Gyu-Hyun Kil

Bidirectional Two-Terminal Switching Device Using Schottky Barrier for Spin-Transfer-Torque Magnetic Random Access Memory

We present a bidirectional two-terminal switching device using a Schottky barrier for spin-transfer-torque magnetic random access memory (STT-MRAM), which is composed of a Schottky barrier contact with a metal/semiconductor/metal (M/S/M) structure. The proposed M/S/M switching device provides a bidirectional current flow sufficient to write STT-MRAM using a punch through with an extended depletion region at a junction under a reverse bias of M/S or S/M. In addition, a high on–off ratio of 105 is confirmed under the read condition, which is acceptable for the operation of STT-MRAM. From this work, it is expected that an M/S/M structure with bilateral Schottky junctions will be a promising switch device for STT MRAM beyond 20 nm.

Bidirectional Two-Terminal Switching Device for Non-Volatile Random Access Memory

A two-terminal N+/P/N+ Si junction device that can replace the conventional selective transistor was studied as a bilateral switching device for spin transfer torque magnetic random access memory (STT-MRAM), by three-dimensional device simulation. An N+/P/N+ junction structure with 30×30 nm2 area provides sufficient bidirectional current flow to write data by a drain-induced barrier lowering (DIBL) under a reverse bias at the N+/P (or P/N+) junction, and high current on/off ratio of 106, which is acceptable for STT-MRAM. In this work, critical parameters such as P-length, P doping, and N+ doping are investigated to elucidate the optimal parameter condition in view of write current and current on/off ratio.

Novel Self-Reference Sense Amplifier for Spin-Transfer-Torque Magneto-Resistive Random Access Memory

A novel self-reference sense amplifier with parallel reading during writing operation is proposed. Read access time is improved compared to conventional self-reference scheme with fast operation speed by reducing operation steps to 1 for read operation cycle using parallel reading scheme, while large sense margin competitive to conventional destructive scheme is obtained by using self-reference scheme. The simulation was performed using standard 0.18 mm CMOS process. The proposed selfreference sense amplifier improved not only the operation speed of less than 20 ns which is comparable to non-destructive sense amplifier, but also sense margin over 150 mV which is larger than conventional sensing schemes. The proposed scheme is expected to be very helpful for engineers for developing MRAM technology.

Macro model for stochastic behavior of resistance distribution of magnetic tunnel junction

In this work, we fabricated MgO-based magnetic tunnel junction (MTJ) samples to observe behavior of resistance variation, and investigated a stochastic behavior model for MTJ resistance from measured real data. We found the relationship between parallel resistance (RP), anti-parallel resistance (RAP), and TMR from the measurements. The variation of barrier thickness affects not only resistance but also TMR. This means that broad RAP distribution is caused by RP distribution. In addition, RAP distribution can be reduced by increasing temperature and bias voltage. We developed a macro model that can evaluate resistance distribution based on the stochastic behavior of MTJ resistance variation from only tox varied. The amount of resistance variation, which is considered with regard to the circuit performance, can be obtained from Δtox designed by designer. In addition, the impact for operating circumstance such as bias and temperature can be considered by using fit equations.

Bidirectional Two-Terminal Switching Device with Metal?Semiconductor?Semiconductor Structures for 4F2 Spin-Transfer-Torque Magnetoresistance Random Access Memory Cell

We proposed a novel bidirectional two-terminal selective device for realizing a 4F2 cell size for spin-transfer-torque magnetoresistance random access memory (STT-MRAM). The proposed switching device is composed of a Schottky-barrier contact and a PN junction with a metal–semiconductor–semiconductor (M/S/S) structure. The proposed M/S/S switching device provides a current density of over 106 A/cm2, which is sufficient to write the magnetic tunnel junction (MTJ), and a bilateral current flow using a punch through with an extended depletion region at a junction. Furthermore, we investigated its switching characteristics by comparing parameters such as doping profile and junction length. From this work, it is expected that an M/S/S structure will be a promising switching device for STT MRAM.

A novel sensing algorithm for Spin-Transfer-Torque magnetic RAM (STT-MRAM) by utilizing dynamic reference

A novel sensing algorithm for non-volatile Spin-Transfer Torque Magneto-resistive Random Access Memory (STT-MRAM) is presented. The dynamic reference sense amplifier (DRSA) improves sensing margin to achieve high reliability and sensitivity by increasing the difference of input voltages of sense amplifier. A dynamic reference sensing algorithm is proposed as a solution for the read margin loss due to variation in magnetic tunneling junction (MTJ) parameters of the STT-MRAM. The proposed sensing method was designed in standard 0.18um process parameters, and simulation results indicate simultaneously increased the read margin compared with the conventional sensing method.