EE 534 summer 2004 University of South Alabama EE534 VLSI Design System summer 2004 Lecture 14:Chapter 10 Semiconductors memories

EE 534 summer 2004 University of South Alabama Chapter 10: SEMICONDUCTOR MEMORIES  Memory classification (more history than technical)  Memory architecture and basic operations  The memory core (data storage units)  Peripheral circuits (decorder, sens-amp, etc.)  Reliability concerns (processing and operational)  General design considerations and future trends

EE 534 summer 2004 University of South Alabama Feynman’s Prediction in 1959  To store Encyclopedia Britannica on the head of a pin (10 -2 inch square), we need a dot every 8nm  To store all books in history (around 25 million copies, which needs bits, or peta-bit) with 8nm in 2D, we need just a few square yards.  If we code the text part and do 3D storage of (5  5  5 atoms), all books in history will be smaller than the sand dust (now you are talking about smart dust).  There is NO physical principles that prohibit this.

EE 534 summer 2004 University of South Alabama Far Away from the End... “What I have demonstrated is that there is room---that you can decrease the size of things in a practical way. I now want to show that there is plenty of room. I will not now discuss how we are going to do it, but only what is possible in principle---in other words, what is possible according to the laws of physics. I am not inventing anti-gravity, which is possible someday only if the laws are not what we think. I am telling you what could be done if the laws are what we think; we are not doing it simply because we haven't yet gotten around to it.” (Richard P. Feynman, There is plenty of room at the bottom, Dec. 1959)

EE 534 summer 2004 University of South Alabama A Word on Terminology for Semiconductor Memories  Different unit to consider in design l circuit designers: bits l chip designers: bytes (8 or 9 bits), gigabyte (10 9 bytes), terabyte (10 12 ), peta bytes (10 15 ), exa bytes (10 18 ), googol bytes ( ). l system designers: words (32 bits now, but many 64-bit system appearing)  ROM (read-only memory), RAM (random-access memory), EEPROM (electrically erasable programmable read-only memory), etc. can take better names l SDRAM (synchronous dynamic RAM), etc. l off-chip access, embedded SRAM, etc.

EE 534 summer 2004 University of South Alabama Semiconductor Memory Classification FIFO: First-in-first-out LIFO: Last-in-first-out (stack) CAM: Content addressable memory

EE 534 summer 2004 University of South Alabama Growth in DRAM Chip Capacity

EE 534 summer 2004 University of South Alabama Memory Architecture: Decoders pitch matched line too long

EE 534 summer 2004 University of South Alabama 2D Memory Architecture A0A0 Row Decoder A1A1 A j-1 Sense Amplifiers bit line word line storage (RAM) cell Row Address Column Address AjAj A j+1 A k-1 Read/Write Circuits Column Decoder 2 k-j m2 j Input/Output (m bits) amplifies bit line swing selects appropriate word from memory row

EE 534 summer 2004 University of South Alabama 3D Memory Architecture Row Addr Column Addr Block Addr Input/Output (m bits) Advantages: 1. Shorter word and/or bit lines 2. Block addr activates only 1 block saving power

EE 534 summer 2004 University of South Alabama Hierarchical Memory Architecture Global Data Bus Row Address Column Address Block Address Block SelectorGlobal Amplifier/Driver I/O Control Circuitry  Advantages: l shorter wires within blocks l block address activates only 1 block: power management

EE 534 summer 2004 University of South Alabama Read-Write Memories (RAM)  Static (SRAM) l Data stored as long as supply is applied l Large (6 transistors per cell) l Fast l Differential signal (more reliable)  Dynamic (DRAM) l Periodic refresh required l Small (1-3 transistors per cell) but slower l Single ended (unless using dummy cell to generate differential signals)

EE 534 summer 2004 University of South Alabama Three Transistors DRAM cell No constraints on device ratios Reads are non-destructive Value stored at X when writing a “1”=V WWL -V Tn ● Binary information is stored in the from of charge in the capacitor C1 ●M2 is storage transistor ●Pass transistors act as access switches for data read and write operation ●All data read and data write operation are performed when PC is low.

EE 534 summer 2004 University of South Alabama Write ‘1’ and read ‘1’ operation ●During write operation signal WS is high As a result M 1 is turned on and allow charge sharing between C 2 and C 1., which turned on M 2. ●During read operation: M 1 is off RS is high, M 3 is on and consequently C 3 discharge through M 2 and M 3. Low level on Dout (C3) is the finger print of stored “1” Read operation Write operation

EE 534 summer 2004 University of South Alabama Write ‘0’ and read ‘0’ operation  For write “0” operation: WS is high, M 1 turn on C 1 and C 2 discharge through M 1 M 2 turned off because of low voltage level of C 1.  During read “0” operation RS is high M 3 turn on but M 2 is off C 3 remains high High level on D out is the finger print of stored “0” bit.

EE 534 summer 2004 University of South Alabama 3-T DRAM cell during operation

EE 534 summer 2004 University of South Alabama 1-Transistor DRAM Cell C S M1 BL WL C BL WL X BL V DD  V T V GND Write "1" Read "1" sensing V DD /2  V BL V PRE – C S C S C BL == Write: C S is charged or discharged by asserting WL and BL. Read: Charge redistribution takes places between bit line and storage capacitance, The direction of which determines the value of data stored. Voltage swing is small; typically around 250 mV. V DD /2 small perturbation VDD/2

EE 534 summer 2004 University of South Alabama DRAM Cell Observations  DRAM cells are single ended in contrast with SRAM cells  Read-out of 1T DRAM is destructive (3T is not), and refresh is necessary after read.  1T DRAM needs an explicit capacitance (3T needs not)  1T DRAM requires a sense amp for each bit line, due to charge redistribution read-out  When writing 1’s into DRAM cells, a V th is lost. This charge loss can be circumvented by bootstrapping the word line (not the bit line) to a higher value than V DD.

