Thomas Windbacher

Reliability Analysis and Comparison of Implication and Reprogrammable Logic Gates in Magnetic Tunnel Junction Logic Circuits

Non-volatile logic is a promising solution to overcome the leakage power issue which has become an important obstacle to scaling of CMOS technology. Magnetic tunnel junction (MTJ)-based logic has a great potential, because of the non-volatility, unlimited endurance, CMOS compatibility, and fast switching speed of the MTJ devices. Recently, by direct communication between spin-transfer-torque-operated MTJs, several realizations of intrinsic logic-in-memory circuits have been demonstrated for which the MTJ devices are used simultaneously as memory and computing elements. Here, we present a reliability analysis of the MTJ-based logic operations and show that the reliability is an essential prerequisite of these MTJ-based logic circuits. It is demonstrated that for given MTJ device characteristics, the implication logic architecture, a new kind of logic based on material implication, significantly improves the reliability of the MTJ-based logic as compared to the reprogrammable logic architecture which is based on the conventional Boolean logic operations AND, OR, etc. Implementing the implication gates in spin-transfer torque magnetic random access memory arrays provides pure electrical read/write and logic operations and also allows fan-out to multiple outputs.

MRAM-based logic array for large-scale non-volatile logic-in-memory applications

A novel non-volatile logic-in-memory (NV-LIM) architecture is introduced to extend the functionality of the spin-transfer torque magnetoresistive random-access memory (STT-MRAM) to include performing logic operations with no extra hardware added. The access transistors of the one-transistor/one-magnetic tunnel junction (1T/1MTJ) cells are used as voltage-controlled resistors. This provides the structural asymmetry required for realizing a fundamental Boolean logic operation called material implication and inherently realizes a NV-LIM architecture which uses MTJs as the main computing elements (logic gate).

Rigorous simulation study of a novel non-volatile magnetic flip-flop

The ever increasing demand in fast and cheap bulk memory as well as electronics in general has driven the scaling efforts in CMOS since its very beginnings. Today, pushing the limits of integration density is still a major concern, but gradually power efficient computing gains more and more interest. A possible way to reduce power consumption is to introduce non-volatility into the devices. Thus power is consumed only, when information is written or read out, while the rest of the time the devices preserve the information with out any power demand. In this work we propose a novel non-volatile magnetic flip-flop which shifts the actual logic operation from the electric signal domain to the magnetic domain, operating via constructive and destructive superposition of spin waves generated by the spin transfer torque effect. Furthermore we carried out a rigorous simulation study for three different device sizes and found them operational between ≈4×1010A/m2 and ≈1012A/m2 at switching times from tens of nanoseconds to picoseconds.

Design and applications of magnetic tunnel junction based logic circuits

By offering zero standby power, non-volatile logic is a promising solution to overcome the leakage current issue which has become an important obstacle, when CMOS technology is shrunk. Magnetic tunnel junction (MTJ)-based logic has a great potential, because of unlimited endurance, CMOS compatibility, and fast switching speed. Recently, several non-volatile MTJ-based circuits have been presented which inherently realize logic-in-memory circuit concepts by using MTJ devices as both memory and the main computing elements. In this work we present a reliability simulation method for designing MTJ-based logic gates integrated with CMOS. As an application example, we study the reliability of a magnetic full adder in two different designs based on the implication and the reprogrammable MTJ logic gates.

Novel bias-field-free spin transfer oscillator

Two versions of magnetic field free spin torque oscillators with in- and out-of-plane spin polarizers are proposed. The field free spin torque oscillators comprise two spin valve stacks with a common free magnetic layer featuring an out-of-plane anisotropy. Their operation frequencies are controlled by the dimensions of the free layer and can also be tuned by the applied currents. Large and stable magnetization precessional motion of the whole shared free layer for both oscillators are obtained. The structure with in-plane polarizers allows more efficient microwave power extraction of the large in-plane magnetization precession of the free layer.

