Effect of external magnetic field on the instability of THz plasma waves in nanoscale graphene field-effect transistors

The instability of plasma waves in the channel of field-effect transistors will cause the electromagnetic waves with THz frequency.Based on a self-consistent quantum hydrodynamic model,the instability of THz plasmas waves in the channel of graphene field-effect transistors has been investigated with external magnetic field and quantum effects.We analyzed the influence of weak magnetic fields,quantum effects,device size,and temperature on the instability of plasma waves under asymmetric boundary conditions numerically.The results show that the magnetic fields,quantum effects,and the thickness of the dielectric layer between the gate and the channel can increase the radiation frequency.Additionally,we observed that increase in temperature leads to a decrease in both oscillation frequency and instability increment.The numerical results and accompanying images obtained from our simulations provide support for the above conclusions.

High-Performance Organic Field-Effect Transistors Based on Two-Dimensional Vat Orange 3 Crystals

The exploration and research of low-cost,environmentally friendly,and sustainable organic semiconductor materials are of immense significance in various fields,including electronics,optoelectronics,and energy conversion.Unfortunately,these semiconductors have almost poor charge transport properties,which range from∼10^(−4) cm^(2)·V^(−1)·s^(−1) to∼10^(−2) cm^(2)·V^(−1)·s^(−1).Vat orange 3,as one of these organic semiconductors,has great potential due to its highly conjugated structure.We obtain high-quality multilayered Vat orange 3 crystals with two-dimensional(2D)growth on h-BN surfaces with thickness of 10–100 nm using physical vapor transport.Raman’s results confirm the stability of the chemical structure of Vat orange 3 during growth.Furthermore,by leveraging the structural advantages of 2D materials,an organic field-effect transistor with a 2D vdW vertical heterostructure is further realized with h-BN encapsulation and multilayered graphene contact electrodes,resulting in an excellent transistor performance with On/Off ratio of 104 and high field-effect mobility of 0.14 cm^(2)·V^(−1)·s^(−1).Our results show the great potential of Vat orange 3 with 2D structures in future nano-electronic applications.Furthermore,we showcase an approach that integrates organic semiconductors with 2D materials,aiming to offer new insights into the study of organic semiconductors.

Effect of spin on the instability of THz plasma waves in field-effect transistors under non-ideal boundary conditions

Terahertz(THz) radiation can be generated due to the instability of THz plasma waves in field-effect transistors(FETs). In this work, we discuss the instability of THz plasma waves in the channel of FETs with spin and quantum effects under non-ideal boundary conditions. We obtain a linear dispersion relation by using the hydrodynamic equation, Maxwell equation and spin equation. The influence of source capacitance, drain capacitance, spin effects, quantum effects and channel width on the instability of THz plasma waves under the non-ideal boundary conditions is investigated in great detail. The results of numerical simulation show that the THz plasma wave is unstable when the drain capacitance is smaller than the source capacitance;the oscillation frequency with asymmetric boundary conditions is smaller than that under non-ideal boundary conditions;the instability gain of THz plasma waves becomes lower under non-ideal boundary conditions. This finding provides a new idea for finding efficient THz radiation sources and opens up a new mechanism for the development of THz technology.

Novel GaN-based double-channel p-heterostructure field-effect transistors with a p-GaN insertion layer

GaN-based p-channel heterostructure field-effect transistors(p-HFETs)face significant constraints on on-state currents compared with n-channel high electron mobility transistors.In this work,we propose a novel double heterostructure which introduces an additional p-GaN insertion layer into traditional p-HFETs.The impact of the device structure on the hole densities and valence band energies of both the upper and lower channels is analyzed by using Silvaco TACD simulations,including the thickness of the upper AlGaN layer and the doping impurities and concentration in the GaN buffer layer,as well as the thickness and Mg-doping concentration in the p-GaN insertion layer.With the help of the p-GaN insertion layer,the C-doping concentration in the GaN buffer layer can be reduced,while the density of the two-dimensional hole gas in the lower channel is enhanced at the same time.This work suggests that a double heterostructure with a p-GaN insertion layer is a better approach to improve p-HFETs compared with those devices with C-doped buffer layer alone.

