Charge-Coupled Device (CCD) Definition/Meaning:
A semiconductor device that has the structure of a
MOSFET
with an extremely long channel and many gates, perhaps 1000, closely
spaced between the source and drain electrodes. A MOS capacitor is
formed between each gate and the substrate; since this capacitor is
capable of storing a charge, CCDs can he used as memory devices. The
CCD essentially acts as a long (high-density) shift register since,
by manipulating the voltages applied to the gates, charge can be
transferred from one MOS capacitor to its neighbor, and so on along
the channel.
The physical structure of the device and the way in which the
gate voltages are manipulated determines the number of gates needed
to store one bit of information, typically two or three gates being
required. Since the stored charge leaks away, CCDs must be
continuously clocked, typically at a frequency of one megahertz.
Charge-coupled memories are particularly suited to applications
where memory contents are accessed in a serial manner, as in
refresh memories for CRT terminals. They are slower than
comparable RAMs but faster than magnetic bulk storage devices.
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