24-01-2012, 11:14 AM
PHASE CHANGE MEMORY
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INTRODUCTION
1.1 Current Non-volatile Memory Technology: FLASH Memory
Non-volatile memory (NVM) has become a very important part of our day
to day life. We see it implemented in everything from portable electronics-USB thumbdrives,
MP3 players, and digital cameras to the program code that makes electronics
around us work-from space shuttles to cars. The NVM known as Flash memory is
everywhere around us.
Flash memory was initially introduced as a functional improvement over the UVerased
EPROM. It was first used in the BIOS memory in personal computers on a
significant high-volume scale.
Need for a new technology
The solid-state memory industry and the semiconductor industry has been
following the Moore`s Law leading to smaller device dimensions enabling everincreasing
system functionality. In the case of memory, increased system functionality is
manifested in the form of more megabytes in the same package size
2.1Theory of Operation
Chalcogenide alloys that are highly resistive semiconductors in the amorphous
phase and highly conductive semimetals in the crystalline phase have been designed. A
two-terminal memory element implementing the chalcogenide material is fabricated
which operates by converting a small volume of the chalcogenide material back and forth
between the crystalline and amorphous phases.
CELL STRUCTURES
In a full memory array, an access device such as a diode or transistor must
be included at each memory cell to ensure that the read and write currents on each bitline
are interacting with one and only one memory device at a time. The amount of current
that this access device can supply must comfortably exceed the required RESET current
of the worst-case PCM element.
