US Patent 10522693 Memory device and electronic device

A memory device with excellent writing performance and excellent storing performance is provided. In the memory device, a first layer overlaps with a second layer. The first layer includes a first transistor including an oxide semiconductor as an active layer. The second layer includes a second transistor and a third transistor each including an oxide semiconductor as an active layer. The off-state current of a transistor formed in the first layer is lower than the off-state current of each of a transistor formed in the second layer. The field-effect mobility of the transistor formed in the second layer is higher than the field-effect mobility of the transistor formed in the first layer.