Semiconductor MemoriesSemiconductor Memories

ECE423ECE423

Xiang YuXiang Yu

12.16.0612.16.06

RAM vs. ROMRAM vs. ROM

VolatileVolatile RAM (random access)RAM (random access)

SRAM (static)SRAM (static)• SynchronousSynchronous

• AsynchronousAsynchronous DRAM (dynamic)DRAM (dynamic)

• FPM DRAMFPM DRAM

• EDO DRAMEDO DRAM

• SDRAMSDRAM

• DDR SDRAMDDR SDRAM

• DDR2 SDRAMDDR2 SDRAM

• Etc..Etc..

Non-volatileNon-volatile ROM (read only)ROM (read only)

System/Video BIOSSystem/Video BIOS

PROMPROM OTP with fuses/anti-fusesOTP with fuses/anti-fuses

EPROMEPROM UV light for erasureUV light for erasure

EEPROMEEPROM Reprogrammable with Reprogrammable with

software and hardwaresoftware and hardware

Flash MemoryFlash Memory

SRAM & DRAMSRAM & DRAM

SRAMSRAM ProsPros

• Extremely FastExtremely Fast

• No refresh cycleNo refresh cycle ConsCons

• Large area - 6T/bitLarge area - 6T/bit

• ExpensiveExpensive ApplicationsApplications

• Memory caches L1 & L2Memory caches L1 & L2

DRAMDRAM ProsPros

• CheaperCheaper

• Higher density - 1T/bitHigher density - 1T/bit ConsCons

• Slower in speedSlower in speed

• Needs refresh cycleNeeds refresh cycle ApplicationsApplications

• Computer memoryComputer memory

Flash MemoryFlash Memory

ProsPros Non-volatileNon-volatile PortabilityPortability High DensityHigh Density

Floating gate transistorFloating gate transistor• CG and FGCG and FG• MLC technologyMLC technology

High IsolationHigh IsolationConsCons Oxide layers near min. limitOxide layers near min. limit High voltage requiredHigh voltage required Slow WRITE cycleSlow WRITE cycle Capacitance CouplingCapacitance Coupling Limited UseLimited Use

Focus For ImprovementFocus For Improvement

Non-volatilityNon-volatility PortabilityPortability Small AreaSmall Area High DensityHigh Density Faster WRITE/ERASE cyclesFaster WRITE/ERASE cycles Lower PowerLower Power Lower CostsLower Costs LongevityLongevity

FRAMFRAM FeRAM (Ferroelectric RAM)FeRAM (Ferroelectric RAM)

Uses ferroelectric characteristics of the capacitorUses ferroelectric characteristics of the capacitor• lead (Pb) zirconate (Zr) titanate (Ti) - PZTlead (Pb) zirconate (Zr) titanate (Ti) - PZT

ProsPros Non-volatileNon-volatile Small sizeSmall size Fast WRITE cyclesFast WRITE cyclesConsCons A potential WRITE after each READA potential WRITE after each READ LongevityLongevity Polarization degradationPolarization degradation Requires high temperaturesRequires high temperatures Want higher densities Want higher densities

MRAMMRAM

Magnetoresistive RAMMagnetoresistive RAM Uses properties of magnetism and the development of the MTJUses properties of magnetism and the development of the MTJ Uses STT technology (Spin Torque Transfer)Uses STT technology (Spin Torque Transfer)

ProsPros Non-volatileNon-volatile Small sizeSmall size Fast WRITE cyclesFast WRITE cycles LongevityLongevityConsCons Requires high temperaturesRequires high temperatures Want higher densitiesWant higher densities

PRAMPRAM Phase-change RAM, PCM, C-RAM (chalcogenide RAM)Phase-change RAM, PCM, C-RAM (chalcogenide RAM)

Uses the special property of chalcogenide alloyUses the special property of chalcogenide alloy• Germanium (Ge), antimony (Sb) and tellurium (Te) – GSTGermanium (Ge), antimony (Sb) and tellurium (Te) – GST• Switches between amorphous (0) and crystallized (1) states with heat from Switches between amorphous (0) and crystallized (1) states with heat from

currentcurrent

ProsPros Non-volatileNon-volatile Small sizeSmall size High densityHigh density Fast switching timesFast switching timesConsCons Requires high current (heat)Requires high current (heat) LongevityLongevity

ComparisonsComparisons

Overall ViewOverall View

ParametersParameters SRAMSRAM DRAMDRAM FlashFlash FRAMFRAM MRAMMRAM PRAMPRAM

Non-volatileNon-volatile NoNo NoNo YesYes YesYes YesYes YesYes

RefreshRefresh NoNo msms NoNo NoNo NoNo NoNo

Cell SizeCell Size 6T6T 1T1C1T1C 1T1T 1T1C1T1C 1TCMTJ1TCMTJ 1T1R1T1R

Read TimeRead Time 2 ns2 ns 10 ns10 ns 70ns70ns 10ns10ns 10ns10ns 10ns10ns

Write TimeWrite Time 2 ns2 ns 10 ns10 ns 1010μμss 20 ns20 ns 5 ns5 ns 5 ns5 ns

Write cyclesWrite cycles > 10> 101515 > 10> 101515 101055 > 10> 101212 > 10> 101515 > 10> 101212