A semiconductor device comprising: a substrate having an AlGaN layer located on a GaN layer for forming a two dimensional electron gas at an interface between the AlGaN layer and the GaN layer; and a plurality of contacts, wherein at least one of the contacts comprises: an ohmic contact portion located on a major surface of the substrate, wherein the ohmic contact portion comprises a first electrically conductive material; and a trench extending down into the substrate from the major surface, wherein the trench passes through the AlGaN layer and into the GaN layer, wherein the trench is at least partially filled with a second electrically conductive material, and wherein the second electrically conductive material is a different electrically conductive material to the first electrically conductive material, wherein the device further comprises: a High Electron Mobility Transistor, HEMT, comprising a gate contact located between a source contact and a drain contact, wherein said at least one of the contacts is the drain contact of the HEMT; characterized in that, the device further comprises at least one island located between the drain and the gate, wherein each island includes a trench extending down into the substrate from the major surface, wherein the trench passes through the AlGaN layer and into the GaN layer, wherein the trench is at least partially filled with said second electrically conductive material. a substrate having an AlGaN layer located on a GaN layer for forming a two dimensional electron gas at an interface between the AlGaN layer and the GaN layer; and a plurality of contacts, an ohmic contact portion located on a major surface of the substrate, wherein the ohmic contact portion comprises a first electrically conductive material; and a trench extending down into the substrate from the major surface, wherein the trench passes through the AlGaN layer and into the GaN layer, wherein the trench is at least partially filled with a second electrically conductive material, and wherein the second electrically conductive material is a different electrically conductive material to the first electrically conductive material, wherein the device further comprises: a High Electron Mobility Transistor, HEMT, comprising a gate contact located between a source contact and a drain contact, wherein said at least one of the contacts is the drain contact of the HEMT; characterized in that, the device further comprises at least one island located between the drain and the gate, wherein each island includes a trench extending down into the substrate from the major surface, wherein the trench passes through the AlGaN layer and into the GaN layer, wherein the trench is at least partially filled with said second electrically conductive material. a High Electron Mobility Transistor, HEMT, comprising a gate contact located between a source contact and a drain contact, wherein said at least one of the contacts is the drain contact of the HEMT; characterized in that, the device further comprises at least one island located between the drain and the gate, wherein each island includes a trench extending down into the substrate from the major surface, wherein the trench passes through the AlGaN layer and into the GaN layer, wherein the trench is at least partially filled with said second electrically conductive material.