THIRD GENERATION SEMICONDUCTOR MEMORIES

OVERVIEW

• WHAT IS MEMORY?• MEMORIES USED IN EARLIER GENERATION OF

COMPUTERS• WHAT ARE SEMICONDUCTOR MEMORIES?• DIFFERENT TYPE OF SEMICONDUCTOR

MEMORIES• DISADVANTAGES• SEMICONDUCTOR TECHNOLOGIES• CONCLUSION

WHAT IS MEMORY?

Memory refers to the physical devices used to store programs (sequences of instructions) or data on a temporary or permanent basis for use in a computer or other digital electronic device.

MEMORIES USED IN EARLIER GENERATION OF COMPUTERS

• FIRST GENERATION computers used vacuum tubes as switching device and magnetic drum for memory.

• SECOND GENERATION computers magnetic-core memory

Magnetic Drum Magnetic Core memory

WHAT ARE SEMICONDUCTOR MEMORIES? Semiconductor memory, a form of electronic data storage

device, is usually used for computer memory, and is implemented onto a semiconductor-based integrated circuit (IC).

DIFFERENT TYPE OF SEMICONDUCTOR MEMORIES

Electronic semiconductor memory can be split into two main types or categories, according to the way in which the memory operates:

• RAM - Random Access Memory• ROM - Read Only Memory

SEMICONDUCTOR TECHNOLOGIES

There are various types of RAM and ROM available. These arise from the variety of applications and also the number of technologies available.

• PROM• EPROM• EEPROM• Flash memory• DRAM• SRAM

PROM (PROGRAMMABLE READ ONLY MEMORY)• Data can be written only once.• It consists of arrays which can be fused according to required

data pattern.• Data is stored as charge on capacitor.

EPROM(ERASABLE PROGRAMMABLE READ ONLY MEMEORY)• Memory can be programmed and then erased at later time.• Data is erased by exposing silicon to ultraviolet light.• To enable this to happen there is circular window present in

EPROM.

EEPROM(ELECTRICALLY ERASABLE PROGRAMMABLE READ ONLY MEMORY)• Uses electric voltage to erase data• Same characteristics as EPROM

FLASH MEMORY• A development of EEPROM.• Non volatile.• Data written and erased in blocks• Therefore it uses programming voltages at level which are

available within the electronic equipment.

DRAM(DYNAMIC RANDOM ACCESS MEMORY)• Type of RAM.• Uses capacitor to store each bit of data.• Level of charge on capacitor determine whether the data bit is

0 or 1.• Needs to be refershed periodically.

SRAM(STATIC RANDOM ACCESS MEMORY)• Unlike DRAM data need not be refreshed again and again.• Faster read and write times than DRAM.• Cycle time is also shorter.• Consumes more power, expensive,less dense.

There are many developments going on in semiconductor technologies to make memories more efficient, some of these are as follows:

• TRAM• ZRAM• SDRAM• MRAM• P-RAM/PCM• FRAM• VRAM

TRAM(THYRISTOR RANDOM ACCESS MEMORY)• Type of DRAM.• Invented and developed by T-RAM semiconductors.• Combine strenght of DRAM and SRAM (high density and high

speed.)• It exploits the electrical property negative differential

resistance.• Highly scalable and has storage density higher than standard 6

transistor SRAM.

ZRAM(ZERO CAPACITY RAM)• Invented and developed by innovative silicon.• Based on floating body effect of silicon on insulator

technology.• It is claimed that ZRAM offers same access speeds as standard

6 transistor SRAM but it uses only 1 transistor.

SDRAM(SYNCHRONOUS DRAM)

• Runs at faster speed than DRAM.• Synchronised with processor clock.• Is capable of keeping 2 sets of memory address open

simultaneously.

MRAM(MAGNETO RESISTIVE RAM)

• Non volatile.• Uses magnetic charges to store data.• Requires low power for active operation.

PRAM(PHASE CHANGE RAM)• Based on the phenomenon of phase change of a form of

chalcogenide glass from amorphous state to poly crystalline state.This is used to store data.

FRAM(FERROELECTRIC RAM)• Similar to DRAM.• Instead of dielectric layer FRAM contains ferroelectric layer of

lead zirconate titanate.• Non volatile.• Consumes ultra low power, single cycle write speed and has

gamma radiation tolerance.

VRAM(VIDEO RAM)• Also called MPDRAM(Multi port DRAM).• Normally VRAM has 2 independent access points instead of 1

allowing CPU and graphics processor to access RAM simultaneously.

• Located on graphics card and comes in variety of formats.• Used for video adapters or 3D accelerators.

DISADVANTAGES

• Reliability issues for RAMs, SRAMs and DRAMs.• Hot-carrier degradation effects• space radiation environment

CONCLUSION

With the requirements for ever larger, faster and lower power memories always increasing, this area of technology will remain one of the most dynamic in the electronics industry.

THANK YOU