Li Ding1,Lian-Mao Peng1
Peking Univ1
Carbon nanotube (CNT) is considered a promising material for building radiofrequency (RF) field-effect transistors (FET) up to terahertz frequencies. This is because CNT has extremely high carrier mobility and saturation velocity, as well as ultra-small intrinsic gate capacitance. Earlier studies on the electronic properties of CNT FETs used individual semiconducting CNTs. But FETs built on individual CNTs can provide at best tens uA which is too low for real applications and the output impedance of the device cannot match the conventional 50 Ω RF measurement standards. Chemical vapor deposition (CVD) based CNT arrays with high-quality were also extensively examined for building RF devices. While promising results were achieved, for example on frequency doublers and mixers, the performance of devices based on CVD arrays of CNTs is severely limited by the low array density and material purity.<br/><br/>Currently, most research efforts are focused on using solution-derived CNTs for preparing thin films. Before 2020, only randomly oriented network of CNTs were widely available for research, and in few cases parallel arrays of CNTs were reported but with limited purity of 99.99% and density of lower than 100 CNT/um. Steady progress on RF performance had been made using these network and low-purity and density arrays, and in particular researchers in Carbonics Inc. pushed the current-gain and power-gain cut-off frequencies of CNT RF FETs to f<sub>T</sub>=75GHz and f<sub>max</sub>=102 GHz using CNT arrays; and researcher from Peking University achieved f<sub>T</sub>=103GHz and f<sub>max</sub>=107 GHz using network of CNTs. An important breakthrough was made in 2020 by reearchers from Peking University, they first developed a multiple-dispersion and sorting process that resulted in extremely high semiconducting purity of better than 99.9999%; and then a dimension-limited self-alignment procedure for preparing well aligned CNT arrays with a tunable array density of 100 to 200 CNTs/um. Top-gate FETs fabricated on the so prepared CNT arrays show a current-gain and power-gain cut-off frequencies of 540 GHz and 306 GHz respectively.<br/><br/>In addition to the channel material, the choice of substrate is also of crucial important for achieving the ultimate performance limit of CNT based RF FETs. The substrate may affect the RF performance of a FET via dielectric permittivity for high speed and dielectric strength, and thermal conductivity for high power. Among all insulating materials, diamond is of particular interest for carbon-based electronics because it is composed of the same carbon element as CNT and has suitable low dielectric permittivity (5.5), strong dielectric strength (1000 kV/cm) and high thermal conductivity (2000 W/mK). Diamond substrate is thus the most suitable substrate for CNT RF devices. Our preliminary results show that we can indeed fabricate well-aligned CNT array-based RF devices on diamond and achieve respectively 245 and 200 GHz of current-gain and power-gain cut-off frequencies. These values are still lower that that achieved using silicon as the substrate, which might be due to the process divergence derived from different substrates. And the device fabrication processes needs to be further optimized on diamond substrate. It is expected that significant improvements on both speed and power could be achieved in RF devices based on aligned CNT arrays on diamond substrate, further pushing the limit into the ballistic terahertz regime in the near future.