A semiconductor device includes tunneling insulating layers on active regions of a substrate, floating gate electrodes on the tunneling insulating layers, an isolation trench within the substrate and the isolation trench defines the active region, spaces the tunneling insulating layers, and isolates the floating gate electrodes. A bottom of the isolation trench is directly in contact with the substrate. The semiconductor device further includes a lower insulating layer on the floating gate electrodes, and a middle insulating layer, an upper insulating layer, and a control gate electrode stacked on the lower insulating layer. The lower insulating layer is configured to hermetically seal a top portion of the isolation trench to define and directly abut an air gap within the isolation trench.