Oxide-semiconductor Channel Transistor DRAM (OCTRAM) with 4F2 Architecture

February 13, 2025

InGaZnO, an oxide semiconductor material, has a large bandgap and high electron mobility. Consequently, an InGaZnO transistor is expected to have ultra-low leakage current and high ON current properties. In addition, a junctionless InGaZnO vertical transistor has immunity to floating-body effect and variable retention time failure. The InGaZnO vertical transistor, therefore, could provide a new type of DRAM having characteristics of low power and high reliability (Figure 1).

To apply the InGaZnO transistor to DRAM access transistor, a high ON current over 1µA is required. We optimized the contact electrode material and spacer thickness, and experimentally achieved 15µA (Figure 2).

The InGaZnO transistor also showed ultra-low leakage current below 10^-18A (1aA) because of its large bandgap (Figure 3).

We developed, in collaboration with Nanya technology, the OCTRAM where the InGaZnO vertical transistor was integrated on top of a capacitor (capacitor-first architecture, Figure 4).

The OCTRAM was operated in the designed voltage range. It also showed long retention over 100 second (Figure 5). The OCTRAM can potentially lower power consumption in a wide range of applications, including AI and post-5G communication systems, and IoT products.