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Non-MOSFET Based Memory. Alex Rodriguez-Triana Terence Frederick April 21, 2008. Outline. MOSFET Based RAM Memory DRAM, SRAM, FLASH Problems with MOSFET Memory Scaling Alternative Memory MRAM FeRAM PCRAM Summary. History of MOSFET Memory. Concept goes back to the 1960s

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non mosfet based memory

Non-MOSFET Based Memory

Alex Rodriguez-Triana

Terence Frederick

April 21, 2008

outline
Outline
  • MOSFET Based RAM Memory
    • DRAM, SRAM, FLASH
  • Problems with MOSFET Memory
    • Scaling
  • Alternative Memory
    • MRAM
    • FeRAM
    • PCRAM
  • Summary
history of mosfet memory
History of MOSFET Memory
  • Concept goes back to the 1960s
  • People were speculative
    • BJT was more advanced and faster
    • Leakage current
  • They were attractive
    • Simple Processing
    • Layout Advantages
      • Leads to high-density integrated circuits
history of mosfet memory1
History of MOSFET Memory
  • SRAM were proposed
    • six MOSFET’s per cell
  • SRAM began to be used in the mid-70s
  • DRAM patented in 1968
    • 1 MOSFET, 1 Capacitor
  • First commercial DRAM
    • 1971 by Intel
dynamic ram
Dynamic RAM
  • Most common type of RAM memory
  • Arranged in a square array
    • one capacitor and transistor per cell
  • Stores one bit per cell
    • Recharging/Refreshing : capacitors lose their charge
  • Rows: Word Lines
  • Columns: Bit Lines
advantages disadvantages of dram
Advantages/Disadvantages of DRAM
  • Advantages
    • Cost
    • Small
      • 1T & 1C vs. 6T for SRAM
    • Number of Read/Write Cycles
      • > 10^15
  • Disadvantages
    • Slow
      • Need to refresh
    • Volatile
      • Data is lost when memory is not powered
static ram
Static RAM
  • Memory cell uses flip-flop to store bit
  • Requires 6 transistors
    • Each bit is stored on 4 transistors that form two inverters
    • Two other transistors control the access to a cell during read and write operations
  • This storage cell has two stable states
    • 0 and 1
advantages disadvantages of sram
Advantages/Disadvantages of SRAM
  • Advantages
    • Performance better than DRAM
      • Faster
      • Less Power Hungry
    • Number of Read/Write Cycles
      • > 10^15
  • Disadvantages
    • Cost
      • More than DRAM
    • Volatile
      • Data is lost when memory is not powered
flash memory
FLASH Memory
  • Invented by Dr. Fujio Masuoka at Toshiba in 1984
  • Stores information in an array of memory cells made from floating-gate transistors
  • Single-Level Cell Devices - each cell stores only one bit
advantages disadvantages of flash
Advantages/Disadvantages of FLASH
  • Advantages
    • Cost
    • Non-Volatile
      • Does not lose information when the power is off
    • Low Power
    • Fast Erase
      • Large blocks rather than one word at a time
  • Disadvantages
    • Number of Read/Write Cycles
      • ~ 10^6
    • Slow Write
      • Entire block must be read, word updated, then entire block written back
future of mosfet memory
Future of MOSFET Memory
  • Current memory technologies are nearing the end
  • Main issue with MOSFET RAMs
    • Scalability
  • Designers put more components onto each chip
    • Width of the smallest features is shrinking
      • 130 nm in 2000 to 45 nm today
  • Existing memory technologies will be good for several more generations
    • Unlikely to make the transition to 22 nm (scheduled for 2011) or 16 nm (2018)
  • New types of technologies
    • MRAM, FeRAM, PCRAM
mosfet scaling
MOSFET Scaling
  • Late 1990s
    • Scaling resulted in great improvement in MOSFET circuit operation
  • Reasons for smaller MOSFETs
    • Same functionality in a smaller area
    • Reduces cost per chip
      • Smaller ICs allow for more chips on a wafer
      • Fab costs for wafer are relatively fixed
mosfet scaling1
MOSFET Scaling
  • Problems when scaling too small
    • Slower chip speed
      • Greater delay due to interconnects
    • Operational problems
      • Higher sub-threshold, increased gate-oxide and junction leakage, lower transconductance, heat production, and process variation
    • Simulation
      • Difficult to predict what the final device will look like
      • Modeling of physical processes
      • Microscopic variations in structure due to the probabilistic nature of atomic processes require statistical predictions
alternative technologies
Alternative Technologies
  • Magnetic RAM (MRAM)
  • Ferroelectric RAM (FeRAM)
  • Phase Change RAM (PCRAM)
magnetoresistive ram
Magnetoresistive RAM
  • Under development since the 1990s
  • Data is stored by magnetic storage elements
    • Formed from two ferromagnetic plates
  • Plates can hold a magnetic field
    • Polarization doesn’t leak away with time like charge
    • Less wear since switching states doesn’t involve movement of electron or atoms
  • One plates is a permanent magnet
    • Set to a certain polarity
    • Second plate’s field will change to match that of an external field
  • A memory device is built from a grid of "cells"
4mb mram
4MB MRAM
  • 1st commercial available MRAM
  • Based on 1T and 1 magnetic tunnel junction
  • Isolates read and write path
  • Separates programming components from the sense circuit
    • Improved performance
read and write of mram
Read and Write of MRAM
  • Read
    • Current is passedthrough the bit
    • resistance of thebit is sensed
  • Write
    • Current is passed through the programming lines
    • Induced magnetic field is created at the junction, which the writable plate picks up
slide18
MRAM
  • Cell works in a toggling mode
    • Same direction
      • Low resistance state (0)
    • Opposite direction
      • High resistance state (1)
mram in embedded systems
MRAM in Embedded Systems
  • Inserted late in the SC fabrication process
  • Low temperature
    • Compatible with CMOS processing
  • Consolidate multiple MRAM into one
    • highly reliable NVRAM
    • Less complexity
    • High performance RD/WR
advantages disadvantages of mram
Advantages/Disadvantages of MRAM
  • Advantages
    • Non-volatile
      • Does not require programming sequences or block erasing
    • Very fast RD/WR and unlimited endurance
    • Simple device Architecture and easy software development
      • Due to easy write and overwrite
  • Disadvantages
    • Scalability of magnetic domain?
      • Might have the same problems as a transistor
    • Disturbance of neighboring cells when put close together
      • Leads to false writes
    • High power needed to write
ferroelectric ram
Ferroelectric RAM

