<p>A thin film transistor is formed in a semi-conductor island on an insulating substrate. The transistor comprises a source (1502) and a drain (1504) of first conductivity type and a channel (1508) of a second opposite conductivity type. The channel is overlapped by one or more insulated gates (1510) and is provided with isolation diodes. Each isolation diode comprises a first region (1506) which is lightly doped and a second region (1512) which is heavily doped and of the second conductivity type. The diodes are not overlapped by the gate (1510). The first and second regions (1506, 1512) extend away from the channel (1508) by less than the length of the adjacent source or drain. The lightly doped region (1506) extends away from the source or drain and the heavily doped region (1512) extends away from the lightly doped region such that the first and second regions (1506, 1512) form a p-n junction with the adjacent source or drain in a direction orthogonal to the main conduction path of the transistor but not parallel to the main conduction path.</p>