P-type cold-source field-effect transistors with TcX_(2) and ReX_(2)(X=S,Se)cold source electrodes:A computational study

Cold-source field-effect transistors(CS-FETs)have been developed to overcome the major challenge of power dissipation in modern integrated circuits.Cold metals suitable for n-type CS-FETs have been proposed as the ideal electrode to filter the high-energy electrons and break the thermal limit on subthreshold swing(SS).In this work,regarding the p-type CS-FETs,we propose TcX_(2) and ReX_(2)(X=S,Se)as the injection source to realize the sub-thermal switching for holes.First-principles calculations unveils the cold-metal characteristics of monolayer TcX_(2) and ReX_(2),possessing a sub-gap below the Fermi level and a decreasing DOS with energy.Quantum device simulations demonstrate that TcX_(2) and ReX_(2) can enable the cold source effects in WSe_(2) p-type FETs,achieving steep SS of 29-38 mV/dec and high on/off ratios of(2.3-5.6)×10^(7).Moreover,multilayer Re S2retains the cold metal characteristic,thus ensuring similar CS-FET performances to that of the monolayer source.This work underlines the significance of cold metals for the design of p-type CS-FETs.

A non-quasi-static model for nanowire gate-all-around tunneling field-effect transistors

Nanowires with gate-all-around(GAA) structures are widely considered as the most promising candidate for 3-nm technology with the best ability of suppressing the short channel effects,and tunneling field effect transistors(TFETs)based on GAA structures also present improved performance.In this paper,a non-quasi-static(NQS) device model is developed for nanowire GAA TFETs.The model can predict the transient current and capacitance varying with operation frequency,which is beyond the ability of the quasi-static(QS) model published before.Excellent agreements between the model results and numerical simulations are obtained.Moreover,the NQS model is derived from the published QS model including the current-voltage(I-V) and capacitance-voltage(C-V) characteristics.Therefore,the NQS model is compatible with the QS model for giving comprehensive understanding of GAA TFETs and would be helpful for further study of TFET circuits based on nanowire GAA structure.

Field-effect transistors based on two-dimensional materials for logic applications

Field-effect transistors （FETs） for logic applications, graphene and MoS2, are discussed. These materials have based on two representative two-dimensional （2D） materials, drastically different properties and require different consider- ations. The unique band structure of graphene necessitates engineering of the Dirac point, including the opening of the bandgap, the doping and the interface, before the graphene can be used in logic applications. On the other hand, MoS2 is a semiconductor, and its electron transport depends heavily on the surface properties, the number of layers, and the carrier density. Finally, we discuss the prospects for the future developments in 2D material transistors.

Unique Characteristics of Vertical Carbon Nanotube Field-effect Transistors on Silicon

A vertical carbon nanotube field-effect transistor(CNTFET) based on silicon(Si) substrate has been proposed and simulated using a semi-classical theory. A single-walled carbon nanotube(SWNT) and an n-type Si nanowire in series construct the channel of the transistor. The CNTFET presents ambipolar characteristics at positive drain voltage(Vd) and n-type characteristics at negative Vd. The current is significantly influenced by the doping level of n-Si and the SWNT band gap. The n-branch current of the ambipolar characteristics increases with increasing doping level of the n-Si while the p-branch current decreases. The SWNT band gap has the same influence on the p-branch current at a positive Vd and n-type characteristics at negative Vd. The lower the SWNT band gap, the higher the current. However, it has no impact on the n-branch current in the ambipolar characteristics. Thick oxide is found to significantly degrade the current and the subthreshold slope of the CNTFETs.

Hot-Carrier Effects on Total Dose Irradiated 65 nm n-Type Metal-Oxide-Semiconductor Field-Effect Transistors

The influence of total dose irradiation on hot-carrier reliability of 65 nm n-type metal-oxide-semiconductor field- effect transistors （nMOSFETs） is investigated. Experimental results show that hot-carrier degradations on ir- radiated narrow channel nMOSFETs are greater than those without irradiation. The reason is attributed to radiation-induced charge trapping in shallow trench isolation （STI）. The electric field in the pinch-off region of the nMOSFET is enhanced by radiation-induced charge trapping in STI, resulting in a more severe hot-carrier effect.

