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Electrochemical Random Access Memory (ECRAM): State of the Art

Introduction

Electrochemical Random-Access Memory (ECRAM) is a type of non-volatile memory technology that uses an electrochemical cell to store and retrieve data. ECRAM implements multiple levels per cell for storing more than a single bit of information per cell. ECRAM is a three-terminal device namely gate, drain, and source. It comprises a conductive channel made of tungsten trioxide, an insulating electrolyte made of lithium phosphorous oxynitride (LiPON), and protons as mobile ions. The resistance of the conductive channel is modulated by the exchange of ions at the interface of the channel and dielectric layer on the application of an electric field. The change in the electrical conductivity of ECRAM on the application of electrical pulses stores information. ECRAM is designed in such a manner to mimic human memory synapses with low power consumption. ECRAM is designed to be used as synaptic memory for artificial intelligence and deep neural networks. Various nonvolatile memories such as resistive random-access memory and phase-change memory can be used for prototype building in neural networks, but due to their non-ideal switching characteristics, such as asymmetric weight update, stochasticity, and limited endurance, ECRAM is considered an attractive alternative for neural networks.


How does ECRAM work?

The principle of operation of ECRAM is based on the resistive switching, where the resistance of material changes in response to the voltage applied across it. ECRAM is composed of two electrodes, an anode, and a cathode, separated by an electrolyte. The electrolyte is a material that conducts ions, which are atoms or molecules that have an electric charge due to the gain or loss of one or more electrons. The anode and cathode are made of conductive materials that are typically coated with a thin layer of active material, such as tungsten oxide, titanium oxide, or nickel oxide. When a voltage is applied to the electrodes, a chemical reaction occurs in the electrochemical layer, causing it to change from a high-resistance state to a low-resistance state. This change in resistance can be detected and used to represent a binary state, with the high-resistance state representing a 0 and the low-resistance state representing a 1.


In an ECRAM, read and write operations are decoupled, hence, allowing for better endurance and low energy switching while maintaining the non-volatility. The electrochemical intercalation in an ECRAM can be precisely and reversibly controlled by controlling the amount of charge through the gate which provides symmetric switching characteristics with plentiful discrete states and reduced stochasticity. The researchers at IBM had fabricated an ECRAM with up to 1000 discrete conductance levels and a large dynamic conductance range of up to 40. Hence, ECRAM emerges as a potential device for high-speed, low-power neuromorphic computing. The write and read operations in ECRAM are performed by applying a voltage across the electrodes.


A.    Write Operation: During the write operation, a negative voltage is applied between the gate and the source. With negative voltage pulses, the intercalated Li ions are released from the channel made up of LiCoO2 or LiPON to the gate that changes the resistance and results in a write operation. The voltage pulse is typically applied for a very short duration and with a very small amplitude. 


B.     Read Operation: The read operation is decoupled from the write operation by applying a voltage between the drain and the source. After applying the voltage between the drain and the source, the resulting current is measured. The magnitude of the current is proportional to the resistance of the cell, which in turn corresponds to the stored data. The read operation is non-destructive, meaning that the data is not lost during the read operation.



The above figure shows the structure of ECRAM, where Li ions are injected or removed from WO3 to change the conductance of ECRAM. The amount of Li ions inserted into WO3 is accurately controlled by the gate current and this process is reversible. During the operation of ECRAM, a series of positive current pulses are fed into the gate for potentiation, and negative gate current pulses are fed into the gate for depression. A typical ECRAM is programmed with 50 up and then 50 down pulses, resulting in good symmetry and a large conductance dynamic range.


Various research institutions have implemented ECRAM cells with a variety of materials, layouts, and performances. The materials for the manufacture of ECRAM include channels of tungsten trioxide, lithium carbonate, and graphene. Based on the type of ions, various ECRAMs are fabricated such as Li-ECRAM having lithium ions, H-ECRAM having hydrogen ions, and MO-ECRAM which is a metal-oxide-based ECRAM. Each of these types of ECRAM has different properties such as different operation speeds, retention capacity, and open circuit potential.  


