CNT transistors, which use single-walled carbon nanotubes (SWCNTs) as the semiconductor material, are now entering a phase of accelerated development. Researchers have begun exploring the integration of CNT-based transistors onto chips, aiming to create early-stage computer systems. Subhaish Mitra, an assistant professor at Stanford University in the U.S., has developed a CNT microprocessor that contains 178 CNT transistors (see Figure 1). This processor can function as a programmable computer when connected to external memory.
The CNT-based computer follows the fundamental structure of the traditional von Neumann architecture. Its operating frequency is around 1 kHz, similar to the early computers from the 1950s. However, this low frequency is not due to limitations in the CNT transistors themselves, but rather because of the large electrode size used for testing and the low operating frequency during verification.
The arithmetic logic unit (ALU), which performs operations like AND and XOR, uses 20 CNT transistors. A D latch circuit, responsible for temporary data storage, uses 9 CNT transistors. The system is fully programmable and can execute 20 basic instructions from the MIPS instruction set, including operations such as AND, OR, JUMP, and more. In addition, it can run a self-contained operating system capable of multitasking.
Figure 1 shows a CNT microprocessor made up of 178 transistors, measuring approximately 7mm by 0.8mm. The image was captured using scanning electron microscopy (SEM) at Stanford University.
One of the key challenges in developing CNT transistors is the presence of metallic CNTs, which can cause short circuits. To address this, Mitra and his team employed a technique called VMR (VLSI-compatible metallic CNT removal). This method involves fabricating CNT transistors on Si/SiO2 wafers, creating temporary gold wiring, and then passing large currents through the circuit to destroy the metallic CNTs. Afterward, the gold wiring is removed, and permanent wiring with materials like platinum is used. This process increases the purity of semiconductor CNTs to over 99.99%.
Another approach to improving CNT transistor performance comes from NEC, TASC, and the University of Tokyo. They developed a coating-based CNT transistor that operates at frequencies above 500 kHz. This level of performance is comparable to Intel's first microprocessors, the 4004 and 8008, from the early 1970s. Their improvements include the use of a "Super Inkjet (SIJ)" method to reduce electrode width from about 50 micrometers to 2 micrometers, significantly reducing parasitic capacitance and increasing operating frequency.
Additionally, they enhanced the separation of semiconductor and metallic CNTs using electrophoresis. This process increased the purity of semiconductor CNTs to over 98%, reducing impurities from dispersion stabilizers by 50 times. As a result, the output current of the transistors improved tenfold, reaching -6.2 μA from -0.6 μA.
These advancements demonstrate that CNT transistors are no longer just experimental components—they are becoming viable alternatives for future computing technologies. With continued progress in fabrication techniques and performance optimization, the potential for CNT-based electronics is growing rapidly.
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