Download How Memristance Works This document describes the physical

How Memristance Works
This document describes the physical mechanics of memristance as it operates in a cross-bar
architecture configuration. Memristance is a theoretical concept that has been developed into a
physical working mechanism by GALACA Enterpises Electronic Research Laboratories (GEERL)
based in Troy, Michigan USA. It bridges the capability gaps that electronics will face in the near
future according to Moore’s Law and will replace the transistor as the main component on
integrated circuit (IC) chips.
FUNCTION – The function of the memristor is to provide the ability to remember current
flow across a resistor permanently. This attribute will allow nano-scale switches to store information
on a grand scale using minimal physical space. Test voltages small enough not the affect the
resistance could read the data from the memristive circuit. These switching attributes resemble
synapse functions in the brain which is a source of intrigue for further development in this area of
research. While memristive circuits will never replace the transistor completely due to power factors,
they will reduce the size of electronics considerably. The potential for nanoscale crossbar memory in
comparison to modern flash technology can be described by placing the entire U.S. Library of
Congress on a single thumb drive.
APPEARANCE – Crossbar type memristive circuits contain a lattice of 40-50nm wide by
2-3nm thick platinum wires that are laid on top of one another perpendicular top to bottom and
parallel of one another side to side. The top and bottom layer are separated by a switching element
approximately 3-30nm in thickness. The switching element consists of two equal parts of titanium
dioxide (TiO2). The layer connected to the bottom platinum wire is initially perfect TiO2 and the
other half is an oxygen deficient layer of TiO2 represented by TiO2-x where x represents the amount
of oxygen deficiencies or vacancies. The entire circuit and mechanism cannot be seen by the naked
eye and must be viewed under a scanning tunneling microscope, as seen in Figure 1, in order to
visualize the physical set up of the crossbar design of the memristive circuit described in this section.
Figure 1 – View of Crossbar Memristor
OPERATION – The memristor’s operation as a switch can be explained in three steps.
These first of these steps is the application of power or more importantly current to the memristor.
The second step consists of the amount of time that the current flows across the crossbar gap and
how the titanium cube converts from a semi-conductor to a conductor. The final step is the actual
memory of the cube that can be read as data.
STEP 1 – As explained above, each gap that connects two platinum wires contains a mixture of two
titanium oxide layers. The initial state of the mixture is halfway between conductance and semiconductance. Two wires are selected to apply power to in either a positive or negative direction. A
positive direction will attempt to close the switch and a negative direction will attempt to open the
switch. The application of this power will be able to completely open the circuit between the wires
but it will not be able to completely close the circuit since the material is still a semi-conductor by
nature. Power can be selectively placed on certain wires to open and close the switches in the
memristor.
STEP 2 – the second step involves a process that takes place at the atom level and is not visible by
any means. It involves the atomic process that the gap material, made from titanium dioxide, goes
through that opens and closes the switch. The initial state of the gap is neutral meaning that it
consists of one half of pure titanium dioxide TiO2 and one half of oxygen starved titanium dioxide
TiO2-x where x in the initial state is 0.05. As positive current is applied, the positively charged oxygen
vacancies push their way into the pure TiO2 causing the resistance in the gap material to drop,
becoming more conductive, and the current to rise. Inversely, as a negative current is applied the
oxygen vacancies withdraw from the pure TiO2 and condense in the TiO2-x half of the gap material
causing the pure and more resistive TiO2 to have a greater ratio slowing the current in the circuit.
When the current is raised the switch is considered open (HI) and for data purposes a binary 1. As
current is reversed and the current is dropped the switch is considered closed (LOW) or a binary 0
for data purposes.
STEP 3 – Step three explains the final step of memristance and is the actual step that makes the
circuit memristive in nature. As explained previously, the concept of memristance is a resistor that
can remember what current passed through it. When power is no longer applied to the circuit
switches, the oxygen vacancies remain in the position that they were last before the power was shut
down. This means that the value of the resistance of the material gap will remain until indefinitely
until power is applied again. This is the true meaning of memristance. With an insignificant test
voltage, one that won’t affect the movement of molecules in the material gap will allow the state of
the switches to be read as data. This means that the memristor circuits are in fact storing data
physically.