Charge coupled devices and memories

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Charge coupled devices and memories

• Charge coupled devices
• Imaging applications, delay line in signal
  processing, filters etc.
• Uses MOS-C
• Semiconductor Memories
• Dynamic random access memories (DRAMs) use MOS-C
  in deep depletion inversion as storage units.
• We will qualitatively discuss the operation of
  these devices.

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MOS-capacitor - review
Consider ideal case, with p-type Si
VG lt 0 accumulation
VG gt 0 depletion, but less than VT
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MOS-capacitor - review
VG gtgt 0 VG gt VT Steady state inversion condition
Depletion layer WT
VG gtgt 0 VG gt VT applied instantaneously. W is
larger than WT.
W
QM
After a few ms, you get steady-state condition,
and W reduces to WT. Inversion layer forms
Depletion of holes
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Time required to form inversion layer

• It take a few milli-seconds to form the inversion
  layer since they are caused by minority carriers.
  Depends on e-h generation rate
• can generate faster by optical means
• can generate by injection of electrons from
  nsource and drain if present in close proximity
• can increase the temperature to increase e-h
  generation rate or by introducing deep levels.
• In CCDs, e-h pairs are generated by optical
  means, and generation by thermal means must be
  kept to a minimum (Why?)

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Concept of CCD imager

• Generation in depletion layer ? light intensity
• i.e., inversion layer charge ? light intensity
• Read out of charge pulse amplitude by operating
  as shift registers
• Need to read out sequentially fast enough such
  that the thermal generation of carriers can be
  neglected.
• 3-? clock provides directionality.

CG
Al
VG
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Memories

• Logic devices that store and manipulate
  information.
• Random access and sequential access.
• Semiconductor Memories
• non-volatile memory
• ROM, PROM, EPROM, E2PROM
• Volatile memories
• DRAM - need to refresh every few ms.
• SRAM - no need to refresh
• One transistor DRAM cell
• high density, but requires complex
  read/write/refresh circuits.

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One transistor DRAM cell
Column bit or data line (BL)
Row Word line (WL)
CS
VP
A stored one will be deep depletion condition. A
stored zero will be the inversion condition.
Note that a stored one will revert to zero if not
refreshed. Need refreshing every few milliseconds.