A semiconductor or power electronic device immersed in a liquid dielectric, the device comprising: a pair of pole pieces (36, 38); a device body located between the pole pieces (36, 38), the device body being subjected to an electric field in use and having a central current carrying region with anode-side and cathode-side contact metallisation and an edge termination region that includes planar edge termination features (48a, 48b) to diffuse the electric field within the device body, the electric field emerging from the edge termination region of the device body; and passivation means having a first part in contact with the edge termination region of the device body and which diffuses the electric field as it emerges from the edge termination region and a second part which further diffuses the electric field as it emerges from the first part of the passivation means; wherein the second part of the passivation means includes a radially outer surface with respect to the central region of the device body, the radially outer surface being in contact with the dielectric liquid; wherein each pole piece (36, 38) has a central region that is substantially flat and a profiled surface (40, 42) at a peripheral edge region to control the diffusion of the electric field within the passivation means (44, 46), the profiled surface of at least one pole piece being curved in a manner that becomes tangential to the outer surface of the contact metallisation of the device body as the surface of the pole piece approaches the central current carrying region of the device body; and wherein the second part of the passivation means provides an interface with the profiled surface (40, 42) of each pole piece (36, 38). a pair of pole pieces (36, 38); a device body located between the pole pieces (36, 38), the device body being subjected to an electric field in use and having a central current carrying region with anode-side and cathode-side contact metallisation and an edge termination region that includes planar edge termination features (48a, 48b) to diffuse the electric field within the device body, the electric field emerging from the edge termination region of the device body; and passivation means having a first part in contact with the edge termination region of the device body and which diffuses the electric field as it emerges from the edge termination region and a second part which further diffuses the electric field as it emerges from the first part of the passivation means; wherein the second part of the passivation means includes a radially outer surface with respect to the central region of the device body, the radially outer surface being in contact with the dielectric liquid; wherein each pole piece (36, 38) has a central region that is substantially flat and a profiled surface (40, 42) at a peripheral edge region to control the diffusion of the electric field within the passivation means (44, 46), the profiled surface of at least one pole piece being curved in a manner that becomes tangential to the outer surface of the contact metallisation of the device body as the surface of the pole piece approaches the central current carrying region of the device body; and wherein the second part of the passivation means provides an interface with the profiled surface (40, 42) of each pole piece (36, 38).