*7.2.4 ASN (atomic switch network): network-based synaptic device*

Inorganic "atomic-switch" synapses embedded in a sophisticated network of silver nanowires make up the more than a billion interconnected synapses used in the self-assembling neuromorphic (brain-like) technology created by researchers in the U.S. and Japan. It has been demonstrated that the atomic switch, a freshly created nanoscale circuit component, displays synapse-like characteristics in an entirely inorganic device. Each square centimetre of the device's silver nanowire network contains a billion junctions that are intricately coupled to one another.

These atomic switch networks (ASN) produce emergent behaviours resembling memristors through distributed, collective interactions, just like biological neural networks. A key feature of biological brain networks and many other complex systems is these emergent behaviours. According to the researchers, experiments are currently being conducted that exploit these emergent behaviours for information processing with the goal of creating a new category of cognitive technology. The architecture of the ASN device is extremely linked and includes synaptic circuit components at each point of nanowire contact. According to the researchers, the collective interactions between these atomic switches provide distinctive emergent features that offer substantial potential for neuromorphic computing.

Comparing these emerging devices, MRAM features non-volatility, as the magnetic properties of ferromagnets do not disappear due to power failure. The magnetism of ferromagnets not only does not disappear due to power failure, but can be considered almost never to disappear, so MRAM can be rewritten infinitely as well as DRAM. The write time of MRAM is as low as 2.3 ns and the power consumption is extremely low, enabling instant on/off and extending the battery life of portable devices. MRAM cells can be easily embedded into logic circuit chips, requiring only one or two additional steps in the back-end metallization process, which requires a photolithographic mask version. In addition, MRAM cells can be completely fabricated in the metal layer of the chip, and even 2–3 layers of cells can be stacked, so it has the potential to construct large-scale memory arrays on logic circuits. However, the biggest drawback of MRAM is the interference between memory cells. When programming the target bit, the free layer in the non-target bit can be easily misprogrammed, especially in the case of high density, the overlap of the magnetic field between adjacent cells will be more serious. For PCM, it does not need to erase previous code or data before writing updated code, so its speed has an advantage over NAND, and the read and write time is more balanced. PCM read and write is non-destructive, so its write resistance far exceeds that of flash memory, and PCM is used to replace traditional mechanical hard drives with higher reliability. PCM has no mechanical rotation device, and no refresh current is required to save code or data, so PCM's power consumption is lower than HDD, NAND, and DRAM. Some PCMs use a non-transistor design, which can realise high-density storage. PCM storage technology is independent of the charged particle state of the material, so it has a strong resistance to space radiation and can meet the needs of national defence and space. While RRAM erase speed is determined by the pulse width of the trigger resistor transition, generally less than 100 ns. Some RRAM materials also have a variety of resistance states, making it possible to store multi-layer data when a memory cell, thus increasing the storage density. The memory matrix of RRAM can be divided into two types: passive matrix and active matrix. The memory cell of the passive matrix is connected by a resistive element and a nonlinear element (usually using a diode),

### *Neuromorphic Computing between Reality and Future Needs DOI: http://dx.doi.org/10.5772/intechopen.110097*

the role of the latter is to make the resistive element get a suitable voltage division, so as to avoid the loss of the memory cell reading and writing information when the resistive element is in a low resistance state. The advantage of this method is that the design is relatively simple and the process is well miniaturised, but the use of a passive matrix will make the adjacent cells inevitably interfere with each other. Active cells are controlled by transistors to read and write and erase resistive components, although good isolation of adjacent cells interference, its design is more complex, and the device can be poorly miniaturised. In terms of capacity, these three new types of memory, MRAM up to 4 Gb, PRAM up to 8 Gb, and RRAM up to 32 Gb, are still very different from flash memory but all three are more than 1000 times faster than flash memory in terms of read and write speed.
