Study And Simulation Of Magnetic Tunnel Junction Based Spin

Transistor

UNDER THE SUPERVISION OFPROF. M. HASAN

PRESENTED BY- TANZEEM IQBAL

10LEM301

Overview

Introduction Why Spin Transistor ? Fundamentals of Spin Transistor Magnetic tunnel junction MTJ model MTJ based Spin Transistor Simulation results Conclusions References

Introduction

Spin transistors are a new concept device that unites an ordinary transistor with the useful functions of a spin (magnetoresistive) device.

The spin transistor comes about as a result of research on the ability of electrons to naturally exhibit one of two (and only two) states of spin: known as "spin up" and "spin down".

Spin transistor shows two different current levels (parallel and anti-parallel state) which is not possible in ordinary Transistor.

Why Spin Transistor ?

Spin Transistor expected to be a potential building block for novel integrated circuits employing spin degrees of freedom.

The useful features of spin transistors are non-volatile information storage and reconfigurable output characteristics: these are very useful and suitable functionalities for various new integrated circuit architectures that are inaccessible to ordinary transistor circuits.

Fundamentals of Spin Transistor

Spin transistor is realized using MOSFET and Magnetic Tunnel junction(MTJ)

Although spin transistor can be realized using MTJ and BJT or FET. But MOSFET is better candidate for integration because of its scalability and also because CMOS is dominant technology

Also MTJ is CMOS-compatible with high stability, reliability and non-volatility .

Magnetic Tunnel Junction(MTJ)

The magnetic tunnel junction (MTJ) is one of the most basic and also most significant spin-based device

The MTJ consists of two layers of ferromagnetic material separated by an extremely thin (typically 1nm) nonconductive tunnelling barrier (MgO, Al2O3 etc.)

One layer is called fixed layer which has a certain layer stack structure fixing its magnetic orientation The other layer whose magnetic orientation can be changed freely according to an external magnetic field is called the free layer

MTJ(contd.)

The MTJ exhibits two resistive states depending on the relative orientation of the magnetization directions of the two ferromagnetic layers due to the spin-dependent tunnelling involved in the electron transport.The resistance change is measured using the tunnel magnetoresistance (TMR) ratio.

Writing operation in MTJ

The conventional writing operation of the MTJ is carried out by using an external Magnetic field

However, the current required in this writing scheme is extremely high, and it scales inversely with the device size

The second method is Current-Induced Magnetization Switching (CIMS)

CIMS is discovered in 1996 . This is also called Spin-Torque-Transfer (STT) writing scheme

Current Induced Magnetization switching

In CIMS writing, the switching between parallel and antiparallel state is controlled by the direction of the writing current

Writing current from the free layer to the fixed layer will write the MTJ into a parallel state (RP), while that owing in the opposite direction will result in an anti-parallel state (RAP )

To ensure switching, the density of writing current has to be higher than the critical current density JC where JC is defined as the minimum current density required to switch the MTJ for a given switching time

Advantage of CIMS over traditional field-driven Switching

The first advantage is that CIMS switching eliminates the need for additional lines, thereby simplifying the circuitry used to control the device

The second advantage is that CIMS switching is dependent on current density (in amperes per square meter). The result is that smaller device sizes require a lower write current.

The correlation between device size and MTJ write current outpaces the correlation between CMOS device size and CMOS output current, thereby making it easier to match MTJ and CMOS technologies as device sizes progress below 100 nm

MTJ Model

The electrical behaviour of a magnetic tunnel junction (MTJ) using spin-torque-transfer (STT) switching can be modelled using a SPICE subcircuit.

The subcircuit is a two-terminal device that exhibits the electrical characteristics of an STT-MTJ. These characteristics include all the major transient characteristics of an MTJ

MTJ Model(contd.)

