AI has been rapidly developed in recent years.
The extremely low off-state current of crystalline oxide semiconductor FET (OSFET®) enables memory devices such as DOSRAM® and NOSRAM®. The memories using OSFETs are nonvolatile and do not degrade over many write cycles in principle. While conventional DRAM needs a refresh operation with an interval of milliseconds or less, the refresh rate of DOSRAM can be reduced to several times per hour to several times per year.
Furthermore, LSI circuits using OSFET (hereinafter, OSLSI®）adopt the memory technology described above and can achieve normally-off operation, shutting off the power when the circuit does not need to operate. Therefore, the power consumption can be reduced to extremely low levels. The crystalline oxide semiconductor technology developed by SEL can increase the processing efficiency of OSLSI, which can also be used for AI operations.
A four-terminal device is more controllable than a two-terminal device as a two-terminal device has one terminal that serves as both an input terminal and an output terminal, whereas a four-terminal device has input and output terminals that can be controlled independently from each other.
The Si FET has four terminals including the body terminal, and it is used for VLSI circuits that are manufactured worldwide owing to its controllability. The OSFET can be provided with a back gate terminal, which will form a pair with the gate terminal on the substrate side. The OSFET with a back gate can function as a four-terminal device.
OSFETs can easily be used in VLSI circuits, and combining the OSFET with the Si FET, we can construct a NOSRAM® device.
The extremely low off-state current of the OSFET enables a rewritable memory device (NOSRAM).
“osMemory Logic” consists of two transistors and one capacitor in the same manner as a basic cell of NOSRAM. The “osMemory Logic” shown below can retain the result of arithmetic operation in a data retention node and output a value corresponding to the product of X and W as I0.
An osMemory Logic cell serves as both an on-site memory and a multiplier circuit. Using the OS-Si hybrid structure, an internal memory (on-site memory) can be fabricated on top of the arithmetic circuits. Moreover, the OS-Si hybrid structure reduces the area of the arithmetic circuits, and the arithmetic part serving also as a memory part offers easy memory access, thereby drastically reducing energy loss in data transmission.
Application of OSLSI that includes AI to arithmetic circuits
Application of OSLSI including AI, #1AI learns an algorithm of image data compressing/decompressing by itself and produces a clear and vivid image. In the 8K era, the high-speed data transfer and low-power consumption of large-sized TV system are expected.
Application of OSLSI including AI, #2AI estimates a period that does not need refreshing display images and thus enables low power consumption in accordance with use environment1).
1) “Low-Power Display System Enabled by Combining Oxide-Semiconductor and Neural-Network Technologies”, H. Kunitake et al., ECS Trans., 79 (1) 177 (2017).
* OSFET, OSLSI, NOSRAM, DOSRAM, and CAAC-IGZO are registered trademarks of Semiconductor Energy Laboratory Co., Ltd.
(Japanese trademark registration No. 5519759, No.5698906, No. 5529056, No.5519752, and No.5494218).