21-02-2010, 01:44 PM
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Memristor “ The Fourth Fundamental Circuit Element
Introduction
Currently known fundamental passive elements “ Resistors, Capacitors & Inductors.
Does a 4th passive element exist..
Leon O. Chua formulated Memristor theory in his paper Memristor-The Missing Circuit Element in 1971.
Memistors are passive two terminal circuit elements.
Behaves like a nonlinear resistor with memory.
History Of Memristor
Four fundamental circuit variables- current i, voltage v, charge q, and flux linkage f
Six possible combinations of these four variables
Five already defined as
Resistor(dv=Rdi), Capacitor(dq=Cdv), Inductor(df=Ldi), q(t)=i(T)dT, f(t)=v(T)dT
The 6th relation defines memristance as df=Mdq
So what is Memristance
Memristance is a property of an electronic component.
When charge flows in one direction, its resistance increases, and if direction is reversed, resistance decreases.
When v=0, charge flow stops & component will Ëœrememberâ„¢ the last resistance it had.
When the flow of charge regains, the resistance of the circuit will be the value when it was last active.
Memristor Theory
Two terminal device in which magnetic flux Fm between its terminals is a function of amount of electric charge q passed through the device.
M(q) = dFm/dq
M(q) = [dFm/dt] / [dq/dt] = V/I
V(t) = M(q(t))I(t)
The memristor is static if no current is applied.
If I(t)=0, then V(t)=0 and M(t) is a constant. This is the essence of the memory effect.
Physical analogy for a memristor
Resistor is analogous to a pipe that carries water.
Water(charge q), input pressure(voltage v), rate of flow of water(current i).
In case of resistor, flow of water is faster if pipe is shorter and/or has a larger diameter.
Memristor is analogous to a special kind of pipe that expands or shrinks when water flows through it
The pipe is directive in nature.
If water pressure is turned off, pipe will retain its most recent diameter, until water is turned back on.
Titanium dioxide memristor
On April 30, 2008, a team at HP Labs led by the scientist R. Stanley Williams announced the discovery of a switching memristor.
It achieves a resistance dependent on the history of current using a chemical mechanism.
The HP device is composed of a thin (5nm) Titanium dioxide film between two Pt electrodes.
Initially there are two layers, one slightly depleted of Oxygen atoms, other non-depleted layer.
The depleted layer has much lower resistance than the non-depleted layer.
Conclusion
The rich hysteretic v-i characteristics detected in many thin film devices can now be understood as memristive behaviour.
This behaviour is more relevant as active region in devices shrink to nanometer thickness.
It takes a lot of transistors and capacitors to do the job of a single memristor.
No combination of R,L,C circuit could duplicate the memristance.
So the memristor qualifies as a fundamental circuit element.
