A planar double-gate transistor is manufactured wherein crystallisation inhibitors are implanted into the channel region (16) of a semiconductor wafer (10), said wafer having a laminate structure comprising an initial crystalline semiconductor layer (14) adjacent an amorphous semiconductor layer (12). Upon heating, partial re-growth of the amorphous semiconductor layer is restricted in the channel area thus allowing for the thickness of the source/drain extension regions to be increased whilst maintaining a thin channel. Any remaining amorphous material is selectively removed leaving a cavityto allow for the forming of gate electrodes (30,32) on opposing sides of the channel region. The invention can be exploited to a greater extent by providing an amorphous layer on both sides of the initial crystalline semiconductor layer thus providing for re-growth limitation in two directions.