an object is to increase the retention characteristics of a memory device formed using a semiconductor with a wide bandgap , such as an oxide semiconductor . a transistor including a back gate is inserted in series at one end of a bit line so that the back gate is constantly at a sufficiently negative potential . the minimum potential of the bit line is set higher than that of a word line . when power is turned off , the bit line is cut off by the back gate transistor , ensuring prevention of outflow of charge accumulated in the bit line . at this time , the potential of a source or a drain of a cell transistor is sufficiently higher than that of a gate of the cell transistor , so that the cell transistor is put in a sufficiently off state ; thus , data can be retained .