Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106.
Electronics industry driven by new discoveries in electronics including electronic circuits, polymer-based electronics, nanotubes, etc. are making tremendous useful advances in electronic devices related speed, capability and cost and indeed, every year tech companies come up with new, faster, smarter and better gadgets. The transistor was invented by three scientists at the Bell Laboratories (USA) in 1947, and it rapidly replaced the vacuum tube as an electronic signal regulator. Transistors, tiny electrical switches, are the fundamental unit that drives all the electronic gadgets. A transistor is a device that regulates current or voltage flow and acts as a switch or gate for electronic signals. Transistors consist of three layers of a
semiconductor material, each capable of carrying a current. A semiconductor is a material somewhere between a real conductor and an insulator that conducts electricity in a “semi-enthusiastic” way. The semiconductor material acquires special properties by doping process. The doping results in a material that either adds extra electrons to the material (called N-type for the extra negative charge carriers) or creates “holes” in the material’s crystal structure (called P-type for more positive charge carriers). The transistor’s three-layer structure contains an N-type semiconductor layer sandwiched between P-type layers (a PNP configuration) or a P-type layer between N-type layers (an NPN configuration).
A small change in the current or voltage at the inner semiconductor layer (which acts as the control electrode) produces a large, rapid change in the current passing through the entire component. The component can thus act as a switch, opening and closing an electronic gate many times per second. Today’s computers use circuitry made with complementary metal oxide semiconductor (CMOS) technology. CMOS uses two complementary transistors per gate (one with N-type material; the other with P-type material). When one transistor is maintaining a logic state, it requires almost no power. Transistors are the basic elements in integrated circuits (IC), which consist of very large numbers of transistors interconnected with circuitry and baked into a single silicon microchip. Today’s transistors are about 70 silicon atoms wide, so the possibility of making them even smaller is itself shrinking. We’re getting very close to the limit of how small we can make a transistor. In the technology world, one of the biggest questions of the 21st century is: how small can we make transistors? If there is a limit to how tiny they can get, we might reach a point at which we can no longer continue to make smaller, more powerful, more efficient devices.
Transistor to memristor
The invention of the transistor represents a major milestone in electronics and science in general. Today transistors power everything from cell phones to supercomputers, all of which take advantage of their numerous advantages. Traditionally, the processing of data in electronics has relied on integrated circuits (chips) featuring vast numbers of transistors and while the size of transistors has reduced to meet the increasing demands of technology, they are now reaching their physical limit. The transistor functions using a flow of electrons, whereas the memristor couples the electrons with ions, or electrically charged atoms. In a transistor, once the flow of electrons is interrupted by, say, cutting the power, all information is lost. Scientists have devised a way to use memristors, described devices with the properties of a memory element and a resistor, to make ICs which can scale better than ICs made with transistors. Memristors could hold the key to a new era in electronics, being both smaller and simpler in form than transistors, low-energy, and with the ability to retain data by ‘remembering’ the amount of charge that has passed through them – potentially resulting in computers that switch on and off instantly and never forget.
A memristor is an electrical component that limits or regulates the flow of electrical current in a circuit and remembers the amount of charge that has previously flowed through it. Memristors are important because they are non-volatile, meaning that they retain memory without power. The original concept for memristors, as conceived in 1971 by Professor Leon Chua at the University of California, Berkeley, was a nonlinear, passive two-terminal electrical component that linked electric charge and magnetic flux. Prof. Chuha conceptualized the existence of a fourth fundamental element in the electronic circuit, besides the three (Resistance, capacitor and inductor) that were already in use at the time for reasons of symmetry
that an extra component could one day be constructed to join these and was called memristor, a portmanteau of the words memory and resistor. Simply putting, the memristor could mean the end of electronics as we know it and the beginning of a new era called “ionics”. Since then, the definition of memristor has been broadened to include any form of non-volatile memory that is based on resistance switching, which increases the flow of current in one direction and decreases the flow of current in the opposite direction. Memristors, which are considered to be a sub-category of resistive RAM, are one of several storage technologies that have been predicted to replace flash memory. Scientists built the first working memristor in 2008 and since that time, researchers in many large IT companies have explored how memristors can be used to create smaller, faster, low-power computers that do not require data to be transferred between volatile and non-volatile memory. If the storage heirarchy could be flattened by replacing DRAM and hard drives with memristors, it would theoretically be possible to create analog computers capable of carrying out calculations on the same chips that store data.
