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bit cell

The bit cell or memory cell used in Dynamic RAMs( DRAM) stores the bit value as a microcharge in a capacitance. In this principle, a MOSFET, a transistor in MOS technology, acts as a switch and simultaneously as a capacitance.

The capacitance is formed from the capacitance of the drain zone and the substrate and is in the range of a few femto-farads (fF), which corresponds to a prefix of `10^-15`.

Structure of a bitcell

Structure of a bitcell

In this smallest capacitance the two possible digital states are stored and read in or out via the bit and word line. When storing, the word line overwrites the charge state, when reading out, the capacitance serves as driver for the bit line. Since the charge in the bit cell is constantly flowing away, the charge states are maintained by a regular refresh.

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