MEMRISTORS- THE FOURTH PASSIVE ELEMENT

Resistors, inductors and capacitors are known as the three basic passive elements in electrical and electronic engineering all along. Resistors are a relation between current and voltage, and can be expressed as R = dv/di (dynamic resistance), linking current and voltage. Inductance is expressed as L = dφ/di, linking flux and current, while capacitance is given by C=dq/dv, linking charge and voltage. Some scientists caught attention to this fact, and fascinated that a relation must exist between flux and charge.

In the year 1971, concept of memristor was postulated in by Leon Ong Chua, professor in departments of computer sciences and electrical engineering at the University of California. He observed the models of capacitor, resistor & inductor, and noted a missing part which he named as a memristor or memory resistor.  

Leon Chua noted relations between Voltage, Current, Charge and Magnetic flux, and the elements R, L, C. in the diagram at left. Then postulated existence of Memristor to relate charge and magnetic flux. Leon Chua considered the plausibility based on symmetry and completeness.

The word memristor stands for memory + resistor, and is now recognized as the fourth basic element. Main feature of memristor is its capability for remembering its state history.  Unit of memristor is Ohms, and is called memristance, in line with resistance, inductance and capacitance. Memristance is charge-dependent resistance, with unit as Ohm. Symbol M has been introduced to represent memristance. Symbols of the four passive elements are now as follows.

It is two-terminal electrical component. It limits or controls the flow of electrical current in a circuit and also remembers or recollects the amount of charge that has previously flowed through it. Memristors thus retain memory without power, and are non-volatile.

Practical representation of this memory resistor was expanded in 2008 by the scientist Stanley Williams. This technology, discovered a few decades ago, got importance in recent times. Practical memristor performance was exploited by the scientists of HP lab while working on crossbar switches.

Memristor principle

This non-volatile electronic memory can be programmed and then stored. Thus, memristors can be useful for data storage. When current flows in one direction, resistance increases, and it decreases when current flows in the opposite direction. Resistance cannot go below zero.

The device would link charge and magnetic flux. Its resistance depends on the time integral of the voltage (i.e. the magnetic flux) and therefore changes its “Memristance” over time, even when the voltage stays fixed. This means memristor has some kind of “memory” of voltage or current levels it has seen in the past and shows this property as instantaneous resistance. One way to produce memristor is by doping part of a semiconductor, as shown in figure.

Over the past decade, researchers have made some progress in developing memristors, and the latest advance comes with an additional feature. A new type of memristor can dissolve in water. This could result in more environmentally friendly electronic devices. It is perhaps most useful where it could prevent valuable information from falling into unwanted hands.

Analogy between the water pipe and memristor

Memristor is sometimes compared to an imaginary water pipe. When water flows in one direction, pipe diameter expands and water flows faster. When water flows in opposite direction, diameter of pipe contracts and flow slows down. If water is closed or shut off, pipe retains its diameter until water is turned back on.

Few other analogies models may be made, depending on construction and working mechanism.

Memristors are today available in a variety of constructions and characteristics. Some memristors are designed to act as switches, while several of them may have binary or multiple identifiable levels,

Today memristors are available for long-term memory as also short-term memory, which can serve different functions in computer devices. This dual capability makes memristors highly versatile for applications in advanced computing and memory systems,

Most common memristors use Titanium dioxide substrate with platinum electrodes, and some other compositions are available. Behaviour of memristors depends on its structure. Typically, V-I behaviour shows hysteresis loop, whih pinches at zero, Pinched hysteresis loop is a defining feature of memristors, which reflects their memory-dependent resistance behaviour. The figure shows basic memristor construction, and possible V-I behaviour.

Current and voltage controlled memristors

Current-Controlled Memristors: Resistance state is governed by magnitude of current flowing through the device. These require a threshold current to trigger resistance switching, meaning no change occurs below this threshold.

Voltage-Controlled Memristors: Resistance state is controlled by applied voltage. A threshold voltage is necessary to initiate switching, and below this voltage, the resistance remains unchanged,

Memristors are also known as matrix switches because they are mainly used for connecting several inputs as well as outputs in the form of a matrix. Each memristor is able to do its own calculation, allowing thousands of operations within a core to be performed simultaneously.

Advantages & Limitations of memristors

Advantages

• Very useful with interfaces of CMOS.
• Do not use power when inactive.
• Consume less energy and generate less heat.
• Capability to memorize flow of charge in a set of time.
• When power is disrupted in data centres, it provides better resiliency and reliability.
• Faster boot-ups
• Capable of restoring both hard drives as well as DRAM

Limitations

• Not yet readily available commercially, only a few manufacturers exist today.
• Speeds of existing versions are at 1/10th of DRAM
• Their performance & speed today do not match transistors and DRAM.
• If all information on PC is non-volatile, rebooting will not resolve any problem.

Memristors Supplement Transistors & Are Not Their Alternative

Memristors are passive devices, while transistors are active devices. Memristors consume energy and drop signal. They cannot amplify. An electronic circuit is always a combination of active and passive devices. Active devices cannot be dispensed.

Active devices like transistors are essential for any computer / data processing. Memristors can take over functions of nonvolatile VLSI and replace resistive random-access memories (RRAM). Memristor + Transistor combinations can make computers smaller and fast, by enabling their capabilities.

Memristor Applications              

• Digital memory, logic circuits, biological and neuromorphic systems.
• Computer technology as digital memory
• Neural networks as well as analog electronics.
• These are applicable for analogic filter applications
• Remote sensing & Low-power applications.
• Programmable Logic & Signal Processing

Memristors have the potential to greatly reduce the energy used in computing. This is very important as the energy demands of computing are increasing rapidly. In essence, memristors hold the potential to

• Improve the speed and efficiency of computing. 
• Enable more advanced AI and machine learning.
• Revolutionize data storage.
• Power the next generation of IoT devices.

Today a lot of research and development going into memristor technology, and it is expected to play a significant role in the future of electronics.