Advantages of ECRAM

ECRAM has several advantages over other non-volatile memory technologies, such as flash memory and phase change memory. It has a faster write speed and lower power consumption than flash memory and does not suffer from the endurance issues of phase change memory. ECRAM also has the potential to store a large number of bits per cell, which can increase memory density and reduce the cost of storage. The advantages of ECRAM can be described as follows:


1.     High speed of operation: ECRAM can achieve high read and write speeds, making it suitable for use in high-performance computing devices. According to the researchers at MIT, the ions in an ECRAM move around in nanoseconds, about 10,000 times as fast as synapses in the brain.


2.     Lower power consumption: ECRAM consumes less power than traditional memory technologies, which makes it more energy-efficient and helps to extend the battery life of portable devices.


3.     High endurance: ECRAM has a high endurance, which means that it can withstand a large number of read and write cycles without degradation in performance. This makes it suitable for use in applications that require frequent memory access. ECRAM is capable of more than 100 million read-write cycles.


4.     Non-volatility: ECRAM is a non-volatile memory technology that retains its data even when the power is turned off. This makes it suitable for use in applications that require persistent storage.


5.     Longer memory: ECRAM is capable of retaining data for long periods. The researchers of the Sandia National Laboratories and the University of Michigan were able to achieve a retention time of 10 years using ECRAM.


6.     Compatibility: ECRAM is designed to be compatible with standard CMOS technology, making it easier to integrate into the existing systems and reducing production costs. 

Feature

ECRAM

RAM

Speed

Nanoseconds

Microseconds

Energy efficiency

Very low

High

Non-volatility

Yes

No

Maturity level

Still under development

Mature technology

Cost

Expected to be more expensive than RAM

Relatively inexpensive

Potential applications

AI accelerators, edge computing devices, IoT devices

General-purpose computing


ECRAM has a wide range of potential applications, particularly in areas where high-speed, low-power memory is required. Some of the potential applications of ECRAM include:


1.     Artificial Intelligence (AI): To improve the performance of AI, the hardware is required to reach a level similar to the human brain. ECRAM is a promising technology for use in AI applications. With the ability of ECRAM to store multiple states within a single cell, it is useful in neural networks, where data storage and processing requirements are intensive.


2.     Internet of Things (IoT) devices: IoT devices often run on battery power and need to consume lower energy to extend their battery life. ECRAM is useful in IoT applications due to its low power consumption and non-volatile memory. ECRAM can offer fast access times, which is essential in IoT devices that often need to process data in real time. The ability of ECRAM to store multiple levels of resistance can also make it useful for edge computing in IoT devices.


3.     Nanotechnology: ECRAM has potential applications in the field of nanotechnology due to its ability to store and process large amounts of data in small spaces with low power consumption. This makes ECRAM an attractive option for use in small devices such as sensors which require high-density memory and low power consumption. ECRAM’s ability to achieve multiple conductance states could be useful in the development of new types of nano-electronics and nano-devices. Researchers could use ECRAM to build artificial synapses and neural networks on a nanoscale, which has a wide range of applications in various fields such as robotics, prosthetics, and brain-computer interfaces.


4.     Medical devices: ECRAM technology has potential applications in medical devices requiring long-term storage and low power consumption. ECRAM could be used in implantable medical devices such as pacemakers, where reliable, non-volatile data storage is essential. ECRAM could help reduce the frequency of device replacements and associated storage with its long-term data retention capability. ECRAM could also be implemented in portable devices, such as glucose meters or blood pressure monitors. The ability of ECRAM to achieve high levels of conductance states makes it useful in pattern recognition applications, such as identifying biomarkers or pathogens.


5.     Autonomous vehicles: ECRAM could be used in autonomous vehicles to store vast amounts of data generated by sensors and cameras. ECRAM could also be used in the processing of data within the autonomous vehicle's control system. Neural networks are often used in autonomous vehicle control systems to analyze sensor data and make decisions about how to maneuver the vehicle. ECRAM's ability to store synaptic weights and conductance states could be useful in implementing these neural networks in a low-power and high-density way.


Conclusion

ECRAM is a promising new memory technology that has the potential to revolutionize the way we think about memory. Compared to conventional non-volatile memories, ECRAM shows many unique characteristics in switching, including linearity and superior symmetry, discrete conductance states with reduced stochasticity, a large dynamic range of conductance, and excellent endurance. By providing a high-speed, low-power alternative to traditional non-volatile RAM, ECRAM could enable new applications and devices that were previously not possible. While ECRAM is still in the early stages of development, it is clear that this technology has the potential to play an important role in the future of computing.

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