CNT transistors, which use single-walled carbon nanotubes (SWCNTs) as the semiconductor material, have finally entered a phase of accelerated development. Researchers are now exploring the integration of these transistors into chip designs as part of early experimental computer systems. Subhaish Mitra, an assistant professor at Stanford University in the U.S., has developed a CNT-based microprocessor that incorporates 178 CNT transistors (Figure 1). The processor is capable of functioning as a programmable computer when connected to external memory.
This CNT-powered computer follows the fundamental architecture of modern von Neumann-style computers. However, its operating frequency is only 1 kHz, comparable to the first transistor-based computers from the early 1950s. 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 relatively low operating speed during verification.
The arithmetic unit, which performs logical operations such as AND and XOR, uses 20 CNT transistors, while the D latch circuit, responsible for temporary data storage, uses 9. The system is fully programmable and can execute 20 basic instructions from the MIPS instruction set, including operations like AND, OR, XOR, JUMP, and more. Additionally, it can run a self-contained operating system that supports multitasking.
Figure 1 shows the CNT microprocessor, consisting of 178 transistors, measuring approximately 7mm x 0.8mm. The image is a scanning electron microscope (SEM) view of the device, highlighting the arrangement of the CNTs used in the design.
One of the key challenges in CNT transistor fabrication 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). After fabricating the transistors on Si/SiOâ‚‚ wafers, they applied high currents through the circuit, effectively destroying the metallic CNTs and achieving a semiconductor purity of over 99.99%. This process significantly improves the reliability and performance of the transistors.
Meanwhile, another approach using coating techniques has also shown promising results. NEC, in collaboration with the Technical Research Alliance and the University of Tokyo, has developed CNT transistors that operate at frequencies above 500 kHz. If used in a CNT-based computer, this could bring performance up to the level of Intel's early microprocessors like the 4004 and 8008 from the 1970s.
A major breakthrough in this research involves the use of the "Super Inkjet (SIJ)" method to create ultra-fine electrodes. This technique reduced electrode width from about 50 μm to just 2 μm, greatly minimizing gate overlap and parasitic capacitance. As a result, the operating frequency was significantly increased.
Another advancement comes from improved electrophoresis techniques for separating metallic and semiconducting CNTs. NEC’s previous method achieved a purity of 95%, but residual impurities from dispersants limited performance. Now, with a purity of over 98%, the amount of residue has been reduced by 50 times, leading to a tenfold increase in output current—rising from -0.6 μA to -6.2 μA.
These developments mark significant progress in making CNT transistors viable for future computing applications, paving the way for more efficient and powerful electronic devices.
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