US Patent 7719905 Semiconductor memory device

A semiconductor memory device has a memory cell having a hierarchical bit line structure for large capacity even in a small cell size. The semiconductor memory device comprises a unit cell configured to read/write data, a cell data sensing unit configured to adjust a current amount of a main bit line depending on a sensing voltage of a sub bit line when data are sensed, and a write control unit configured to store data in the corresponding unit cell depending on a current level applied from the main bit line to the sub bit line.