Types of memristors
Memristors can be classified into different types, depending on how they are built. Memristors are classified into two types, and a brief overview of different memristors is explained below:
- Ionic thin film and molecular memristors
- Magnetic and spin based memristorsIonic thin film and molecular memristors: Molecule and Ionic thin-film memristors mostly rely on different material properties of the thin film atomic lattices that display hysteresis below the application of charge. These memristors are classified into different types:
- Titanium Dioxide Memristors: These types of memristors are broadly explored for designing and modeling.
- Ionic or Polymeric Memristors: Ionic and Polymeric memristors utilize dynamic doping of inorganic die-electric type or polymer materials. In this type of memristors, the charge carriers’ solid state ionic’s move all over the structure.
- Resonant Tunneling Diode Memristors: These types of memristors use specially doped quantum well diodes of the space layers between the sources and drain regions.
- Manganite Memristors: These types of memristors use a substrate of bilayer oxide films based on manganite as opposite to titanium dioxide memristors.Magnetic and Spin Based Memristors: Spin based memristors are opposite to ionic nanostructure and molecule based systems, and rely on the property of degree in electronic spin. In this type of system, the polarization of electronic spin is aware. These types of memristors are classified into two types:
- Spintronic Memristors: In these types of memristors, the route of spin of electrons changes the magnetization state of the device which consequently changes its resistance.
- Spin Torque Transfer Memristors: In these types of memristors, the comparative magnetization position of the two electrodes affect the magnetic state of a tunnel junction which in turn changes its resistance.
Recently, ultra-dense resistive memory arrays built from various two-terminal semiconductor or insulator thin film devices have been demonstrated among these, bipolar voltage-actuated switches have been identified as physical realizations of ‘memristors’ or memristive devices, combining the electrical properties of a memory element and a resistor. Accordingly, if incorporated within an appropriate circuit, memristive switches can thus perform ‘stateful’ logic operations for which the same devices serve simultaneously as gates (logic) and latches (memory) that use resistance instead of voltage or charge as the physical state variable. The memristor works by using electrical current to change the location of atoms in a thin film of titanium dioxide. Moving the atoms causes a change in resistance. That changed state remains after the current is turned off. The scientists say that memristors could be used for both non-volatile memory and for logic. Memristors are said to be stackable allowing 3D circuits. It is said that memristors’ switching speed matched that of today’s transistors and had read/write endurance measured in the ‘hundreds of thousands’ of cycles. However, the leakiness and expense of transistor scaling are suggesting fundamental limits are three or four generations away. Therefore, memristors could be competitive with transistors within three years, and could scale for ‘a very long time’. Then, memristors could deliver non-volatile storage as dense as 20GByte per sq cm within three years which he reckons would be a factor of two better than what conventional flash memory can achieve in that timeframe. The University of Southampton has demonstrated a new memristor technology that can store up to 128 discernible memory states per switch, almost four times more than previously reported. Memristor market key players are: Toshiba Corporation, SanDisk Corporation, Intel Corporation, Fujitsu Ltd., Samsung Electronics Co., Ltd., Cypress Semiconductor Corporation, IBM, Hewlett Packard, Seagate Technology LLC., SK Hynix and Sony Corp.
