Improving Carbon Nanotube-Based Radiofrequency Field-Effect Transistors by the Device Architecture and Doping Process

The semiconducting carbon nanotube (CNT) has been considered a promising candidate for future radiofrequency (RF) electronics due to its excellent electrical properties of high mobility and small capacitance. After decades of development, great progress has been achieved on CNT-based RF field-effect transistors (FETs). However, almost all elevations are owing to advancement of the CNT materials and fabrication process, while the study of device architecture is seldom considered and reported. In this work, we innovatively combined device architecture and related doping processes to further optimize CNT-based RF FETs by guiding process or materials with collaborative optimization for the first time and explore their effect on device performance carefully and statistically. Based on more mature random-oriented CNT materials, we fabricated CNT-based RF FETs having three different gate positions of device architecture variation accompanied by suitable doping schemes. The optimized FETs obtained 2-3 times of current density (transconductance) and 1.3 times the cutoff frequency and maximum oscillation frequency compared with unoptimized devices at the same channel length. After transistor-level verification of effect, we further built a CNT RF amplifier and demonstrated almost 10 dB of transducer gain improvement operating at 8 GHz for X-band application. The achieved results from this work would help further improve CNT RF performance beyond the materials and process point of view.

Aligned Carbon Nanotubes-Based Radiofrequency Transistors for Amplitude Amplification and Frequency Conversion at Millimeter Wave Band

Aligned carbon nanotubes (ACNTs) have been considered as a promising candidate semiconductor with great potential in radiofrequency (RF) electronics due to their high carrier mobility/saturation velocity and small intrinsic capacitance. However, almost all of previously reported works focused on only the cutoff frequency, which is far from enough for practical RF application. In this work, given the speed advantage of ACNTs, we further explore amplitude amplification and frequency conversion capability of ACNTs based RF devices simultaneously, which are two basic functions in RF electronics. Considering there is no de-embedding process for amplification/conversion and reduction power loss, multifinger configuration RF transistors (still having current density around 1 mA/μm) were fabricated with cutoff frequency and maximum oscillation frequency exceeding 150 and 130 GHz, respectively. Based on dedicated ACNTs based RF FETs, we demonstrate almost 7 dB power gain (S₂₁) with over 40 GHz 3-dB bandwidth for amplification and from -12.7 to -17 dB of conversion gain with over 25 dBm IIP3 (input third-order intercept point) of linearity for conversion simultaneously operating at 30 GHz in millimeter wave (mmWave) band both without any tuning instruments and matching technology assistance. The performance achieved here is the best among all the nanomaterials at the mmWave band.

Performance enhancement of charge plasma-based junctionless TFET (JL-TFET) using stimulated n-pocket and heterogeneous gate dielectric

Junctionless tunneling field-effect transistor (JL-TFET) is an excellent potential alternative to conventional MOSFET and TFET due to the lack of a steep doping profile, which makes it easier to fabricate. JL-TFET not only offers a lower subthreshold swing (SS) compared to MOSFET, but mitigates the low on-current problem associated with conventional TFET. The DC and analog characteristics of JL-TFET can be further improved by design modifications. In this research, we have presented two novel structures of JL-TFET: stimulated n-pocket JL-TFET (SNPJL-TFET) and SNPJL-TFET with heterogeneous gate dielectric. The performance of these devices has been gauged against conventional JL-TFET. Both novel structures exhibit excellent performance including point SS around 20 mV/dec, highI_ON/I_OFFin the order of 10¹⁴and lower threshold voltage (V_T). By analyzing RF and linearity parameters such as the transconductance generation factor,F_T, transit time, total factor productivity, second-order voltage intercept point, third-order voltage intercept point, third-order input intercept point and third-order intermodulation distortion, it is observed that the proposed devices are more suitable for RF applications since they show superiority in most of the analyzed parameters.