EE 534 summer 2004 University of South Alabama Read-Write Memories (RAM)  Static (SRAM) l Data stored as long as supply is applied l Large (6 transistors per cell) l Fast l Differential signal (more reliable)  Dynamic (DRAM) l Periodic refresh required l Small (1-3 transistors per cell) but slower l Single ended (unless using dummy cell to generate differential signals)

EE 534 summer 2004 University of South Alabama 6-transistor SRAM Cell !BLBL WL M1 M2 M3 M4 M5 M6Q Q Note that it is identical to the register cell from static sequential circuit - cross-coupled inverters Consumes power only when switching - no standby power (other than leakage) is consumed The major job of the pullups is to replenish loss due to leakage Sizing of the transistors is critical!

EE 534 summer 2004 University of South Alabama SRAM Cell Analysis (Read) BL=1 WL=1 M1 M4 M5 M6 Q=1 Q=0 C bit Read-disturb (read-upset): must carefully limit the allowed voltage rise on Q to a value that prevents the read-upset condition from occurring while simultaneously maintaining acceptable circuit speed and area constraints

EE 534 summer 2004 University of South Alabama SRAM Cell Analysis (Read) BL=1 WL=1 M1 M4 M5 M6 Q=1 Q=0 C bit Cell Ratio (CR) = (W M1 /L M1 )/(W M5 /L M5 ) V Q = [(V dd - V Tn )(1 + CR  (CR(1 + CR))]/(1 + CR) To avoid read-disturb, the voltage on node Q should remain below the trip point of the inverter pair for all process, noise, and operating conditions.

EE 534 summer 2004 University of South Alabama Read Voltages Ratios V dd = 2.5V V Tn = 0.5V The voltage rise inside the cell will not rise above the threshold if Cr>1.2

EE 534 summer 2004 University of South Alabama SRAM Cell Analysis (Write) BL=1 BL=0 WL=1 M1 M4 M5 M6 Q=1 Q=0 Pullup Ratio (PR) = (W M4 /L M4 )/(W M6 /L M6 ) V Q = (V dd - V Tn )  ((V dd – V Tn ) 2 – (  p /  n )(PR)((V dd – V Tn - V Tp ) 2 ) In order to write the cell, the pass gate M6 must be more conductive than the M4 to allow node Q to be pulled to a value low enough for the inverter pair (M2/M1) to begin amplifying the new data. The maximum ratio of the pullup size to that of the pass gate required to guarantee that the cell is writable – M6 in linear, M4 in saturation State shown is that before write takes effect (1 is stored, trying to write a 0)

EE 534 summer 2004 University of South Alabama Write Voltages Ratios V dd = 2.5V |V Tp | = 0.5V  p /  n = 0.5

EE 534 summer 2004 University of South Alabama Design Issues: Cell Sizing  Keeping cell size minimized is critical for large caches  Minimum sized pull down fets (M1 and M3) l Requires minimum width and longer than minimum channel length pass transistors (M5 and M6) to ensure proper CR l But sizing of the pass transistors increases capacitive load on the word lines and limits the current discharged on the bit lines both of which can adversely affect the speed of the read cycle  Minimum width and length pass transistors l Boost the width of the pull downs (M1 and M3) l Reduces the loading on the word lines and increases the storage capacitance in the cell – both are good! – but cell size may be slightly larger

EE 534 summer 2004 University of South Alabama 6T-SRAM — Layout V DD GND Q Q WL BL M1 M3 M4M2 M5M6  Actually all transistors in 6-T SRAM cell can be minimum- sized regardless of the writability concerns, as long as the differential signal is maintained (one side will be able to write in, and then the bi- stability takes over)  The bit lines are usually pre- charged to V DD /2 instead of V DD to take full advantage of the differential signal.

EE 534 summer 2004 University of South Alabama Resistance-load SRAM Cell

EE 534 summer 2004 University of South Alabama MOS NOR ROM W0: 1011 W1: 0110 W2: 1010 W3: 1111 Logic 1 bit is stored as a the absence of an active transistor While a logic 0 bit is stored as the presence of an active transitor. In actual ROM layout 1 bit is programmed by omitting the drain or source connection 0 bit is programmed by connecting the drain to the ground or metal to diffusion contact

EE 534 summer 2004 University of South Alabama Erasable Programmable ROM(EPROM): Floating-gate transistor (FGMOS)

EE 534 summer 2004 University of South Alabama Floating-Gate Transistor Programming: EPROM  Hot-carrier injection is self-limiting: no detailed control circuitry necessary  Writing and sensing on the same transistor: simplified  Erase by UV may have residual effects Virtually all nonvolatile memories are currently based on the floating gate approach.

EE 534 summer 2004 University of South Alabama Floating gate tunneling oxide (FLOTOX): EEPROM  F-N tunneling is not as self- limiting, and read/write need to be carefully controlled.  V th variation is large: one more control transistor  Other geometry possible (split- gate, etc.)

EE 534 summer 2004 University of South Alabama Floating-gate transistor (FGMOS) :Flash Memory  Hot-carrier injection write operation (self-limiting)  Block erase by F-N tunneling with careful tuning on the block level  Sensing by the same transistor (small cell footprint)

EE 534 summer 2004 University of South Alabama Cross-sections of NVM cells EPROMFlash Courtesy Intel

EE 534 summer 2004 University of South Alabama Characteristics of State-of-the-art NVM

EE 534 summer 2004 University of South Alabama Next Lecture and Reminders  Next lecture l Project report due on 1 st December l Project oral exam: 1 st December full adder group.