Hyein Lim

Unified Analytical Model for Switching Behavior of Magnetic Tunnel Junction

The switching time of the magnetic tunnel junction for spin-transfer torque magnetoresistive random access memory is investigated as a function of the current. We present a unified analytical model for switching time so that the problem of discontinuity around the critical current is solved. The suggested unified model shows excellent agreement with the experimental data of parallel/antiparallel states simultaneously. Furthermore, only one set of parameters is used to represent all switching regime in our model.

Analytic Model of Spin-Torque Oscillators (STO) for Circuit-Level Simulation

Spin-torque oscillators (STO) is a new device that can be used as a tunable microwave source in various wireless devices. Spin-transfer torque effect in magnetic multilayered nanostructure can induce precession of magnetization when bias current and external magnetic field are properly applied, and a microwave signal is generated from that precession. We proposed a semi-empirical circuit-level model of an STO in previous work. In this paper, we present a refined STO model which gives more accuracy by considering physical phenomena in the calculation of effective field. Characteristics of the STO are expressed as functions of external magnetic field and bias current in Verilog-A HDL such that they can be simulated with circuit-level simulators such as Hspice. The simulation results are in good agreement with the experimental data.

Advanced Circuit-Level Model for Temperature-Sensitive Read/Write Operation of a Magnetic Tunnel Junction

A circuit-level model of a magnetic tunnel junction (MTJ) that accurately depicts the spin-transfer torque switching behavior of an MTJ along with its voltage/temperature dependency is presented. The unified switching model that was suggested in our previous work is suitable for the circuit-level model because the model seamlessly covers the entire current pulsewidth region. Based on this switching behavior model, a method of dynamic current monitoring is proposed for an advanced circuit-level model. The voltage/temperature characteristics of an MTJ are also integrated for a more accurate circuit-level simulation. The proposed model corresponds well with the experimental data for switching characteristics at different temperatures. The design margin in the read/write circuits with an MTJ is analyzed to demonstrate the effectiveness of the model.

Advanced Circuit-Level Model of Magnetic Tunnel Junction-based Spin-Torque Oscillator with Perpendicular Anisotropy Field

Interest in spin-torque oscillators (STOs) has been increasing due to their potential use in communication devices. In particular the magnetic tunnel junction-based STO (MTJ-STO) with high perpendicular anisotropy is gaining attention since it can generate high output power. In this paper, a circuit-level model for an in-plane magnetized MTJSTO with partial perpendicular anisotropy is proposed. The model includes the perpendicular torque and the shift field for more accurate modeling. The bias voltage dependence of perpendicular torque is represented as quadratic. The model is written in Verilog-A, and simulated using HSPICE simulator with a current-mirror circuit and a multi-stage wideband amplifier. The simulation results show the proposed model can accurately replicate the experimental data such that the power increases and the frequency decreases as the value of the perpendicular anisotropy gets close to the value of the demagnetizing field.

A New Circuit Model for Spin-Torque Oscillator Including Perpendicular Torque of Magnetic Tunnel Junction

Spin-torque oscillator (STO) is a promising new technology for the future RF oscillators, which is based on the spin-transfer torque (STT) effect in magnetic multilayered nanostructure. It is expected to provide a larger tunability, smaller size, lower power consumption, and higher level of integration than the semiconductor-based oscillators. In our previous work, a circuit-level model of the giant magnetoresistance (GMR) STO was proposed. In this paper, we present a physics-based circuit-level model of the magnetic tunnel junction (MTJ)-based STO. MTJ-STO model includes the effect of perpendicular torque that has been ignored in the GMR-STO model. The variations of three major characteristics, generation frequency, mean oscillation power, and generation linewidth of an MTJ-STO with respect to the amount of perpendicular torque, are investigated, and the results are applied to our model. The operation of the model was verified by HSPICE simulation, and the results show an excellent agreement with the experimental data. The results also prove that a full circuit-level simulation with MJT-STO devices can be made with our proposed model.

Investigation of power dissipation for ReRAM in crossbar array architecture

Power consumption of large-scale crossbar array architecture is investigated by the comprehensive crossbar array matrix model. The power dissipation is examined as functions of array size, leakage current of selector, and various bias schemes. The power consumption increases as the array size and the leakage current of selector increases. In addition, 1/3 bias scheme shows power consumption about 1~2 orders of magnitude larger than other bias schemes. This phenomenon is induced from the unselected cells which is delivered with voltage about Vdd/3, whereas the voltage of unselected cells are almost 0 V for 1/2 bias and floating bias schemes.

A Compact Model of Gate-Voltage-Dependent Quantum Effects in Short-Channel Surrounding-Gate Metal-Oxide-Semiconductor Field-Effect Transistors

In this paper, we present a compact model of gate-voltage-dependent quantum effects in shortchannel surrounding-gate (SG) metal-oxide-semiconductor field-effect transistors (MOSFETs). We based the model on a two-dimensional (2-D) analytical solution of Poisson’s equation using cylindrical coordinates. We used the model to investigate the electrostatic potential and current sensitivities of various gate lengths (Lg) and radii (R). Schrodinger’s equation was solved analytically for a one-dimensional (1-D) quantum well to include quantum effects in the model. The model takes into account quantum effects in the inversion region of the SG MOSFET using a triangular well. We show that the new model is in excellent agreement with the device simulation results in all regions of operation.

Guideline model for the bias-scheme-dependent power consumption of a resistive random access memory crossbar array

The 1/2 and 1/3 bias schemes are commonly used to select a cell in a resistive random access memory (ReRAM) crossbar array. The 1/3 bias scheme is advantageous in terms of its write margin but typically requires a higher power consumption than the 1/2 bias scheme. The power consumption of ReRAM can vary according to the nonlinearity of the selector device. In this paper, we propose a power guideline model that suggests selector nonlinearity requirements to guarantee a lower power consumption for the 1/3 bias scheme than for the 1/2 bias scheme. Therefore, the selector nonlinearity requirements for the low power consumption of the 1/3 bias scheme can be immediately obtained using this guideline model without simulation.

An analysis of the read margin and power consumption of crossbar ReRAM arrays

The read margin and power consumption for various selector characteristics and bias schemes are analyzed during read operation. The 1/2 and 1/3 bias schemes exhibit different read operation properties. There is a trade-off between the read margin and power consumption that depends on the bias scheme and characteristics of the selector. The read margin of the 1/2 bias scheme is decreased at low on/off ratios because of the output voltage drop across the LRS cell. This drop is due to a rapid increase in the leakage current when a selector with low on/off ratio is used in the 1/2 bias scheme. Therefore, the bias scheme and selector should be selected appropriately according to the purpose of the application.

Physics-Based SPICE Model of Spin-Torque Oscillators

The spin-torque oscillator (STO) is a new compact device operating as a tunable RF oscillator in the tens of gigahertz range whose characteristics are determined by the applied current and magnetic field. In this paper, we present a physics-based empirical circuit-level model of an STO that is compatible with circuit-level simulators such as SPICE. The characteristics of an STO are modeled as physics-based analytic functions of the applied current and external magnetic field. The validity of our model was verified by the HSPICE simulation of a current mirror circuit that contains an STO element. The simulation results are in good agreement with the experimental data in the normal operation range. High-order nonlinear effects at large currents are not included in our model because there is no theoretical equation available yet that can precisely explain these effects.