Semiconductor memory

Semiconductor memory is a

As of 2017[update], sales of semiconductor memory chips are $124 billion annually, accounting for 30% of the semiconductor industry.^[6] Shift registers, processor registers, data buffers and other small digital registers that have no memory address decoding mechanism are typically not referred to as memory although they also store digital data.

Description

In a semiconductor memory chip, each

Data is accessed by means of a binary number called a

The two basic operations performed by a memory chip are "read", in which the data contents of a memory word is read out (nondestructively), and "write" in which data is stored in a memory word, replacing any data that was previously stored there. To increase data rate, in some of the latest types of memory chips such as DDR SDRAM multiple words are accessed with each read or write operation.

In addition to standalone memory chips, blocks of semiconductor memory are integral parts of many computer and data processing integrated circuits. For example, the

Types

Volatile memory

RAM (Random-access memory) – This has become a generic term for any semiconductor memory that can be written to, as well as read from, in contrast to ROM (below), which can only be read. All semiconductor memory, not just RAM, has the property of random access.

• DRAM (
  MOS capacitor to store each bit. This type of RAM is the cheapest and highest in density, so it is used for the main memory in computers. However, the electric charge that stores the data in the memory cells slowly leaks out, so the memory cells must be periodically refreshed
  (rewritten) which requires additional circuitry. The refresh process is handled internally by the computer and is transparent to its user.
  • FPM DRAM (
    Fast page mode DRAM
    ) – An older type of asynchronous DRAM that improved on previous types by allowing repeated accesses to a single "page" of memory to occur at a faster rate. Used in the mid-1990s.
  • EDO DRAM (
    Extended data out DRAM
    ) – An older type of asynchronous DRAM which had faster access time than earlier types by being able to initiate a new memory access while data from the previous access was still being transferred. Used in the later part of the 1990s.
  • VRAM (
    video adapters
    (video cards).
  • SDRAM (
    memory bus. This allowed the chip to process multiple memory requests simultaneously using pipelining
    , to increase the speed. The data on the chip is also divided into banks which can each work on a memory operation simultaneously. This became the dominant type of computer memory by about the year 2000.
    • double pumping
      (transferring data on both the rising and falling edges of the clock pulse). Extensions of this idea are the current (2012) technique being used to increase memory access rate and throughput. Since it is proving difficult to further increase the internal clock speed of memory chips, these chips increase the transfer rate by transferring more data words on each clock cycle
      • DDR2 SDRAM – Transfers 4 consecutive words per internal clock cycle
      • DDR3 SDRAM – Transfers 8 consecutive words per internal clock cycle.
      • DDR4 SDRAM – Transfers 16 consecutive words per internal clock cycle.
    • RDRAM (
      Rambus DRAM
      ) – An alternate double data rate memory standard that was used on some Intel systems but ultimately lost out to DDR SDRAM.
      • XDR DRAM (Extreme data rate DRAM)
    • SGRAM (
      bit masking
      and block write, and can open two pages of memory at once.
      • GDDR SDRAM (Graphics DDR SDRAM)
        • GDDR2
        • GDDR3 SDRAM
        • GDDR4 SDRAM
        • GDDR5 SDRAM
        • GDDR6 SDRAM
    • HBM (High Bandwidth Memory) – A development of SDRAM used in graphics cards that can transfer data at a faster rate. It consists of multiple memory chips stacked on top of one another, with a wider data bus.
  • PSRAM (
    game consoles such as the Wii
    .
• SRAM (
  cache memories
  in computers.
• CAM (
  cache memory
  .