Electronics, Electronics components, Nanotechnology, New technology

Memristor and memristence

This post’s subject is the memristor, the fourth fundamental passive electronic component, alongside resistor, capacitor, and coil.

Memristor theory

The four important physical quantities for electronics are: Voltage, current, charge, and flux. The values of fundamental passive components, resistance, capacitance, and inductance, depend on the ratio between two of these quantities.

Passives components don’t increase current or voltage intensity. In 1971, Leon Chua realized that a passive element is missing, whose value depends on electric charge and magnetic flux, to complete the symmetry between fundamental passive components. This missing component was called memristor, whose name came from the junction of word memory and resistor.

The first memristor

A new component was created in 2005 by HP Labs, was called Crossbar Latch. In 2008, was announced that Crossbar Latch was actually a memristor. It can only be built with nanotechnology, whose behavior cannot be reproduced by any combination of commercial components.

The first memristor
17 memristors created by HP, seen by an atomic force microscope. Source: Device-Boom.

How does it work?

The structure built by HP Labs consist in a sandwich of 2 titanium dioxide layers, one pure (TiO_{2}) and another doped with oxygen deficiency (TiO_{2-x}), and 2 platinum electrodes (Pt). The electrodes and TiO_{2} layers have only a few nanometers of width and length.  

memristor structure
Source: (Johnsen, 2012).

The TiO_{2} possesses high electric resistance, while TiO_{2-x} is a good conductor. When there is a positive voltage in the doped side, volume of TiO_{2-x} increases (b). Because the gaps, which act like positive charges, are repelled by the positive pole. Therefore, electric resistance is lowered. When applies a negative voltage, in relation to the doped layer, these gaps are attracted and the volume of TiO_{2-x} is decreased, increasing electric resistance (c).

operation of memristor
The memristor can be modeled as two potentiometers connected as shown on the right. Source: ECE Emperors.
Memristor's conductor filament growth
The volume variation of TiO_{2-x} occurs by increasing or decreasing a conductor filament, built by electrochemical reactions. Source: (Dongale et al., 2016).

What is interesting is that the memristor saves the value of resistance after the current. Memristence value is in ohms (Ω). It’s possible to create memristors based on other materials, for example, lithium niobate (LiNbO_{3}).

Memristor made from another material
A conductor filament formation in a lithium niobate memristor. Source: (Yakopcic et al., 2017).
Curves of 4 passive component
The characteristic curves of 4 passive components, in V x I graphics. Source: The Register.

What is the memristor for?

  • The property to store the current value in form of resistance allows creating faster memories that store information even without energy. Imagine shut down your computer without save, when turns it on, programs are open.
A memristor array can store more information in smaller area. Source: Stack Exchange.
Memristor array as a neuron’s synapses. Source: Extreme Tech.
  • Logic doors can be made with memristors.

Exist many types of memristors. To talk about them all, I would have to write one or more posts.

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About Pedro Ney Stroski

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