Block diagram of MTJ SPICE model

MTJ Model(contd.) The two terminals n+ and n− are connected by a

voltage controlled resistor (VCR). The value of the resistance is the numerical magnitude of the signal VTMR

The decision circuit determines when the device switches from the parallel to the antiparallel state, or vice versa

The bistable circuit consists of a bistable multivibrator with amplitude control and initial condition. The amplitude control is implemented using a behavioral description of an ideal amplifier.

The curve-fitting circuits allows the to directly specify a parallel and antiparallel resistance

MTJ based Spin Transistor MTJ is combined with MOSFET to realize MTJ

based Spin Transistor MTJ is connected to the source of a MOSFET

feeds back its voltage drop to the gate, and the degree of negative feedback depends on the resistance states of the MTJ.

MTJ based Spin Transistor

Advantage of Spin Transistor based on CMOS/MTJ Hybrid Circuit

By introducing the MTJ's non- volatility into CMOS We can reduce stand-by power.

Therefore, CMOS/MTJ hybrid circuits may be able to support ultra-low-power operation at more advanced technology nodes, as their advantage of saving leakage power will become increasingly significant with technology scaling.

 

MTJ based Spin Transistor(contd.)

The MTJ based Spin Transistor shows high and low current drivabilities that is controlled by the magnetization configurations of the MTJ

Magnetocurrent ratio γMC is parameter that measure the difference in parallel and antiparallel current. γMC is defined as

γMC increased with decreasing Drain Voltage and also increased with increasing Gate Voltage of MOSFET

Simulation Results MTJ Characteristics

Transient Simulation of Drain Current of Spin Transistor with fixed Gate

Voltage

VG =1V, VD =1V

γMC =0.74

Transient Simulation of Drain Current of Spin Transistor with Varying Gate Voltage

VG=3V For parallel state and VG=1V for antiparallel

state VD=3V γMC

=9.22

Spin Transistor Output Characteristics

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

20

40

60

80

100

120

DRAIN VOLTAGE(VOLTS)

DR

AIN

CU

RR

EN

T(u

A)

PARALLEL STATE

ANTI-PARALLEL STATE

Conclusions

MTJ based spin transistor shows different current level in parallel and anti-parallel state

The most important characteristic of the MTJ based spin transistor is non-volatile nature of its parallel and anti-parallel state i.e it can retain its state, even when the power is switched off

Non-volatile logic and reconfigurable logic architectures employing spin transistors would be a promising path for spintronic integrated circuits

References

[1] S. Sugahara and J.Nitta, “Spin-Transistor Electronics: An Overview and its Outloook”, Invited Paper Proceedings of the IEEE Vol. 98, No. 12, December 2010.[2] J. D. Harms, F. Ebrahimi, X.Yao, and J. P Wang “SPICE Macromodel of Spin-Torque-Transfer-Operated Magnetic Tunnel Junctions” IEEE Transactions on Electron devices, Vol. 57, NO. 6, June 2010.[3] L. B. Faber,W. Zhao, J. O. Klein, T. Devolder, and C. Chappert, “Dynamic compact model of spin-transfer torque based magnetic tunnel junction (MTJ),” in Proc. IEEE-DTIS, 2009, pp. 130–135.[4] S. Sugahara, “Spin metal-oxide-semiconductor field-effect transistors (spin MOSFETs) for integrated spin electronics,” Inst. Electr. Eng. Proc.- Circuits Devices Syst., vol. 152, no. 4, pp. 355–365, Aug. 2005.

References(contd.)

[5] S. Sugahara, “Perspective on field-effect spin-transisotrs,” Phys. Stat. Sol. C, vol. 3, no. 12, pp. 4405–4413, Jan. 2006.[6] J. Fabian, A. Matos-Abiaguea,C. Ertlera,P. Stano, and I. Zutic, “Semiconductor spintronics,” Acta Physica Slovaca, vol. 57, no. 4–5, pp. 565–907, Aug.–Oct. 2007.[7] G. Zorpette, "The quest for the Spin transistor," IEEE Spectrum, vol. 39, no. 12, pp. 30-35, Dec. 2001.  

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