Q. What is Charge-coupled Devices?
CCDs are employed for storing information. They have arrays of cells that can hold charge packets of electron. A word is signified by a set of charge packets. The presence of every charge packet signifies bit-value 1. Charge packets don't remain stationary and cells pass charge to neighbouring cells with subsequent clock pulse. Thus cells are arranged in tracks with a circuitry for writing the data at beginning as well as a circuitry for reading the data at end. Logically tracks (one for every bit position) can be conceived as loops because read circuitry passes information back to write circuit that then re-creates bit values in track unless new data is written to circuit.
These devices come under category of semi-random operation since devices should wait till data has reached circuit for detection of charge packets. Access time to these devices isn't very high. At present this technology is used only in specific applications and commercial products are not available.