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 multiple 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 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. As of 2017, sales of semiconductor memory chips are $124 billion annually, accounting for 30% 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 2M, each containing an N bit word. Consequently, the amount of data stored in each chip is N2M bits. The memory storage capacity for M number of address lines is given by 2M, 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. As of 2014 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.
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