REMEMBERING WITHOUT POWER

A transistor with a second gate

Nonvolatile The bit survives with the power off, for years

The cell An ordinary transistor with an extra gate, the floating gate, fully wrapped in oxide

Programming A strong field makes electrons tunnel through the oxide onto the floating gate

Erasing The reverse field pulls them off again

No wires touch the floating gate. The charge simply has nowhere to go.

STORED CHARGE SHIFTS THE THRESHOLD

The same rule as a sensor gate

ΔV_th = −Q_FG ÷ C_cg

Electrons on the floating gate shield the channel from the control gate, so a higher control-gate voltage is needed to turn the cell on. A read voltage placed between the erased and programmed thresholds tells them apart: conducting means 1, not conducting means 0. It is exactly the charge-shifts-the-threshold rule from Unit 5, used deliberately.

A memory cell is a charge detector that someone loaded on purpose.

MORE BITS PER CELL

Levels, gaps and wear

Instead of two states, a cell can store one of 4, 8 or 16 charge levels: 2, 3 or 4 bits. The price is smaller gaps between the levels, so the same tunnel-oxide wear that barely matters for one bit causes errors with four. That is why denser Flash has shorter endurance and needs stronger error correction.

Each extra bit per cell doubles the number of levels and halves the safety margin.

THE MODERN CELL

NAND, charge traps and 3D stacking

NAND strings Cells connected in series share contacts, giving the densest storage but block-based access

Charge trap A silicon-nitride layer holds the charge in many separate traps instead of one floating island

3D NAND Strings are built vertically in tall stacks of well over 100 layers, so density grows by building up

Wear leveling A controller spreads writes evenly because each cell survives only a limited number of erase cycles

Flash scaled by going vertical when shrinking the cells ran out of room.

CHECK THE CELL

How does Flash store a bit?

A trapped charge shifts the threshold, and the cell reads that shift.

UNIT 15 STUDY COMPLETE

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Ready for the Fab Challenge?

You've trapped charge on a floating gate, packed several bits into one cell, and seen where Flash gets its density and its wear.