How Floating‑Gate Transistors Enable NAND Flash Write and Read Operations

Flash memory has two categories; NAND flash is mainly used for storage.

Write Operation

MOS characteristics

Applying a high gate voltage turns the MOS on.

Applying a low gate voltage turns it off.

Adding a floating‑gate layer and a tunneling layer to the MOS creates a floating‑gate transistor, the basic unit for storing one bit.

Floating‑gate transistor write operation (logic 0)

Applying a relatively high gate voltage makes electrons tunnel into the floating gate, so the floating gate holds electrons, representing logic 0.

Floating‑gate transistor write operation (logic 1)

Applying a high voltage to the substrate pulls electrons out of the tunneling layer, leaving none on the floating gate, representing logic 1.

Summary

The presence or absence of electrons on the floating gate determines the two states.

Read Operation

The state can be read by checking whether the floating gate contains electrons.

How to detect electrons on the floating gate?

With the gate at low voltage, a channel forms; current flows between drain and source, which can be measured with an ammeter.

If electrons are present on the floating gate, they repel incoming electrons, preventing channel formation and thus no current.

Matrix Control

NAND flash read/write unit is a page; erase unit is a block.

Two floating‑gate transistors share one N‑channel and are connected to the same substrate, so operations are performed per block.

To write logic 0 to a cell, apply a high voltage to the selected row and a low voltage to the column, avoiding a current path that would block electron flow to the floating gate.

To write logic 1, apply a high voltage to both the row and the column, creating a current path that blocks electron flow to the floating gate.