Non-volatile variable threshold memory cell

Claim 1

In the operation of a non-volatile memory cell comprising an insulated gate field-effect semiconductor device having a source and drain and having a gate isolation means including two gate isolation materials, one of which is characterized by the ability to preferentially trap charges of one polarity, the other being characterized by the ability to preferentially trap charges of the opposite polarity, the improved method comprising the steps of: a. writing into the memory cell by increasing the positive voltage on the drain junction to a level which causes avalanche breakdown, while simultaneously applying a small voltage of source polarity to the gate terminal, thereby driving some of the avalanching carriers through only one of said gate isolation materials whereby a trapped charge is stored at the interface between the two gate isolation materials; then at an appropriate later time b. erasing the memory cell by increasing the voltage on the gate to a sufficiently high level to initiate injection of charges of opposite polarity from the gate electrode into the isolation layers, while holding the source, drain and substrate electrodes at substantially ground potential, thereby causing said injection to occur preferentially at the interface between the gate isolation layers, whereby the stored charge at that location is neutralized to restore the threshold voltage to substantially its intrinsic value.