DRAM

DRAM
Name	DRAM
Caption	A typical DDR4 SDRAM module
Invented	1966
Inventor	Robert Dennard
Manufacturer	Samsung Electronics, SK Hynix, Micron Technology

DRAM. Dynamic random-access memory is a type of semiconductor memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged, representing the two values of a bit. Because the charge in the capacitors leaks away, the information eventually fades unless the capacitor charge is refreshed periodically, which is the origin of the term "dynamic." This refresh requirement differentiates it from SRAM, which does not require refreshing.

Overview

The fundamental storage cell in a DRAM chip consists of one transistor and one capacitor, a structure often referred to as a "1T1C" cell. This simple design allows for very high memory density and low cost per bit, making DRAM the predominant technology for the main memory in modern computing systems, from personal computers to supercomputers. Its primary role is to serve as the working memory, or RAM, for the central processing unit, holding the operating system, application software, and active data. Major manufacturers of DRAM include Samsung Electronics, SK Hynix, and Micron Technology, who continuously advance the technology in a highly competitive market.

Operation

A DRAM cell operates by storing a logic value as an electrical charge on its capacitor. To read the stored value, the word line is activated, connecting the capacitor to the bit line. A sense amplifier then detects the minute voltage change on the bit line to determine if the charge represents a logical 1 or 0. This read operation is destructive, draining the capacitor's charge, so the data must be immediately rewritten after each read. To prevent data loss from charge leakage, a periodic refresh cycle is required, typically managed by an external memory controller that reads and rewrites each row of cells thousands of times per second. The timing of these operations is governed by strict protocols like those defined in the JEDEC standards.

Types and variants

DRAM technology has evolved through numerous architectures and interfaces. Early types included Fast Page Mode DRAM and Extended Data Out DRAM. The most significant evolution was the move to synchronous DRAM, or SDRAM, which synchronizes operations with the system clock. This led to the ubiquitous DDR SDRAM family, including successive generations like DDR2 SDRAM, DDR3 SDRAM, DDR4 SDRAM, and DDR5 SDRAM, each doubling data transfer rates. Other specialized variants include Graphics DDR SDRAM for video cards, Low Power DDR for mobile devices, and High Bandwidth Memory which stacks DRAM dies for applications in artificial intelligence and high-performance computing.

History and development

The invention of the one-transistor DRAM cell is credited to Robert Dennard at IBM's Thomas J. Watson Research Center in 1966, with a patent granted in 1968. The first commercial DRAM chip was the Intel 1103, introduced in 1970, which heralded the end of magnetic-core memory dominance. The 1980s saw the rise of Japanese manufacturers like NEC and Fujitsu during the so-called "Memory Wars." The 1990s brought standardization with SDRAM and the establishment of JEDEC as the defining standards body. Continued scaling, driven by Moore's Law, has faced significant challenges related to capacitor leakage and power consumption, leading to innovations in materials and 3D structures like the trench capacitor and stacked capacitor.

Applications

DRAM is the universal solution for main system memory across virtually all computing platforms. It is the essential working memory in servers, desktop computers, and laptops, where it interfaces with the CPU via motherboard slots. In the consumer electronics space, it is found in smartphones, tablet computers, and gaming consoles. Specialized high-performance DRAM is critical for data centers running cloud services and for the GPUs used in scientific simulation and cryptocurrency mining. Its speed and volatility make it unsuitable for long-term storage, a role filled by non-volatile memory like NAND flash in solid-state drives.

Performance characteristics

Key metrics for evaluating DRAM include latency, bandwidth, and throughput. Latency, often measured in nanoseconds, includes critical timings like CAS latency, which is the delay between a read command and data availability. Bandwidth, measured in gigabytes per second, has been dramatically increased through architectural advances like dual-channel architecture and higher-speed interfaces. Performance is also heavily influenced by the memory controller design within modern microprocessors like those from Intel and AMD. Engineers constantly balance these characteristics against constraints of power dissipation, signal integrity, and thermal management in system design.

Category:Computer memory Category:Semiconductor devices Category:American inventions