A non-volatile electrically erasable
programmable read only memory (
EEPROM) capable of storing two bit of information having a nonconducting charge
trapping dielectric, such as
silicon nitride, sandwiched between two
silicon dioxide
layers acting as electrical insulators is disclosed. The invention includes a method of
programming, reading and erasing the two bit
EEPROM device. The nonconducting
dielectric layer functions as an electrical charge
trapping medium. A conducting gate layer is placed over the upper
silicon dioxide layer. A left and a right bit are stored in physically different areas of the charge
trapping layer, near left and right regions of the
memory cell, respectively. Each bit of the memory device is programmed in the conventional manner, using
hot electron programming, by applying
programming voltages to the gate and to either the left or the right region while the other region is grounded.
Hot electrons are accelerated sufficiently to be injected into the region of the trapping
dielectric layer near where the programming voltages were applied to. The device, however, is read in the opposite direction from which it was written, meaning voltages are applied to the gate and to either the right or the left region while the other region is grounded. Two bits are able to be programmed and read due to a combination of relatively low gate voltages with reading in the reverse direction. This greatly reduces the potential across the trapped charge region. This permits much shorter programming times by amplifying the effect of the charge trapped in the localized
trapping region associated with each of the bits. In addition, both bits of the
memory cell can be individually erased by applying suitable erase voltages to the gate and either left or right regions so as to cause electrons to be removed from the corresponding charge
trapping region of the
nitride layer.