Patent attributes
NVM cell for storing three levels of charge: one erased and two programmed states. The cell comprises a transistor structure providing a gate current versus gate voltage curve having a shape with a flat region or a second peak. To provide such a structure, one embodiment combines two parallel transistors having different threshold voltages, and another embodiment uses one transistor with variable doping. The gate current curve provides two programming zones. Programming the first state includes applying a voltage across a channel, ramping up a gate voltage in the first programming zone, followed by ramping it back down. Programming the second state comprises applying a voltage across a channel, ramping up a gate voltage past the first programming zone and into the second programming zone, followed by ramping it back down. Ramping the voltage back down may optionally be preceded by turning off the voltage across the channel.
