In a non-volatile semiconductor memory device composed of flating gate type memory cells, after the drain or source is charged, it is placed in an electrically floating state and a signal with alternately changing positive and negative potentials is applied to the control gates of the ...http://www.google.com/patents/US5748530?utm_source=gb-gplus-sharePatent US5748530 - Non-voltile memory device, non-volatile memory cell and method of adjusting the threshold value of the non-volatile memory cell and each of plural transistors 
