The Missing Memristor Found!

By Samuel Chege.

The long-sought after memristor, the "missing link" in electronic circuit theory, has been invented by Hewlett Packard Senior Fellow R. Stanley Williams at HP Labs. Memristors: the fourth passive component type after resistors, capacitors and inductors, were postulated in a seminal 1971 paper "Memristor - The missing circuit element" in the IEEE Transactions on Circuit Theory by professor Leon Chua at the University of California (Berkeley) reasoning from symmetry, but their first realization was announced on April 30, 2008 by HP, almost four decades later.

Since the birth of electronics, engineers have made circuits using a combination of 3 basic elements resistor, capacitor and inductor. The discovery of the memristor has sparked a new wave of enthusiasm and optimism in revolutionizing circuit design, marking a new era for the advancement Memristor The Missing Memristor Found! of both digital and analogue applications.

Memristor - The fourth basic element Pinched hysteresis curve for memristor. Memristor is a passive two-terminal electronic device that is built to express the property of memristance (just as a resistor expresses resistance). Memristance is a property of an electronic component such that if charge flows in one direction through a circuit, the resistance of that component will increase, and if charge flows in the opposite direction, the resistance will decrease. If the flow of charge is stopped by turning off the applied voltage, the component will 'remember' the last resistance that it had, and when the flow of charge starts again the resistance of the circuit will be what it was when it was last active. Thus its name memristor (short for memory resistor).

You can think of it as a kind of pipe that when water passes through in one direction, its diameter increases and when water flow in the opposite way, its diameter decreases. After water stops flowing the most recent diameter is retained. Thus having memory characteristics.

The most recognizable signature of a memristor is that when an AC voltage is applied to the device, the current - voltage characteristic is a Lissajous curve. This is commonly referred to as pinched hysteresis. The effect had been observed by electrical engineers for a long time but they could not explain it. They observed that titanium oxide changed resistance in presence of oxygen. With this limited know-how, they could only build the simplest of a device: oxygen sensor.

The memristor invented by HP Labs consists of an active film of Titanium dioxide (TiO2) sandwiched between two platinum electrodes. The film is made of two layers, one of which has a slight depletion of oxygen.

"This new circuit element solves many problems with circuitry today-since it improves in performance as you scale it down to smaller and smaller sizes"
Prof Leon Chua University of California (Berkeley).

The oxygen vacancies acts as charge carriers such that the depleted layer has a much lower resistance than the non-depleted layer. When an electric field is applied, the oxygen vacancies drift. This changes the boundaries between the high and low resistance regions and thus the resistance of the whole layer. The resistance of the layer is dependent on how much charge has passed through it in a particular direction which can be reversed by changing direction of current. At the moment several patents have already been applied for various applications of memristors technology. HP has already tested the material in its ultra-high- density crossbar switches, which use nanowires to pack a record 100 G-bits onto a single die--compared with 16 G-bits for the highest density flash memory chips extant. They even proposed a 3D design with 1000 layers or 1 Peta-bits of memory in a 1cm cube. Moreover, access time almost 100 times faster and energy usage is only one percent compared to the flash memories in use today.

"This new circuit element solves many problems with circuitry today--since it improves in performance as you scale it down to smaller and smaller sizes," said Chua. "Memristors will enable very small nanoscale devices to be made without generating all the excess heat that scaling down transistors is causing today." Beyond the obvious application in memory, memristor patents include applications in programmable logic, signal processing, neural networks, control systems, reconfigurable computing, RFID and brain computer interfaces.

An interesting application of memristor is in building machine modeled on the nervous system. In biological neural networks, each nerve cell communicates with other cells through synapses; adjustments to the strength of the synaptic connections is thought to be one mechanism of learning. In an artificial neural network, synapses must be small, simple structures if they are to be provided in realistic numbers. Memristor meets those requirements. Moreover, its native mode of operation changing its resistance in response to the currents suggests a direct way of modeling the adjustment of synaptic strength. This will lead to development of brain-like computers or what is called neuromorphic computing with application in pattern recognition and machine learning.

With only a few universities teaching memristor at the moment, every electronic book will have to be revised to appreciate the paradigm shift in electronics. Soon the more efficient memristors will replace transistors in various applications such as logic and memories. As engineers understands this technology, you should just expect a new spectrum of devices that you have never seen before!!