MOS Memory
NON-volatile Memory
- Flash memory -
3
Flash memory cell VS MOSFET
• Flash memory cell has a charge storage layer (Floating gate, F/G) which change the flat band voltage and threshold voltage of a cell can be changed
➔ Memorize information in the charge storage layer
Flash memory cell structure
• B/L : Bit Line
• C/G : Control Gate, (Word Line)
• F/G: Floating Gate
• ONO: Oxide-Nitride-Oxide layer
• Vth of cell changes depending on the amount of charge in F/G
• Electrons can be injected into (or ejected out of) the F/G through tunnel oxide with electric field
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Flash memory operation
• Write (or read) binary data to a flash cell
• Data ‘0’ → ‘off’ state (program)
• Data ‘1’ → ‘on’ state (erase)
➢ [Charge injection]
• By hot electrons
• By Fowler-Nordheim tunneling
By hot electron injection By Fowler-Nordheim tunneling
Charge injection methods
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Coupling ratio
𝑉
𝐹/𝐺= 𝛼
𝑂𝑁𝑂∙ 𝑉
𝐶/𝐺+ 𝛼
𝐷∙ 𝑉
𝐷+ 𝛼
𝐵∙ 𝑉
𝐵+ 𝛼
𝑆∙ 𝑉
𝑆where 𝐶
𝑡𝑜𝑡= 𝐶
𝑂𝑁𝑂+ 𝐶
𝐷+ 𝐶
𝐵+ 𝐶
𝑆𝛼
𝑂𝑁𝑂= 𝐶
𝑂𝑁𝑂/𝐶
𝑡𝑜𝑡; ONO coupling ratio 𝛼
𝐷= 𝐶
𝐷/𝐶
𝑡𝑜𝑡; Drain coupling ratio 𝛼
𝐵= 𝐶
𝐵/𝐶
𝑡𝑜𝑡; Body coupling ratio 𝛼
𝑆= 𝐶
𝑆/𝐶
𝑡𝑜𝑡; Source coupling ratio
From 𝑸 = 𝑪 ∙ 𝑽 and charge conservation law
• ONO coupling ratio means gate controllability
• For fast programming, high 𝑉𝐹/𝐺 required
➔ If 𝛼𝑂𝑁𝑂 is large, high 𝑉𝐹/𝐺 can be achieved with same 𝑉𝐶/𝐺
Endurance; lifespan of Flash memory
Fig. Schematic illustrating the collapse of the memory window as a function of the number of program (write) and erase cycles
• Programming and erasure of an Flash memory device requires much higher electric field than encountered in the normal operation of a MOSFET.
• Tunnel Oxide degradation → Memory window collapse (closer Vth between programmed and erased states)
• Endurance: measured in terms of the number of program and erase cycles before the memory window is reduced to the point of inadequate margin.
• Most Flash memories on the market : 103 ~ 106 cycles of endurance.
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Endurance improvement; Wear leveling
• If certain cells used intensively → those cells wear out quickly
→ storage capacity decrease
• Wear leveling; Each time a block of data is re-written to the flash memory
→ written to a new location
