Reprogrammable nonvolatile semiconductor memory formed of MOS transistors and reprogramming method thereof

Claim 1

A reprogrammable nonvolatile semiconductor memory having storage cells each of which is formed of an MOS transistor and arranged in a matrix array on a semiconductor substrate of a first electroconductive type, each of said transistors comprising: a source region and a drain region formed in a surface area of said substrate, both of which are of a second electroconductive type inverse in polarity to said first electroconductive type; a floating gate formed through a first gate insulator film on the surface of said substrate between said source region and said drain region; and a control gate formed through a second gate insulator film on said floating gate; and said substrate including word lines respectively connected to said control gates of said transistors belonging to each row of said matrix; digit lines respectively connected to said drain regions of said transistors belonging to each column of said matrix; sources lines respectively connected to said source regions of one of a plurality of groups which are obtained by dividing said matrix array; a first decoder for selecting at least one of said word lines; a second decoder for selecting at least one of said digit lines; and means for controlling threshold voltages of each of said cells wherein cells that have been reprogrammed have substantially the same threshold voltage; thereby a reprogramming data value being written to said desired cell by applying a voltage through said corresponding source line.