Coherent Electron Transport in Air-Stable, Printed Single-Crystal Organic Semiconductor and Application to Megahertz Transistors

Organic semiconductors (OSCs) have attracted growing attention for optoelectronic applications such as field-effect transistors (FETs), and coherent (or band-like) carrier transport properties in OSC single crystals (SCs) have been of interest as they can lead to high carrier mobilities. Recently, such p-type OSC SCs compatible with a printing technology have been used to achieve high-speed FETs; therefore, developments of n-type counterparts may be promising for realizing high-speed complementary organic circuits. Herein, coherent electron transport properties in a printed SC of a state-of-the-art, air-stable n-type OSC, PhC₂ -BQQDI, by means of variable-temperature gated Hall effect measurements and X-ray single-crystal diffraction analyses in conjunction with band structure calculations, are reported. Furthermore, the SC FET is tested for high-speed operations, which obtains a cutoff frequency of 4.3 MHz at an operation voltage of 20 V in air. Thus, PhC₂ -BQQDI is shown as a new candidate for practical applications of SC-based, organic complementary devices.

Carbon Nanotube Film-Based Radio Frequency Transistors with Maximum Oscillation Frequency above 100 GHz

Carbon nanotubes (CNTs) have been considered a preferred channel material for constructing high-performance radio frequency (RF) transistors with outstanding current gain cutoff frequency (f_T) and power gain cutoff frequency (f_max) but the highest reported f_max is only 70 GHz. Here, we explore how good RF transistors based on solution-derived randomly oriented semiconducting CNT films, which are the most mature CNT materials for scalable fabrication of transistors and integrated circuits, can be achieved. Owing to the significantly reduced number of CNT/CNT junctions obtained by scaling the channel length down to below 100 nm, we realized RF field-effect transistors (FETs) with maximum transconductance G_m up to 0.38 mS/μm, which is the record among CNT-based RF FETs. After de-embedding the pad-induced capacitances and resistances, the CNT FETs with different gate lengths (L_g) exhibit f_T as high as 103 GHz (intrinsically 281 GHz) or f_max up to 107 GHz (intrinsically 190 GHz), which are the records among CNT-based RF FETs. In particular, the CNT FETs with an L_g of 50 nm present pad de-embedding f_T of 86 GHz and f_max of 85 GHz, and represent the best CNT RF transistor in terms of comprehensive performance to date. To demonstrate the actual high-speed and scalable fabrication of our CNT RF FETs, we fabricated CNT FET-based five-stage ring oscillators with oscillation frequencies above 5 GHz.

High-Performance Organic Vertical Thin Film Transistor Using Graphene as a Tunable Contact

Here we present a general strategy for the fabrication of high-performance organic vertical thin film transistors (OVTFTs) based on the heterostructure of graphene and different organic semiconductor thin films. Utilizing the unique tunable work function of graphene, we show that the vertical carrier transport across the graphene-organic semiconductor junction can be effectively modulated to achieve an ON/OFF ratio greater than 10(3). Importantly, with the OVTFT design, the channel length is determined by the organic thin film thickness rather than by lithographic resolution. It can thus readily enable transistors with ultrashort channel lengths (<200 nm) to afford a delivering current greatly exceeding that of conventional planar TFTs, thus enabling a respectable operation frequency (up to 0.4 MHz) while using low-mobility organic semiconductors and low-resolution lithography. With this vertical device architecture, the entire organic channel is sandwiched and naturally protected between the source and drain electrodes, which function as the self-passivation layer to ensure stable operation of both p- and n-type OVTFTs in ambient conditions and enable complementary circuits with voltage gain. The creation of high-performance and highly robust OVTFTs can open up exciting opportunities in large-area organic macroelectronics.

Sub-100 nm channel length graphene transistors

Here we report high-performance sub-100 nm channel length graphene transistors fabricated using a self-aligned approach. The graphene transistors are fabricated using a highly doped GaN nanowire as the local gate with the source and drain electrodes defined through a self-aligned process and the channel length defined by the nanowire size. This fabrication approach allows the preservation of the high carrier mobility in graphene and ensures nearly perfect alignment between source, drain, and gate electrodes. It therefore affords transistor performance not previously possible. Graphene transistors with 45-100 nm channel lengths have been fabricated with the scaled transconductance exceeding 2 mS/μm, comparable to the best performed high electron mobility transistors with similar channel lengths. Analysis of and the device characteristics gives a transit time of 120-220 fs and the projected intrinsic cutoff frequency (f(T)) reaching 700-1400 GHz. This study demonstrates the exciting potential of graphene based electronics in terahertz electronics.