Transcript Flash Memory

COEN 180
Flash Memory
Floating Gate Fundamentals
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Floating Gate between control gate and
channel in MOSFET.
Not directly connected to an outside
line.
Floating Gate Fundamentals
Floating Gate Fundamentals
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First used in Erasable Programmable
Read Only Memory
EPROM
Floating Gate Fundamentals
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To function, Floating Gate EPROM cell
needs to be able to maintain a charge
on the floating gate.
Charge completely isolated, hence can
be stored for long times.
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Floating: gate not connected to the outside.
Floating Gate Fundamentals
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No charge in floating gate.
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Assume control gate, source, drain at GND.
Increase voltage in control gate:
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Floating gate voltage also increases, but at a
lower rate.
Raises the threshold value of the transistor.
When threshold voltage is high enough, creates
a channel between source and drain.
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Threshold value about twice as high.
Floating Gate Fundamentals
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Floating gate (negatively) charged:
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Isolates control gate from forming a
channel at normal threshold values.
Floating Gate Fundamentals
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Discharging the floating gate:
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Exposure to UV light for twenty minutes.
Chips have a crystal cover that allows UV
light to hit the chip.
UV light charges electrons on the floating
gate so that they can break through the
isolation layer around the floating gate.
Floating Gate Fundamentals
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To charge the floating gate
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Apply 12V (or higher) to control and drain
Maintain source and substrate at ground
For a few hundred microseconds.
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Creates a large drain current.
Accelerates electrons to high velocity: hot electrons.
Break through the silicon substrate SiO2 barrier.
Some get caught in the floating gate.
Floating gate charge causes channel inversion.
Electrons remain trapped on floating gate.
Potential about -5V.
Floating Gate Fundamentals
Floating Gate Fundamentals
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Need to avoid trapping electrons in SiO2
after several charge / discharge cycles.
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Could raise threshold value of transistor.
Careful growth of SiO2 layer.
Floating Gate Fundamentals