Novel very high IE structures based on the directed BBHE mechanism for ultralow-power flash memories

In NOR flash memories, there is a strong desire to scale the drain programming voltage and operating power for use in portable applications. Low power consumption in these memories can be achieved by increasing their injection efficiency (IE) (=I_G/I_D), either by using novel device structures or programming mechanisms. Device structures based on the band-to-band hot electron (BBHE) concept have traditionally shown the highest IE but the IE drops dramatically at low drain voltages. In this letter, we analyze the reasons for this reduction in IE at low V_DS and propose new silicon-on-insulator (SOI)-based cells that circumvent the above limitation. Experimental structures fabricated show extremely high IE (10^-3), up to 5-10 times higher than conventional BBHE cells at very low gate and drain voltages.