A few examples should help clarify these rules.
Memory Stickand xD-Picture Card.
For example, the microSD card has an area of just over 1. In single-level cell SLC devices, each cell stores only one bit of information. The floating gate may be conductive typically polysilicon in most kinds of flash memory or non-conductive as in SONOS flash memory.
The cells can be seen as an electrical switch in which current flows between two terminals source and drain and is controlled by a floating gate FG and a control gate CG. On top is the control gate CGas in other MOS transistors, but below this, there is a floating gate FG insulated all around by an oxide layer.
Because the FG is electrically isolated by its insulating layer, electrons placed on it are trapped until they are removed by another application of electric field e.
Counter-intuitively, placing electrons on the FG sets the transistor to the logical "0" state. Once the FG is charged, the electrons in it screen partially cancel the electric field from the CG, thus, increasing the threshold voltage VT1 of the cell.
This means that now a higher voltage VT2 must be applied to the CG to make the channel conductive. If the channel conducts at this intermediate voltage, the FG must be uncharged if it was charged, we would not get conduction because the intermediate voltage is less than VT2and hence, a logical "1" is stored in the gate.
If the channel does not conduct at the intermediate voltage, it indicates that the FG is charged, and hence, a logical "0" is stored in the gate. The presence of a logical "0" or "1" is sensed by determining whether there is current flowing through the transistor when the intermediate voltage is asserted on the CG.
In a multi-level cell device, which stores more than one bit per cell, the amount of current flow is sensed rather than simply its presence or absencein order to determine more precisely the level of charge on the FG.
This, in turn, changes the drain-source current that flows through the transistor for a given gate voltage, which is ultimately used to encode a binary value.
The Fowler-Nordheim tunneling effect is reversible, so electrons can be added to or removed from the floating gate, processes traditionally known as writing and erasing. Over half the energy used by a 1. NOR flash continues to be the technology of choice for embedded applications requiring a discrete non-volatile memory device.
Erasing[ edit ] To erase a NOR flash cell resetting it to the "1" statea large voltage of the opposite polarity is applied between the CG and source terminal, pulling the electrons off the FG through quantum tunneling. The erase operation can be performed only on a block-wise basis; all the cells in an erase segment must be erased together.
Programming of NOR cells, however, generally can be performed one byte or word at a time. Compared to NOR flash, replacing single transistors with serial-linked groups adds an extra level of addressing.Big Ideas Math® and Big Ideas Learning® are registered trademarks of Larson Texts, Inc.
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