A semiconductor device such as a DPAM memory device is disclosed. A, Substrate (12) of semiconductor material is provided with energy band modifying means in the form of a box region (38) and is covered by an insulating layer (14). A semi-conductor layer (16) has source (18) and drain (20) regions formed therein to define bodies (22) of respective field effect transistors. The box region (38) is more heavily doped than the adjacent body (22), but less highly doped than the corresponding source (18) and drain (20), and modifies the valence and/or conduction band of the body (22) to increase the amount of electrical charge which can be stored in the body (22).