A transformer comprising primary and secondary windings (2, 3), each winding (2, 3) having first and second metal capping layers (33,34,35,37) coupled together electrically in parallel by metal connectors (31,32,39,40) passing through trenches in a substrate lying between the first and second metal capping (8, 9, 14, 15) layers, characterized in that either: (a) the substrate is a first silicon substrate lying between second and third silicon substrates (36, 38), each of the first and second metal capping layers (33, 34) for the primary winding lying on top and bottom surfaces of the first silicon substrate respectively and each of the first and second metal capping layers (35, 37) for the secondary winding lying on top and bottom surfaces of the second and third silicon substrates (36, 38) respectively, the metal connector (31, 32) for the primary winding passing through the first silicon substrate and the metal connector (39a-c, 40a-c) for the secondary winding having first (39a-c) and second (40a-c) parts passing in parallel through the first, second and third silicon substrates (30, 36, 38) such that the secondary winding surrounds the primary winding; or (b) the substrate is a first oxide layer lying between second and third oxide layers (72,79), each of the first and second metal capping layers (73a, 73b, 78a, 78b) for the primary winding lying on top and bottom surfaces of the first oxide layer respectively and each of the first and second metal capping layers (71a, 71b, 83a, 83b) for the secondary winding lying on top and bottom surface of the second and third oxide layers (72, 79) respectively, the metal connector (76a-d) for the primary winding passing through the first oxide layer and the metal connector (82b, 82c) passing in parallel through the first, second and third oxide layers (72, 74, 79) such that the secondary winding surrounds the primary winding. (a) the substrate is a first silicon substrate lying between second and third silicon substrates (36, 38), each of the first and second metal capping layers (33, 34) for the primary winding lying on top and bottom surfaces of the first silicon substrate respectively and each of the first and second metal capping layers (35, 37) for the secondary winding lying on top and bottom surfaces of the second and third silicon substrates (36, 38) respectively, the metal connector (31, 32) for the primary winding passing through the first silicon substrate and the metal connector (39a-c, 40a-c) for the secondary winding having first (39a-c) and second (40a-c) parts passing in parallel through the first, second and third silicon substrates (30, 36, 38) such that the secondary winding surrounds the primary winding; or (b) the substrate is a first oxide layer lying between second and third oxide layers (72,79), each of the first and second metal capping layers (73a, 73b, 78a, 78b) for the primary winding lying on top and bottom surfaces of the first oxide layer respectively and each of the first and second metal capping layers (71a, 71b, 83a, 83b) for the secondary winding lying on top and bottom surface of the second and third oxide layers (72, 79) respectively, the metal connector (76a-d) for the primary winding passing through the first oxide layer and the metal connector (82b, 82c) passing in parallel through the first, second and third oxide layers (72, 74, 79) such that the secondary winding surrounds the primary winding.