Memory device and method for operating the same

A method, comprising:

• forming a first MIS structure and a second MIS structure over a semiconductor substrate;
• reverse biasing the first MIS structure to induce a first flat band voltage shift in the first MIS structure;
• forward biasing the second MIS structure to induce a second flat band voltage shift in the second MIS structure, wherein an absolute value of the second flat band voltage shift is greater than an absolute value of the first flat band voltage shift;
• detecting a first read current flowing through the first MIS structure under a reading voltage pulse;
• and
• detecting a second read current flowing through the second MIS structure under the reading voltage pulse, wherein the second read current is greater than the first read current.

A memory device, comprising:

• a semiconductor substrate;
• a first MIS structure comprising a first continuous insulating layer over and in contact with the semiconductor substrate and a first metal electrode layer over and in contact with the first continuous insulating layer;
• a second MIS structure comprising a second continuous insulating layer over and in contact with the semiconductor substrate and a second metal electrode layer over and in contact with the second continuous insulating layer;
• and
• a third MIS structure comprising a third continuous insulating layer over and in contact with the semiconductor substrate and a third metal electrode layer over and in contact with the third continuous insulating layer, wherein the first to third continuous insulating layer have substantially the same thickness and the same material, and the first to third MIS structures are at three different resistance states.