FinFET SRAM Design

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FinFET SRAM Design

• Zheng Guo
• 11/09/2004

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Increase SRAM Stability

• Need to broaden operating margin or suppress
  variations.
• Cut/boost node voltages
• Difficult to apply and lower performance.
• Increase ß-ratio
• Difficult to only forward bias NMOS.
• New devices?

Yamaoka, Hitachi, 2004
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Double-gate MOSFET

• Two gates achieve greater channel control.
• Improved drive current from higher mobility and
  steeper sub-threshold swing.
• More Scalable than a single gate Ultra-thin body
  (UTB) SOI device.

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Thin-body double gate MOSFETs
Gate length Lg
Gate
Source
Fin Width TSi
Drain
Fin Height HFIN W/2

• Back-gated (BG) MOSFET
• Independent front and back gates
• One switching gate and Vth control gate

Double-gated (DG) MOSFET
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Compact Device Model

• Semi-empirical device model based on a subset of
  BSIM3
• Captures the effect of supply change, threshold
  shift, mobility enhancement
• Parameters can be used to create Spectre AHDL
  models for DC simulation

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6T SRAM Cell

• All FETs double-gated.

• Access NFETs back-gated w/ feedback to
  dynamically increase ß-ratio by weakening access
  NFETs.
• All other FETs double-gated.

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4T SRAM Cell

• Data retention leakage current usually need to be
  at least 1000xIleakage to compensate for the
  widely fluctuating leakage current.
• Data retention leakage current flows on both
  sides but only needs to flow in one side for
  dataretention.

Yamaoka, Hitachi, 2004
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4T SRAM Cell

• All FETs double-gated.

• Access PFETs back-gated w/ feedback to
  selectively increase compensation current and to
  dynamically increase ß-ratio.
• All other FETs double-gated.

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4T Write Issue

• When writing to a neighboring cell (sharing
  common bit-lines), ICOMP is reversed and will
  discharge the 1 storage node!
• Level of discharge depends on VT of the access
  PMOS transistors.
• Use high VT devices.

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4T Write Issue

• Moderate VT access devices
• 110 surface conduction

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4T Write Issue

• High VT access devices
• 110 surface conduction
• WL-0.2V on Read/Write.

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Layouts
6T Cell
4T Cell
6.6um X 8um
5um X 8um