Development of novel circuits consisting of magnetic tunnel junction and negative differential resistance devices

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The objective of this project is to develop novel magnetic random access memory (MRAM) cells consisting of magnetic tunnel junction (MTJ) and negative differential resistance (NDR) devices, such as an interband tunnel diode (ITD) or a resonant tunnel diode (RTD), connected in parallel or in series. The NDR characteristics of the ITD or RTD can increase the tunneling magnetoresistance (TMR) ratio of the MTJ. The peak voltages (currents) of the NDR characteristics are changed depending on the magnetization directions of the MTJ when the NDR device is connected in series (parallel), resulting in large MR ratio.We successfully confirmed the basic operation of the proposed memory cells by both simulation and experiment. The fabricated circuit consisting of the CoFe based MTJ and GaAs based tunnel diode showed that the effective MR ratio was enhanced from its original value of 15 to 890% in the series circuit.We also optimized the device structures. As for the MTJs, we fabricated the MTJs using half-metallic materials, such as Co-based full Heusler alloys, Perovskite-type manganites, or ferromagnetic semiconductors, and achieved relatively high TMR ratios of up to 90% at room temperature. The use of MTJs with its original TMR ratio of more than 50% results in sufficient operating margin of the bias current/voltage. As for the NDR devices, we fabricated the GaAs-based RTD with high peak-to-valley current ratio.We developed the SPICE models for both MTJs and tunnel diodes based on the experimental results and analyzed that how the circuit performance, such as cell area, access speed, power dissipation, and operating margin, would be changed as increasing the integration density of the memory cell. Furthermore, we designed the power supply circuit based on the reference cell architecture, which was robust to the variation of device characteristics, and successfully confirmed its proper operation.