An organic field-effect transistor has a gate insulating layer (4) comprising a cured epoxy resin. The epoxy resin has a lower concentration of trapping centres (8) compared with a conventional epoxy resin in which trapping centres are provided by hydroxyl (OH) groups. The lower concentration of trapping centres can be achieved by reducing the number of hydroxyl groups throughout the layer and/or by reducing the number of hydroxyl groups in a surface region (9).