Novel MTJ-based shift register for non-volatile logic applications

The increasing costs and leakage losses have become the major concerns for CMOS technology scaling. A possible way to address in particular the standby power problem is to introduce non-volatility into the devices and circuit blocks so that unused devices or even entire circuit blocks do not waste energy, and power is only spent, when information is read or written. Recently, we proposed a non-volatile magnetic flip flop which moves the information storage and processing from the CMOS domain to the magnetic domain. Here, we propose a way to extend the functionality of the device to a shift register; computing via spin wave superposition and passing information by spin torque transfer (STT). The presented shift register and its operation allows an extremely dense layout, is CMOS compatible, and non-volatile.

Simulation of field-effect Biosensors (BioFETs)

In this paper a bottom-up approach for modeling field-effect biosensors (BioFETs) is developed. Starting from the given positions of charged atoms, of a given molecule, the charge and the dipole moment of a single molecule are calculated. This charge and dipole moment are used to calculate the mean surface density and mean dipole moment at the biofunctionalized surface, which are introduced into homogenized interface conditions linking the Angstrom-scale of the molecule with the micrometer-scale of the FET. By considering a single-stranded to double-stranded DNA reaction, we demonstrate the capability of a BioFET to detect a certain DNA and to resolve the DNA orientation.

CMOS-compatible spintronic devices: a review

For many decades CMOS devices have been successfully scaled down to achieve higher speed and increased performance of integrated circuits at lower cost. Today’s charge-based CMOS electronics encounters two major challenges: power dissipation and variability. Spintronics is a rapidly evolving research and development field, which offers a potential solution to these issues by introducing novel ‘more than Moore’ devices. Spin-based magnetoresistive random-access memory (MRAM) is already recognized as one of the most promising candidates for future universal memory. Magnetic tunnel junctions, the main elements of MRAM cells, can also be used to build logic-in-memory circuits with non-volatile storage elements on top of CMOS logic circuits, as well as versatile compact on-chip oscillators with low power consumption. We give an overview of CMOS-compatible spintronics applications. First, we present a brief introduction to the physical background considering such effects as magnetoresistance, spin-transfer torque (STT), spin Hall effect, and magnetoelectric effects. We continue with a comprehensive review of the state-of-the-art spintronic devices for memory applications (STT-MRAM, domain wallmotion MRAM, and spin–orbit torque MRAM), oscillators (spin torque oscillators and spin Hall nano-oscillators), logic (logic-in-memory, all-spin logic, and buffered magnetic logic gate grid), sensors, and random number generators. Devices with different types of resistivity switching are analyzed and compared, with their advantages highlighted and challenges revealed. CMOScompatible spintronic devices are demonstrated beginning with predictive simulations, proceeding to their experimental confirmation and realization, and finalized by the current status of application in modern integrated systems and circuits. We conclude the review with an outlook, where we share our vision on the future applications of the prospective devices in the area.

Biotin-Streptavidin Sensitive BioFETs and Their Properties

In this work the properties of a biotin-streptavidin BioFET have been studied numerically with homogenized boundary interface conditions as the link between the oxide of the FET and the analyte which contains the bio-sample. The biotin-streptavidin reaction pair is used in purification and detection of various biomolecules; the strong streptavidin-biotin bond can also be used to attach biomolecules to one another or onto a solid support. Thus this reaction pair in combination with a FET as the transducer is a powerful setup enabling the detection of a wide variety of molecules with many advantages that stem from the FET, like no labeling, no need of expensive read-out devices, the possibility to put the signal amplification and analysis on the same chip, and outdoor usage without the necessity of a lab.

Spin injection and diffusion in silicon based devices from a space charge layer

We have performed simulations on electron spin transport in an n-doped silicon bar with spin-dependent conductivity with or without the presence of an external electric field. We further consider three cases like charge neutrality, charge accumulation, and charge depletion at one boundary and found substantial differences in the spin transport behavior. The criteria determining the maximum spin current are investigated. The physical reason behind the transport behavior is explained.