1385055 Surface wave devices DEFENCE SECRETARY OF STATE FOR 28 April 1972 [5 May 1971] 17563/72 Divided out of 1372235 Heading H3U Acoustic surface or interface waves passing between input and output transducers are amplified by interaction with a set of spaced filamentary conductors, a portion of which cross semi-conductor material insulated therefrom, in which an electron drift velocity exceeding the acoustic surface wave velocity is established. The substrate 139 is piezo-electric, or a piezolayer on a base, or electrostrictive in which case a biasing electric field is provided. As shown in Fig. 2, a plurality 141a-141d of semi-conductors each interact with a proportion of the filamentary conductors 143, which pass over a thin insulating film (not shown) between them and the semi-conductor. In Fig. 1 (not shown) a single semi-conductor (141) shares a common base (not shown) with piezo substrate (139). Conductor spacing may differ on the piezo substrate, semi-conductor, respectively. In Fig. 3 (not shown), a non-piezoelectric, sapphire substrate (147) supports the surface waves, and aluminium nitride piezoelectric material (149, 157, 161) is provided under or over the transducers (149, 155) and filamentary conductors 165, which also pass over or under silicon semiconducting thin film 163 with an insulating film between it and the conductors. In Fig. 4 (not shown) the semi-conductor material (209) is above the conductors (207) which need thus be provided only adjacent the surface wave track. In Fig. 5, to reduce electro-acoustic coupling between conductors 177, and that part of the substrate 173 removed from track A, a low dielectric constant silica layer 175 is provided under the conductors, see also Fig. 6 (not shown). Alternatively the semi-conductor 179 may be made narrower than track A, Fig. 8 (not shown), or if layer 175 is omitted, a suitable choice of semi-conductor layer and biasing reduces electro-acoustic coupling. The narrower semiconductor layer may be mounted under or over the conductors. Completed assemblies may have a protective cover. The filamentary conductors may form closed loops, a perpendicular biasing magnetic field, possibly biasing a magnetostrictive substrate, being provided. The filamentary conductor spacings may be equal, monotonically changing, or random. If a pair of side-by-side output transducers (171, 183), Fig. 7 (not shown), are provided, amplification and track changing can be achieved for suitable semi-conductor biasing.