A SONOS (silicon-oxide-nitride-oxide-silicon) device is a non-volatile memory
where the charge is stored in a layer of trap-rich dielectric material instead
of a floating gate as in an EEPROM. Fig. 5.31 shows an example where a
layer of SiN
is sandwiched between two layers of SiO
. Electrons tunneling
from the substrate are trapped and redistribute themselves in separate
trapping centers. This has the advantage that the charge is stored
independently in the traps. A leaky path in the tunnel dielectric cannot lead
to full charge loss, as it is the case in conventional EEPROM
devices. Therefore, reliability and retention time is
increased [282,283,209,284,285,168,286,287,288,289,290,291].
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The trap-assisted tunneling model can be applied to simulate device
characteristics of this device, where three layers of SiO have been used and
the trap concentration and trap energy level in the middle layer was chosen to
resemble a layer of silicon nitride. The transient trap occupancy for a
discharging process starting from an initial condition of 2V at the gate
contact is shown in Fig. 5.33. Initially the traps are
filled. Over time, the electrons leak through the lower dielectric into the
channel. After
s almost no more charge is stored in the trap-rich
dielectric.
A. Gehring: Simulation of Tunneling in Semiconductor Devices