SRAM

1. SRAM Mohammad Sharifkhani

11. Effect of Mismatch

12. Data Retention Voltage

13. DRV Mote-carlo simulation

18. Inv. with VL Input @ Read load Init cond. VL=1, VH=0 Inv. with VR Input @ Write load

19. Shmoo plot • A shmoo plot is a graph that represents how a particular test passes or fails when parameters like frequency, voltage, or temperature are varied and the test is executed repeatedly. • The shape of the failing region is meaningful and helps in determining the cause of the failure. • A shmoo plot of normal circuit operation shows better high-frequency performance as supply voltage increases, as shown in Fig. 1a. • Other shapes frequently seen include the curlback (Fig. 1b), ceiling (Fig. 1c), floor (Fig. 1d), wall (Fig. 1e), finger (Fig. 1f), and breaking wave (Fig. 1g).

24. Techniques for improving reliability • Read assist circuits • Write assist circuits • Error correction methods

25. Multiple voltages for NM improvement

26. Threshold voltage variation Monte-carlo simulation Access (reduction in the BL-differential produced), Higher Vt  lower BL swing Read (data flipping while reading), lower v-trip write (unsuccessful write) Higher Vt  weaker PU ration hold (data flipping at a lower supply voltage in standby mode) S. Mukhopadhyay, JSSC 2007

27. Solution: Adaptive body bias

28. Dynamic VDD Selection Higher write noise margin  more power

29. Read Write-assist circuits Keeps WL voltage in check (lower for stable read) Charge redistribution between cell VDD and down Vdd

30. Pulsed WL and BL Minimization of WL activation Threats write: Read Modify Write is used for all columns

31. Pulsed WL and BL On pseudo read columns the BLs are precharged to a lower voltage than VDD to maintain stability of the cell  Weaker access  lower delta V to trip the cell

32. Dynamic Body Bias On-chip programmable voltage generator with N-well resistors Forward BB: stronger PMOS makes a wider butter fly curve more SNM during read operation Higher leakage (only applied on selected banks)

33. Read assist Local SA Pilo, JSSC’07 Divided BLs

34. Read assist When enabled the half selected BL/read BL get full swing Masks the BL of the half selected columns that do not need full amplification to save power 10% more power at nominal voltage Yet allows for 1.20.9 VDD reduction and keeps the array stable  saves power at the end Mask registers are loaded during power up

35. Write assist Weaker PMOS is needed Supply reduction of the to be written cells by ~200mV Only the columns to be written on get the lower supply voltage: a power decoder is needed

36. Reference generator Bi-directional/data dependent current flow Write voltage Old data: VDDVWR New data: sink data from VWR to charge up the new 1 node Writing old data pulls up VWR  push-pull is needed at Ref. Generator

37. Redundancy in SRAMs

38. Redundancy

39. Error Correction Code

40. Multi-bit errors

41. Multi-bit errors: Interleaving

42. Future trends • More than 6T cells • Change in technology • eDRAM

43. More transistors

44. Thin Body MOSFETs

45. Double Gate FinFET

46. Double Gate vs. Tri-Gate

47. Independent Gate operation

48. Applications

49. Independent Gate Operation6-T SRAM in Bulk-Si

50. 6T SRAM with FinFET