding layer of a first material in contact with the first waveguide layer; and an active region of the semiconductor structure comprises a second waveguide layer, different to the first waveguide layer, as a second portion of the core of the waveguide, for guiding light through the active region, and a second cladding layer, of a second material different to the first material of the first cladding layer, in contact with the second waveguide layer, the semiconductor structure characterised in that a first butt joint of a surface of the first cladding layer in contact with a surface of the second cladding layer is offset from a second butt joint of a surface of the first waveguide layer in contact with a surface of the second waveguide layer. a substrate; and a waveguide on the substrate, wherein a passive region of the semiconductor structure comprises: a first waveguide layer as a first portion of a core of the waveguide, for guiding light through the passive region, and a first cladding layer of a first material in contact with the first waveguide layer; and an active region of the semiconductor structure comprises a second waveguide layer, different to the first waveguide layer, as a second portion of the core of the waveguide, for guiding light through the active region, and a second cladding layer, of a second material different to the first material of the first cladding layer, in contact with the second waveguide layer, the semiconductor structure characterised in that a first butt joint of a surface of the first cladding layer in contact with a surface of the second cladding layer is offset from a second butt joint of a surface of the first waveguide layer in contact with a surface of the second waveguide layer. a passive region of the semiconductor structure comprises: a first waveguide layer as a first portion of a core of the waveguide, for guiding light through the passive region, and a first cladding layer of a first material in contact with the first waveguide layer; and a first waveguide layer as a first portion of a core of the waveguide, for guiding light through the passive region, and a first cladding layer of a first material in contact with the first waveguide layer; and an active region of the semiconductor structure comprises a second waveguide layer, different to the first waveguide layer, as a second portion of the core of the waveguide, for guiding light through the active region, and a second cladding layer, of a second material different to the first material of the first cladding layer, in contact with the second waveguide layer, the semiconductor structure characterised in that a first butt joint of a surface of the first cladding layer in contact with a surface of the second cladding layer is offset from a second butt joint of a surface of the first waveguide layer in contact with a surface of the second waveguide layer. a second waveguide layer, different to the first waveguide layer, as a second portion of the core of the waveguide, for guiding light through the active region, and a second cladding layer, of a second material different to the first material of the first cladding layer, in contact with the second waveguide layer, the semiconductor structure characterised in that a first butt joint of a surface of the first cladding layer in contact with a surface of the second cladding layer is offset from a second butt joint of a surface of the first waveguide layer in contact with a surface of the second waveguide layer.