SONOS
SONOS, short for "Silicon-Oxide-Nitride-Oxide-Silicon", is a type of non-volatile computer memory closely related to Flash RAM. It is one of Charge trap flash variants. It is distinguished from mainstream flash by the use of silicon nitride (Si3N4) instead of polysilicon for the charge storage material. A further variant is "SHINOS" ("Silicon Hi-k Nitride Oxide Silicon"). SONOS promises lower programming voltages and higher program/erase cycle endurance than polysilicon-based flash, and is an area of active research and development effort. Companies offering SONOS-based products include GlobalFoundries, Cypress Semiconductor, Macronix, Toshiba, United Microelectronics Corporation and Floadia .
A SONOS memory cell is formed from a standard polysilicon N-channel MOSFET 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.
When the polysilicon control gate is biased positively, electrons from the transistor source and drain regions tunnel through the oxide layer and get trapped in the silicon nitride. This results in an energy barrier between the drain and the source, raising the threshold voltage Vt (the gate-source voltage necessary for current to flow through the transistor). The electrons can be removed again by applying a negative bias on the control gate.
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