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

该项目的目标是开发新型磁随机存取存储器（MRAM）单元，由磁隧道结（MTJ）和负差分电阻（NDR）器件组成，如带间隧道二极管（ITD）或谐振隧道二极管（RTD），并联或串联连接。过渡段或RTD的NDR特性可以提高MTJ的隧穿磁阻（TMR）比。当NDR器件串联（并联）时，NDR特性的峰值电压（电流）会随着MTJ的磁化方向而变化，从而导致较大的MR比。我们通过仿真和实验成功地证实了所提出的存储单元的基本工作原理。由CoFe基MTJ和GaAs基隧道二极管组成的串联电路表明，在串联电路中，有效磁导比由原来的15%提高到890%。我们还优化了器件结构。对于MTJs，我们使用半金属材料，如钴基全Heusler合金、钙钛矿型锰矿或铁磁半导体来制备MTJs，并在室温下获得了高达90%的相对较高的TMR比率。使用原始TMR比超过50%的MTJs，可以获得足够的偏置电流/电压工作裕度。对于NDR器件，我们制作了具有高峰谷电流比的gaas基RTD。基于实验结果，我们建立了MTJs和隧道二极管的SPICE模型，并分析了随着存储单元集成密度的增加，电路性能（如单元面积、存取速度、功耗和工作裕度）的变化。此外，我们基于参考单元架构设计了电源电路，该架构对器件特性的变化具有鲁棒性，并成功地验证了其正常工作。

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.