Memory Chip

Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a silicon integrated circuit memory chip. There are numerous different types using different semiconductor technologies. The two main types of random-access memory (RAM) are static RAM (SRAM), which uses several transistors per memory cell, and dynamic RAM (DRAM), which uses a transistor and a MOS capacitor per cell. Non-volatile memory (such as EPROM, EEPROM and flash memory) uses floating-gate memory cells, which consist of a single floating-gate transistor per cell.

Most types of semiconductor memory have the property of random access, which means that it takes the same amount of time to access any memory location, so data can be efficiently accessed in any random order. This contrasts with data storage media such as hard disks and CDs which read and write data consecutively and therefore the data can only be accessed in the same sequence it was written. Semiconductor memory also has much faster access times than other types of data storage; a byte of data can be written to or read from semiconductor memory within a few nanoseconds, while access time for rotating storage such as hard disks is in the range of milliseconds. For these reasons it is used for primary storage, to hold the program and data the computer is currently working on, among other uses.

, semiconductor memory chips sell annually, accounting for % of the semiconductor industry. 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 bit of binary data is stored in a tiny circuit called a '' memory cell'' consisting of one to several transistors. The memory cells are laid out in rectangular arrays on the surface of the chip. The 1-bit memory cells are grouped in small units called ''words'' which are accessed together as a single memory address. Memory is manufactured in word length that is usually a power of two, typically ''N''=1, 2, 4 or 8 bits.

Data is accessed by means of a binary number called a memory address applied to the chip's address pins, which specifies which word in the chip is to be accessed. If the memory address consists of ''M'' bits, the number of addresses on the chip is 2^''M'', each containing an ''N'' bit word. Consequently, the amount of data stored in each chip is ''N''2^''M'' bits. The memory storage capacity for ''M'' number of address lines is given by 2^''M'', which is usually in power of two: 2, 4, 8, 16, 32, 64, 128, 256 and 512 and measured in kilobits, megabits, gigabits or terabits, etc. the largest semiconductor memory chips hold a few gigabits of data, but higher capacity memory is constantly being developed. By combining several integrated circuits, memory can be arranged into a larger word length and/or address space than what is offered by each chip, often but not necessarily a power of two.

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 microprocessor chips that run computers contain cache memory to store instructions awaiting execution.

Types

Volatile memory

Volatile memory loses its stored data when the power to the memory chip is turned off. However it can be faster and less expensive than non-volatile memory. This type is used for the main memory in most computers, since data is stored on the hard disk while the computer is off. Major types are:

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 (''Dynamic random-access memory'') This uses memory cells consisting of one MOSFET (MOS field-effect transistor) and one 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 random access memory'') An older type of dual-ported memory once used for the frame buffers of video adapters (video cards).
** SDRAM (''Synchronous dynamic random-access memory'') This added circuitry to the DRAM chip which synchronizes all operations with a clock signal added to the computer's 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.
*** DDR SDRAM (''Double data rate SDRAM'') This could transfer twice the data (two consecutive words) on each clock cycle by 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 (''Synchronous graphics RAM'') A specialized type of SDRAM made for graphics adaptors (video cards). It can perform graphics-related operations such as 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 ('' Pseudostatic RAM'') This is DRAM which has circuitry to perform memory refresh on the chip, so that it acts like SRAM, allowing the external memory controller to be shut down to save energy. It is used in a few game consoles such as the Wii.
* SRAM (''Static random-access memory'') This stores each bit of data in a circuit called a flip-flop, made of 4 to 6 transistors. SRAM is less dense and more expensive per bit than DRAM, but faster and does not require memory refresh. It is used for smaller cache memories in computers.
* CAM (''Content-addressable memory'') This is a specialized type in which, instead of accessing data using an address, a data word is applied and the memory returns the location if the word is stored in the memory. It is mostly incorporated in other chips such as microprocessors where it is used for cache memory.

Non-volatile memory

Non-volatile memory (NVM) preserves the data stored in it during periods when the power to the chip is turned off. Therefore, it is used for the memory in portable devices, which don't have disks, and for removable memory cards among other uses. Major types are:

* ROM (''Read-only memory'') This is designed to hold permanent data, and in normal operation is only read from, not written to. Although many types can be written to, the writing process is slow and usually all the data in the chip must be rewritten at once. It is usually used to store system software which must be immediately accessible to the computer, such as the BIOS