Variable resistance nonvolatile storage device and method of forming memory cell

A variable resistance nonvolatile storage device comprising:

• a semiconductor substrate;
• a variable resistance element including:
• a first electrode;
• a second electrode;
• and a variable resistance layer which is provided between said first and second electrodes and in contact with said first and second electrodes and which has a resistance value that reversibly varies based on voltage signals each of which has a different polarity and is applied between said first and second electrodes;
• and
• an N-type MOS transistor formed on a main surface of said semiconductor substrate, said N-type MOS transistor including:
• a first N-type diffusion layer region;
• a gate;
• and a second N-type diffusion layer region formed on a side of said gate opposite said first N-type diffusion layer region,
• wherein said variable resistance layer includes:
• a first region which is in contact with said first electrode and contains a first oxygen-deficient transition metal oxide having a composition MOx;
• and a second region which is in contact with said second electrode and contains a second oxygen-deficient transition metal oxide having a composition MOy (where x<y),
• wherein said first N-type diffusion layer region of said N-type MOS transistor is connected with said first electrode included in said variable resistance element to form a memory cell so that a substrate bias effect in said N-type MOS transistor is smaller when one of the voltage signals having a polarity that causes a resistance change to high resistance state in said variable resistance layer is applied to said N-type MOS transistor and said variable resistance element, than when another one of the voltage signals having a polarity that causes a resistance change to low resistance state in said variable resistance layer is applied to said N-type MOS transistor and said variable resistance element, and
• wherein the first region of the variable resistance layer and the first electrode of the variable resistance element are each connected between the first N-type diffusion layer region and the second region of the variable resistance layer.