The current memory device

3.4.1 Dynamic random-access memory (DRAM)

Dynamic random-access memory (DRAM) is the memory type most commonly used in desktop and larger computers. Each elementary DRAM cell comprises a single MOS transistor and a storage capacitor (see Fig. 13). Each storage cell contains one bit of information. However, this charge leaks off the capacitor because of the sub-threshold current of the cell transistor and must be refreshed several times each second.

Figure 13. Dynamics random access memory (DRAM) cell.

DRAM works by sending a charge through the appropriate column (CAS) to activate the transistor at each bit in the column. When writing, the row lines specify the state the capacitor should take on. When reading, the sense amplifier determines the level of charge in the capacitor; if this is more than 50%, it is read as a 1; otherwise it is read as a 0. The counter tracks the refresh sequence based on which rows have been accessed in what order. The total length of time this

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takes is so short that it is expressed in nanoseconds. Fig.14 shows a simplified DRAM diagram.

Figure 14. Simplified DRAM diagram (from [59]).

3.4.2 Ferroelectric random-access memory (FRAM)

Ferroelectric random-access memory (FRAM) is a high-performance, low-power, non-volatile memory type that combines the benefits of conventional non-volatile memory types (Flash, EEPROM) and high-speed RAM (SRAM, DRAM). This universal memory outperforms existing types like EEPROM and Flash, consumes less power, is many times faster and exhibits greater endurance in multiple read-and-write operations. While the maximum number of read/write cycles for Flash and EEPROM is about 100,000, the lifetime of FRAM memory is essentially unlimited, with more than 1 trillion (10¹³) read/write cycles.

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FRAM stores information using polarized ferroelectric film material placed between two electrodes. The FRAM cell structure is like the transistor and capacitor structure of a DRAM cell but does not require the same high programming voltages as Flash or EEPROM. This means that FRAM offers non-volatile data storage but is significantly more energy-efficient than other conventional non-volatile memories. Specifically, FRAM uses ferroelectric film as a capacitor for storing data and commonly uses PZT (Pb[ZrTi]O3), which has a perovskite-type structure (ABO3) (Fig. 15. When an electric field is applied, the Zr/Ti atom shifts up or down, and this polarization remains when the electric field is removed. This is the property that provides non-volatility and reduces the power requirement for data storage.

Figure 15. PZT cell structure.

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3.4.3 Static random-access memory (SRAM)

Static random-access memory has stationary and access functions that can save data without refresh. Although SRAM exhibits high performance, it also has some disadvantages, including low integration level, large volume and large power consumption. Because SRAM is a static memory cell, it has more parts and takes up more space than a dynamic memory cell. That means less memory per chip, making SRAM more expensive.

The SRAM cell consists of a bi-stable flip-flop connected to the internal circuitry by two access transistors (Fig. 16. When the cell is not addressed, the two access transistors are closed, and the data is kept in a stable state, latched within the flip-flop. The flip-flop needs the power supply to retain the information. While the data in an SRAM cell are volatile (i.e. are lost when power is removed), the data do not “leak away” as in DRAM, and no refresh cycle is needed.

Figure 16. SRAM Cell (from [60]).

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3.4.4 Flash memory

Flash memory technology is a mix of EPROM and EEPROM technologies. The term “flash” refers to how a large chunk of memory can be erased at one time.

This distinguishes flash devices from EEPROMs, where each byte is erased individually. Flash is now a mature memory technology and competes strongly against other non-volatile memory types such as EPROMs and EEPROMs, as well as some DRAM applications. There are two forms of flash memory: NOR (not or) and NAND (not and). These names refer to the type of logic gate used in each storage cell. Flash memory wears out after multiple erases, with oxidation due to wear on the insulating oxide layer around the charge storage mechanism used to store data.

3.4.5 Comparison of MRAM and Redox RAM

Redox RAM combines good potential for scaling below 10 nm generation with fast read and write times (<10 ns) and relatively low write current (in the microampere range). It is stackable in three-dimensional cross-bar architectures and offers multilevel capabilities by controlling the growth or dissolution of conducting filaments, which determines cell resistance. However, the underlying physical mechanisms are based on statistical phenomena that are difficult to control at dot scale, leading to relatively large dot-to-dot variability. Predictive models of reliability are more difficult to establish for this technology, and their endurance (∼108 cycles) is sufficient for flash-type applications but not for working memory in microprocessors, which require > 1015 cycles. Because it allows continuous variation of resistance between a minimum and maximum value in hysteretic fashion, redox RAM seems best suited to intermediate storage class memory applications that fall between hard disk drives, solid-state drives (SSD) and DRAM and memristor applications.

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Today, STT-MRAM seems the most credible candidate for embedded flash and SRAM replacement, combining CMOS compatibility, high retention time (10 years), endurance (> 1015 cycles), and fast write/read time (1–30 ns, depending on embodiments). A further goal is DRAM replacement, although the technology is not yet mature enough for this high-density application. The main difficulty relates to the etching of MTJ stacks at small pitch (sub-20 nm) and the associated cell-to-cell variability. This variability is caused mainly by edge defects generated during patterning of the cells. Whenever the MgO barrier is damaged by patterning, this yields local changes in barrier resistance, tunnel magnetoresistance, magnetic anisotropy and, correlatively, cell retention.

Redeposition on the MTJ sidewalls during etching also contributes to variability.

As the number of laboratories (including major equipment suppliers) working on this technology has increased substantially, technological progress is much faster.

Table 2 compares various non-volatile memory types.

Table 2 Performance comparison of various non-volatile memories [61]

SRAM eDRAM DRAM eFlash

(NOR)

Flash (NAND)

FRAM PCM

STT-MRAM

RRAM

Endurance (cycles)

Unlimited Unlimited Unlimited 10⁵ 10⁵ 10¹⁴ 10⁹ Unlimited 10⁹

Read/

write access time(ns)

< 1 1-2 30 10/10³ 100/10⁶ 30 10/100 2-30 1-100

Density 18.4 20.3 0.8 0.0021

Write power

0.6 1.8 0.025 0.08

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