Aligned Carbon Nanotube Arrays with Germanium Protective Layers for Improving the Performance of Radio Frequency Transistors

Abstract

Aligned carbon nanotube arrays with high semiconducting purity and high density have been regarded as an ideal channel material for constructing high-frequency field effect transistors and high-speed integrated circuits. However, contaminants (mainly resist residues) that leave the surface of carbon nanotubes during the fabrication process seriously deteriorate the interface contact quality, which will trigger large contact resistance and thus inhibit the ultimate performance of the carbon nanotube-based transistors. Here, we demonstrate a germanium protective layer technique that can keep a clean surface of carbon nanotube films from the contaminants during device fabrication and thus boost the quality of ohmic contact between carbon nanotubes (CNTs) and metal. The transfer length method (TLM) is used to evaluate the effect of the germanium protective layer technique on the ohmic contact resistance. The contact resistance from metal–CNTs extracted using the TLM was reduced by 93%, from 1.66 to 0.123 KΩ μm before and after the germanium process treatment. Owing to the great improvement of contact between metal and CNTs by introducing the germanium protective layer technique, the fabricated aligned CNT array-based radio-frequency field-effect transistors with a gate length of 110 nm on the quartz substrate present excellent DC performance, with an on-state current of 1.28 mA/μm and maximum transconductance of up to 1.05 mS/μm at a bias of −1.2 V. Meanwhile, the as-measured current gain cut-off frequency (fT,EXT) and power gain frequency (fMAX,EXT) increase from 12 to 110 GHz and 13 to 105 GHz, respectively. This approach provides a simple and reproducible means of improving the interface quality and reducing contact resistance in CNT-based devices to enhance performance.