A comparison: Transistor & Memristor
|· Higher reliability and greater physical ruggedness· Extremely long life· Smaller mechanical sensitivity· Lower cost· Smaller in size especially in small-signal circuit
· Low operating voltages for greater safety
· No warm-up period for cathode heaters required after power application
· Lower power dissipation and generally greater energy efficiency
|· Has properties which cannot be duplicated by the other circuit elements (resistors, capacitors, and inductors· Capable of replacing both DRAM and hard drives· Smaller than transistors while generating less heat· Works better as it gets smaller which is the opposite of transistors· Devices storing 100 gigabytes in a square centimeter have been created using memristors
· Quicker boot-ups
· Requires less voltage (and thus less overall power required)
|· Silicon transistors can age and fail· High-power and high-frequency operation· Solid-state devices are more vulnerable to electrostatic discharge in handling and operation· Solid-state devices have less mass to absorb the heat due to overloads, in proportion to their rating· Sensitivity to radiation and cosmic rays.||· Not currently commercially available· Current versions only at 1/10th the speed of DRAM· Has the ability to learn but can also learn the wrong patterns in the beginning· Since all data on the computer becomes non-volatile, rebooting will not solve any issues as it often times can with DRAM· Suspected by some that the performance and speed will never match DRAM and transistors|
Applications of memristor
With a faster speed, lower power consumption and a higher density of information per volume, memristors offer many advantages over the old transistors. This new technology could allow creating computers that operate in a way similar to the synapses in our brains. Some of the interesting applications of memristors are highlighted as:
Non-volatile memory applications: Memristors can retain memory states, and data, in power-off modes. Non-volatile random access memory (NVRAM) is pretty much the first to-market memristor application. The fab prototypes resistance is read with alternating current, so that the stored value remains unaffected.
Low-power and remote sensing: coupled with memcapacitors and meminductors, the complementary circuits to the memristor which allow for the storage of charge, memristors can possibly allow for nano-scale low power memory and distributed state storage, as a further extension of NVRAM capabilities.
Crossbar Latches as Transistor Replacements or Augmentors: The hungry power consumption of transistors has been a barrier to both miniaturization and microprocessor controller development. Solid-state memristors can be combined into devices called crossbar latches, which could replace transistors in future computers, taking up a much smaller area.
Analog computation and circuit Applications: Though, digital use in electronics rose to dominance because analog computations are not as scalable, reproducible, or dependable as digital solutions. However, there still exist some very important areas of engineering and modeling problems which require extremely complex and difficult workarounds to synthesize digitally. Analog was great, but required management for scalability beyond what even the extremely complex initial digital vacuum tube computers could provide. Memristor applications will now allow us to revisit a lot of the analog science that was abandoned for digital.
Circuits which mimic neuromorphic and biological systems (Learning Circuits): A large part of the analog science has to do with advances in cognitive psychology, artificial intelligence modeling, machine learning and recent neurology advances. The ability to map peoples brain activities under MRI, CAT, and EEG scans is leading to a treasure trove of information about how our brains work. But modeling a brain using ratiocinated mathematics is like using linear algebra to model calculus. Simple electronic circuits based on an LC network and memristors have been built, and used recently to model experiments on adaptive behavior of unicellular organisms. These types of learning circuits find applications anywhere from pattern recognition to Neural Networks.
Others: Programmable logic and signal processing, and a variety of control system memristor patents are out there, waiting for the microchips to fall where they may. The memristive applications in these areas will remain relatively the same, because it will only be a change in the underlying physical architecture, allowing their capabilities to expand. Researchers have found a way of enhancing the capabilities of an emerging nanotechnology that could open the door to a new generation of electronics. Memristors don’t require a silicon layer and different materials can be used as a substrate. This could create a new class of microchips, that could eventually be integrated in everyday items such as windows, clothes or even coffee cups.
The transistor era is soon to be gone and the memristor, its high-tech ultra-modern counterpart could soon revolutionized electronics. Memristor chips will soon be integrated in textiles, windows, even coffee cups and any imaginable items used in daily life. This is a really exciting discovery, with potentially enormous implications for modern electronics. By 2020, there are expected to be more than 200 billion interconnected devices within the Internet of Things framework- these will generate an incredible amount of data that will need processing. According to the researchers, memristors could hold the key to a new era in electronics, being both smaller and simpler in form than transistors, low-energy, and with the ability to retain data by ‘remembering’ the amount of charge that has passed through them- potentially resulting in computers that switch on and off instantly and never forget. Memristors are a key enabling technology for next-generation chips, which need to be highly reconfigurable yet affordable, scalable and energy-efficient.