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PCRAM

PCRAM (phase change RAM, PRAM, PCM)

PCRAM (Phase Change Random Access Memory), also known as PCM (Phase Change Memory) and OUM (Ovonic Unified Memory), is a type of non-volatile RAM. The data is therefore retained even after the power supply is interrupted.

The PCRAM stores data by changing the state of a part of the phase change material on a microscopic level: The information is represented by the amorphous or crystallized state.

Let's start by looking at the individual components of the term, as they already give us valuable insights into the characteristics of this storage technology.

Phase change
Basically, a "phase change" is a change in the physical state of a substance between solid, liquid and gas. These changes are normally caused by changes in temperature or pressure. In PCRAM, the "phase change" refers to the ability of the material to change between an amorphous (disordered) and crystalline (ordered) state. The phase change is caused by electrical impulses. The physical states achieved in this way then represent different memory states (0 and 1).

Random Access
Like any other RAM, PCRAM is a type of computer memory that allows random data access. In contrast to sequential storage systems such as tape drives or optical media, where data must be accessed in a specific order, RAM allows each memory cell to be accessed directly and in any order, which speeds up data access. RAM is used in computers as temporary memory to store the system's current working data. Data in RAM is volatile, i.e. it is lost if the power supply is interrupted.

Table of contents

Properties of the PCRAM

Non-volatility
As already mentioned in the introduction, PCRAM is a non-volatile memory technology. This means that the data is retained even after the power supply is switched off.

Fast write access
PCRAM offers fast write access, similar to that of DRAM and SRAM. As briefly mentioned in the introduction, the memory state is achieved by changing the phase state of a special material between disordered (amorphous) and ordered (crystalline) form. The speed of the writing process in PCRAM is ultimately determined by the speed at which the material can switch between these phases. As research in this field continues to be very active, it is quite possible that other materials will be discovered or developed in the future that have even better properties for PCRAM technology.

High memory density
PCRAM can achieve a higher memory density than DRAM, SRAM and in many cases also FRAM, which gives it a decisive advantage over these memory technologies in some applications.

Low power consumption
Compared to DRAM and SRAM, PCRAM requires less power, which makes it more energy efficient.

Interesting additional information
PCRAM has the potential to gain a large market share among non-volatile memory technologies. Externally, PCRAM works in a similar way to the widespread NOR flash memory that is often found in smartphones. However, although PCRAM has been available as a series product for several years, only models with a relatively low storage capacity have been available to date.

However, intensive research is still being carried out in this area. The researchers include well-known companies such as Samsung, the market leader in the field of NAND flash.

Areas of application for PCRAM

A brief note in advance: Whether the use of PCRAM is recommended for certain applications cannot be said in general terms and always depends on a variety of application-specific factors. With the following information on the suitability of PCRAM for various application areas, we therefore do not wish to make any purchase recommendations, but rather to put the properties of PCRAM memory technology into context.

Memory in mobile devices
PCRAM is fundamentally ideal for use in mobile devices such as smartphones and tablets. Due to its non-volatility, high storage density and fast access times, PCRAM can improve the performance of these devices while extending battery life.

Memory for artificial intelligence (AI) and machine learning (ML)
AI and ML systems often require a large amount of fast, non-volatile memory. PCRAM is a good choice here as it offers high storage density and fast read and write times. In particular, PCRAMs could be used as part of a hierarchical memory system, in which they provide an intermediate layer between the fast but small and expensive SRAM and the large but slower NAND flash.

Hierarchical storage systems are systems that consist of several levels or layers of storage. Each with its own specific characteristics and costs. They are designed to offer a compromise between speed, size and cost.
In such a system, SRAM would be the top layer. SRAM is extremely fast, but also very expensive and has a low storage density, so it is only used for very small amounts of data that need to be available extremely quickly, such as the cache of a processor. At the other end of the spectrum is NAND flash. NAND flash has a high storage density and is much cheaper than SRAM, but also much slower. NAND flash is typically used for mass storage where large amounts of data need to be stored and speed is less important. In this hierarchical memory model, PCRAM could represent a middle layer. PCRAM is faster than NAND flash and has a higher storage density than SRAM, so it could serve as a kind of "intermediate storage". It could store data that is accessed more frequently than that in NAND flash, but not so frequently that it needs to be held in SRAM.

By using PCRAM in this way, systems in the future could take advantage of SRAM's speed, NAND flash's storage density and PCRAM's balance between the two to create an efficient and cost-effective storage solution.

Automotive and industrial applications
PCRAM could also be useful in vehicles and industrial systems, especially in applications that require a robust, reliable and energy-efficient storage solution. For example, advanced driver assistance systems (ADAS) or autonomous vehicles could benefit from the use of PCRAM by storing and retrieving data quickly and reliably, even in extreme conditions such as high or low temperatures.

How the PCRAM works

The structure of the PCRAM is similar to that of the DRAM. Essentially, the PCRAM consists of many memory cells, which are best imagined as being arranged in rows and columns. However, the memory element in the PCRAM is not a capacitor (as in the DRAM), but a resistive phase change element.

Let's take a closer look at the phase change element:

The phase change material is located between two metallic electrodes. At room temperature, the material is stable in both the amorphous and crystalline phases. A small part of the phase change material (a chalcogenide alloy) is brought into one of the two states by the heating electrode. As with MRAM, the information is then read out on the basis of the distinguishable resistances in the amorphous and crystalline states by applying a voltage to the resistive electrode.

Think of it a bit like connecting a wire to a battery: The wire gets hot. In the PCRAM, the heat is used to heat the material to a certain temperature.

If the material is heated to a high temperature, it melts and becomes amorphous, i.e. disordered. As soon as it cools and solidifies, it retains this amorphous state. In this way, we represent the state that corresponds to the number "0".

If we heat the material only slightly, it does not become completely amorphous, but remains partially ordered, i.e. crystalline. As soon as it cools and solidifies, it retains this crystalline state. This represents the state that corresponds to the number "1".

In this way, we can use different amounts of heat generated by different strengths of electrical impulses to create the different states of the material and thus store data. And because we can change the state of each memory cell independently of the others, we can write and read data very quickly and accurately.

Interesting additional information
PCRAMs are faster and have a longer service life than flash memory. In addition, the phase change material has the property of being able to be in more than just two distinguishable intermediate states, so that it has the potential to store several bits in a single cell. However, this potential is not yet being exploited.

For these reasons too, PCRAM technology could gain in importance in the future.

Sources

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