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-C 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-C - review
VG gtgt 0 VG gt VT Steady state inversion condition
Depletion layer thickness 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 milliseconds 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 CCD, e-h are generated by optical means, and
  generation by thermal means should be kept to a
  minimum.

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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.

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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.
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Vbi
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