This article takes a look at the basics of a NAND flash cell, the building block of almost every solid state drive. It is the first of several articles that will describe the basics of Solid State Drives (SSD).
In order to store a single bit of data on a solid state drive, you need the smallest building block - a single NAND flash cell. The simplest NAND cell can be set to either a 0 or 1 state. It will continue to store that state even after power has been removed.
A simple NAND Flash Cell diagram is shown above. The NAND flash cell is made from a floating gate transistor. Electrical charge is stored on the floating gate which is isolated above and below by oxide insulating layers.
In its simplest form when the floating gate is charged, it is programmed and recognized as a binary 0. When the floating gate has no charge it is erased and recognized as a binary value of 1.
|Floating Gate State||Referred to As||Binary Value Assigned|
|Charged||Programmed||Zero - 0|
|No Charge||Erased||One - 1|
The floating gate remains in its charged or uncharged state until it is changed by surrounding circuitry. Removing power from the NAND device does not affect the state of the floating gate which is why it is such a valuable device for data storage.
To read a cell, voltage is applied to the control gate and current flow from the source to drain is attempted.
If there is no current flow, it signifies the floating gate is charged (binary 0) - as in the diagram above. If there is current flow, the floating gate is not charged (binary 1) - as in the diagram below.
To write a cell, a high voltage is applied to the control gate and electrons move from the silicon substrate to the floating gate. This process is called tunneling since the electrons “tunnel” through the oxide insulator to reach the floating gate. See diagram below.
To erase a NAND cell, a high voltage is applied to the silicon substrate and electrons move from the floating gate to the silicon substrate. This uses the same tunneling process as the writing process. See diagram below.
The tunneling process described in the Write and Erase functions above cause stress on the oxide insulator layer. Over time this stress breaks down the oxide layer and the floating gate becomes unable to maintain a charge. At some point the cell is no longer usable and must be retired. This is what is responsible for the finite number of writes/erases per cell of NAND flash.
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Continue to Blog #2: SLC, MLC & TLC NAND Flash or learn all about Solid State Drives: