27-04-2011, 11:17 AM
A SONOS memory cell is formed from a standard polysilicon NMOS transistor with the addition of a small sliver of silicon nitride inserted inside the transistor's gate oxide. The sliver of nitride is non-conductive but contains a large number of charge trapping sites able to hold an electrostatic charge. The nitride layer is electrically isolated from the surrounding transistor, although charges stored on the nitride directly affect the conductivity of the underlying transistor channel. The oxide/nitride sandwich typically consists of a 2 nm thick oxide lower layer, a 5 nm thick silicon nitride middle layer, and a 5—10 nm oxide upper layer.SONOS promises lower programming voltages and higher program/erase cycle endurance than polysilicon-based flash. SONOS distinguished from mainstream flash by the use of silicon nitride (Si3N4) instead of polysilicon for the charge storage material.