3 via the drain electrode 7 the layer 10 1 is formed simultaneously with a manufacturing step for preparing the gate electrode 9 in fig3 and 4 another type of each of the conductive layers 10 2 and 10 3 also made of al polycrystalline silicon or the like is provided on the insulating layer 8 each of them extend from either of the drain electrode 7 or the source electrode 9 they are disposed like an island each is formed simultaneously with the preparation of the gate electrode 9 each of the layers 10 2 and 10 3 is connected to each of the field plate portions 6 39 and 7 39 in fig5 on the insulating layer 8 34 a further field plate layer 10 4 made of al polycrystalline silicon or the like is provided substantially covering region a as explained in fig1 the layer 10 4 is coupled to the source electrode 6 via the field plate layer 6 39 alternatively the layer 10 4 may be coupled to the drain electrode 7 via the field plate layer 7 39 in fig6 there is provided a further field plate layer 10 5 on the insulating layer 8 34 the layer 10 5 extends from a wiring element 11 disposed on the drain electrode 7 the layer 10 5 substantially covers the region a as explained above it is composed of al polycrystalline silicon or the like as described above since any of the layers 10 1 10 2 10 3 10 4 and 10 5 is necessarily connected to any of the electrode means such as the source electrode 6.