Borrows concepts from DRAM

most popular design follows the 1T1C design concept

similar/same write process

write accomplished by applying charge that is stored in capacitor

Similarity to Floating Gate Design

1T design

Also reminiscent of MRAM

focuses on ferroelectric properties, whereas MRAM techniques often focus on ferromagnetic properties

both characteristics take form of hysteresis loop

structure
Structure

1T type

Similar to normal transistor

Identical to floating gate design where floating gate is ferroelectric material

1T1C type

ferroelectric material serves ONLY as capacitor

introduction
Introduction

Two major focuses in the paper

developing a better material to deal with leakage currents in 1T1C FeRAM

replace some Fe in lattice with Mn

Improve upon 1T FeRAM design

create MFIS-FET

Introduce a new 1T2C FeRAM design

1t2c design
1T2C Design

2 Ferroelectric capacitors of the same size connected to the gate of the transistor

capacitors polarized opposite the gate

Good performance

non-destructive data reads

good data retention time

high on/off current ratio

advantages disadvantages of feram
Advantages/Disadvantages of FeRAM

Advantages

lower power usage

faster write speed

greater number of rewrites

already being mass-produced

Disadvantages

still more research to be done on reliability (i.e. high NRE cost)

only applicable to a small niche

study of phase change random access memory pcram at the nano scale

“Study of Phase Change Random Access Memory (PCRAM) at the Nano-Scale”

by R. Zhao, L.P. Shi, W.J. Wang, H.X. Yang, H.K. Lee, K.G. Lim, E.G. Yeo, E.K. Chua and T.C. Chong

introduction1
Introduction

RAM based on floating-gate design (i.e. Flash memory) will soon meet physical limitations

interpoly tunneling

intercell crosstalk

Flash memory is the most prevalent non-volatile memory on the market

a viable option must be found soon

PCRAM may be that option

fabrication design
Fabrication/Design

“Bybrid” process used to etch the layers

Electronic Beam Lithography (EBL)

Optical Lithography

Electrodes made of TiW

Dielectric is common SiO2

Phase Change material is Ge2Sb5Te2

Feature size refers to contact between PC and bottom electrode

how it works
How it Works

Unique Phase Change material has two states

Crystalline state has low resistance and represents a stored ‘1’

Amorphous state has high resistance and represents a stored ‘0’

To change bit from 1 to 0 (i.e. RESET), a relatively high voltage is applied for a short time such that the compound melts but is not able to recrystallize

To change bit from 0 to 1 (i.e. SET), a lower voltage is applied for a longer time so that compound can crystallize

simulation
Simulation

Pulse generator created to produce short (<10ns) signal

Known resistance placed in circuit

Voltages measured to determine drop across resistor

Current into PCRAM approximately (V1-V2)/Rload

Cells with feature sizes ranging from 40 to 200 nm created

same wafer used

advantages disadvantages of pcram
Advantages/Disadvantages of PCRAM

Advantages

great scalability

fast for both reads and writes

low current required to program

Disadvantages

as of yet, only in the research phase

still limited read/write accesses (108)