Dynamic RAM, DRAM Memory Technology Tutorial

- an overview or tutorial covering the basics of dynamic RAM - DRAM memory technology including notes about structure, operation and usage.

Dynamic RAM, or DRAM is a form of random access memory, RAM. It is widely used in PCs and other processor based systems as the basic form of working memory within the system.

Although a variety of other different types of semiconductor memory that are available, dynamic RAM - DRAM is widely used in all forms of microprocessor and computer equipment.

Over the years, DRAM has been one of the most widely used memory technologies and is also in widespread use today.

What is DRAM technology?

As the name DRAM, or dynamic random access memory, implies, this form of memory technology is a type of random access memory. It stores each bit of data on a small capacitor within the memory cell. The capacitor can be either charged or discharged and this provides the two states, "1" or "0" for the cell.

Since the charge within the capacitor leaks, it is necessary to refresh each memory cell periodically. This refresh requirement gives rise to the term dynamic - static memories do not have a need to be refreshed.

The advantage of a DRAM is the simplicity of the cell - it only requires a single transistor compared to around six in a typical static RAM, SRAM memory cell. N view of its simplicity, the costs of DRAM are much lower than those for SRAM, and they are able to provide much higher levels of memory density. However the DRAM has disadvantages as well, and as a result, most computers use both DRAM technology and SRAM, but in different areas.

In view of the fact that power is required for the DRAM to maintain its data, it is what is termed a volatile memory. Memory technologies such as Flash are non-volatile and retain data even when the power is removed.

DRAM technology history

Being a form of memory technology, the dynamic RAM arose out of the developments of the first microprocessors and the accompanying integrated circuit developments.

In the middle to late 1960s integrated circuits started to appear in some advanced electronics products - previously for computer memories, a form of magnetic memory was used. These memories used a single small ferrite toroid for each memory element. Naturally this "core" memory was very expensive, and integrated versions were for more attractive for the long term.

The idea for the DRAM technology appeared relatively early in the integrated circuit semiconductor timeline. An early form was found in a Toshiba calculator that was made in 1966 out of discrete component, and then two years later the idea of the DRAM was we know it today was patented.

The next stage of DRAM technology development came in 1969 when Honeywell who had entered the computer market in a large way asked Intel to fabricate a DRAM using a three transistor cell idea they had developed.

The resulting DRAM IC was designated the Intel 1102 and it appeared in early 1970. However the device had a number of problems and this set Intel to develop a new DRAM technology that operated more reliably. The resulting new device appeared in late 1970 and was called the Intel 1103.

DRAM technology moved a stage further ahead, when in 1973, MOSTEK produced their MK4096. As the part number indicates, this device had a 4 k capacity. However its main advantage was that it incorporated a multiplexed row and column address lines approach. This new approach enabled these memories to fit into packages with fewer pins. The resulting a cost advantage grew when compared to the previous approaches with each increase in memory size. This enabled the MOSTEK DRAM technology to gain more than 75% of the world market share.

Ultimately MOSTEK lost out to Japanese manufacturers of DRAM technology as they were able to manufacture higher quality devices at a lower cost.

DRAM advantages and disadvantages

As with any technology, there are various advantages and disadvantages to using it. Balancing the advantages and disadvantages of using DRAM against another form of technology ensures that the optimum format is chosen.

Advantages Disadvantages
  • Very dense
  • Low cost per bit
  • Simple memory cell structure
  • Complex manufacturing process
  • Data requires refreshing
  • More complex external circuitry required (read and refresh periodically)
  • Volatile memory
  • Relatively slow operational speed

Dynamic RAM, DRAM applications & usage

Dynamic RAM has been available for many years and was used with the earliest home and personal computers. Over a period of around 20 years, internal processor speeds have risen by a factor of around 40, yet in this time the speed of dynamic RAM technology has barely risen - they have seen a factor of two increase at the most.

The mismatch between processor and RAM is more pronounced for dynamic RAM, DRAM than for Static RAM, SRAM. This is because the dynamic RAM is optimised for low leakage and not for speed. However the cost of dynamic RAM is much lower and falling. The cost of a dynamic RAM is about an eighth that of static RAM per bit, although these comparisons vary with time and can only be taken as a rough guide.

As a result SRAMs are used where speed is required, while dynamic RAM, DRAM is used where cost efficient storage of large amounts of data is needed.

DRAM memory is a particularly important type of electronics component in today's electronics and computer technology scene. The DRAM memory allows fast and dense memories to be made that are suitable for use with many processor and computer applications. More recently other forms of DRAM, notably the SDRAM have improved the basic technology and enable greater levels of speed and density to be achieved in line with the requirements of modern computing technology. As a result these newer forms of semiconductor memory are now more widely used that the more traditional DRAM chips. Nevertheless DRAM memory chips are still be produced in very large quantities and used in many new applications.

By Ian Poole

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