The memristic theory was developed in 1971 by Professor Leon Chua. The behavior of memristor was discovered by a HP Lab Scientists while he was trying to find out cross bar switches. Memristor is a switch used to connect multiple inputs to multiple outputs in a matrix – that’s why it is also known as matrix switch. Professor Leon Chua had looked at the samples of resistor, capacitor and inductor – from which, he saw a missing component and called it as a memory resistor or memristor. The practical model of memristor was developed by Stanley in the year 2006. Memristor technology was invented several decades ago, but it got fabricated recently.
Until now, to design electronic circuits, passive elements are used such as capacitors, resistors and inductors, but a fourth fundamental element also exists, which is called as “memristor”. Memristor is a semiconductor that joins a capacitor, resistor and inductor to make a fourth new kind of element whose resistance is called as memristance that varies as a function of current and flux. Memristance is a simply charge dependent resistance and the unit of memristor is Ohm. The following are the benefits of memristor technology:
- It utilizes less energy and generates less heat.
- When power is interrupted in data centers, it gives greater reliability and resiliency.
- Memristors do not consume power when idle and are comfortable with CMOS interfaces.
- Density permits additional information to be stored
A physical memristor consists of two platinum electrodes, and the resistance of this device depends on the polarity, magnitude and length. When the voltage is turned off, the resistance remains as it did just before it was turned off. This makes the memristor a nonvolatile memory device.
In the above figure, two terminal memristor utilize titanium dioxide as the resistive material. In other memristors, silicon dioxide material can be used. However, titanium dioxide works better. When a voltage is applied across the two terminals, as shown above – the oxygen atoms in the material disperse towards left or right. And then, the material will become thinner or thicker depending on the polarity of the voltage, thus causing a change in the resistance.
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 memristors
1. Ionic 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.
2. 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.
Memristor Spice Model
In recent times the research on memristors has increased rapidly due to their potential applications as well as the demonstration of memristor manufacturing. As of now the Research is in full swing to use memristors in analog circuits, digital or programmable logic controllers, computers and sensors. The new models of memristors need to be available for the design engineers to use them as a circuit element throughout design exploration. There are three types of models available for the design engineers: Verilog-A, MATLAB and Spice model. Verilog-A and MATLAB models can be used for a high-level abstracted simulation only. But, the recent memristor spice model can be used for a circuit level simulation.
This spice model makes it possible to design and simulate memristor circuits. In this model, we simulate two circuits – a low-pass filter in which a memristor is in series with an inductor, and a resistor with an Operational Amplifier. The results are contrasted with the inductor circuits, in which the memristor is replaced by an inductor. The contrast shows that a memristor acts like an inductor under some conditions. Memristor has immense performance in terms of size and power dissipation.
A memristor is a two-terminal device with a variable resistance, which can be used in many applications including neuromorphic systems, logic circuits and digital memory.
The most observable application of a memristor is memory. A memristor can store a single bit of data in DRAM – where the capacitors are restored with memristors. When compared to DRAM and SRAM, this kind of memory has many benefits like – it is non-volatile; it displays good scalability, and it has no leakage power. This type of memory is superior to flash memory in terms of scalability and speed.
Memristors perform equally well like the biological synapses. This feature makes good building blocks in neuromorphic systems, where synapses and neurons are formed as electronic systems. This kind of memory has many benefits when we compare it with the DRAM and SRAM.
Another feasible application of memristors is logic circuits. These can be used as a standalone logic gate, or used in hybrid CMOS memristor circuits. One notable logic application of memristors is its usage in an FPGA as configurable switch, and in connecting the CMOS logic gates.
Thus, in future memristors can be used to do digital logic using implication instead of NAND. Though hundreds of memristors have been built, but still, there are many more to be perfected. The information provided in this article is simple, concise and correct. We believe that you might have got an idea about the memristors. Furthermore, for any help regarding this topic, give your suggestions, comments in the comment section below.
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