Evaluation of a gate-first process for AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with low ohmic annealing temperature

In this paper, TiN/A1Ox gated A1GaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors （MOS- HFETs） were fabricated for gate-first process evaluation. By employing a low temperature ohmic process, ohmic contact can be obtained by annealing at 600 ℃ with the contact resistance approximately 1.6 Ω.mm. The ohmic annealing process also acts as a post-deposition annealing on the oxide film, resulting in good device performance. Those results demonstrated that the TiN/A1Ox gated MOS-HFETs with low temperature ohmic process can be applied for self-aligned gate AIGaN/GaN MOS-HFETs.

Ge Complementary Tunneling Field-Effect Transistors Featuring Dopant Segregated NiGe Source/Drain

Ge complementary tunneling field-effect transistors（TFETs） are fabricated with the NiGe metal source/drain（S/D） structure. The dopant segregation method is employed to form the NiGe/Ge tunneling junctions of sufficiently high Schottky barrier heights. As a result, the Ge p-and n-TFETs exhibit decent electrical properties of large ON-state current and steep sub-threshold slope（S factor）. Especially, I_d of 0.2 μA/μm is revealed at V_g-V_（th） = V_d = ±0.5 V for Ge pTFETs,with the S factor of 28 mV/dec at 7 K.

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors （GFETs） fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density I DS, improved transconductance g m, reduced sheet resistance lion, and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasifree-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

Fabrication and Characterization of 1700 V 4H-SiC Vertical Double-Implanted Metal-Oxide-Semiconductor Field-Effect Transistors

The fabrication and characterization of 1700 V 7 A 4H-SiC vertical double-implanted metal-oxide-semiconductor field-effect transistors （VDMOSFETs） are reported. The drift layer is 17μm in thickness with 5 × 10^15 cm^-3 n-type doping, and the channel length is 1μm. The MOSFETs show a peak mobility of 17cm2/V.s and a typical threshold voltage of 3 V. The active area of 0.028cm2 delivers a forward drain current of 7A at Vcs = 22 V and VDS= 15 V. The specific on-resistance （Ron,sv） is 18mΩ.cm2 at VGS= 22 V and the blocking voltage is 1975 V （IDS 〈 lOOnA） at VGS = 0 V.

Thickness dependence of surface morphology and charge carrier mobility in organic field-effect transistors

With the aim of understanding the relationships between organic small molecule field-effect transistors （FETs） and organic conjugated polymer FETs, we investigate the thickness dependence of surface morphology and charge carrier mobility in pentacene and regioregular poly （3-hexylthiophene） （RR-P3HT） field-effect transistors. On the basis of the results of surface morphologies and electrical properties, we presume that the charge carrier mobility is largely related to the morphology of the organic active layer. We observe that the change trends of the surface morphologies （average size and average roughness） of pentacene and RR-P3HT thin films are mutually opposite, as the thickness of the organic layer increases. Further, we demonstrate that the change trends of the field-effect mobilities of pentacene and RR-P3HT FETs are also opposite to each other, as the thickness of the organic layer increases within its limit.

The study on mechanism and model of negative bias temperature instability degradation in P-channel metal-oxide-semiconductor field-effect transistors

Negative Bias Temperature Instability （NBTI） has become one of the most serious reliability problems of metaloxide-semiconductor field-effect transistors （MOSFETs）. The degradation mechanism and model of NBTI are studied in this paper. From the experimental results, the exponential value 0.25-0.5 which represents the relation of NBTI degradation and stress time is obtained. Based on the experimental results and existing model, the reaction-diffusion model with H^＋ related species generated is deduced, and the exponent 0.5 is obtained. The results suggest that there should be H^＋ generated in the NBTI degradation. With the real time method, the degradation with an exponent 0.5 appears clearly in drain current shift during the first seconds of stress and then verifies that H^＋ generated during NBTI stress.

Fabrication and characterization of the normally-off N-channel lateral 4H–SiC metal–oxide–semiconductor field-effect transistors

In this paper, the normally-off N-channel lateral 4H-SiC metal-oxide-semiconductor field-effect transistors （MOSF- FETs） have been fabricated and characterized. A sandwich- （nitridation-oxidation-nitridation） type process was used to grow the gate dielectric film to obtain high channel mobility. The interface properties of 4H-SiC/SiO2 were examined by the measurement of HF l-V, G-V, and C-V over a range of frequencies. The ideal C-V curve with little hysteresis and the frequency dispersion were observed. As a result, the interface state density near the conduction band edge of 4H-SiC was reduced to 2 x 1011 eV-l.cm-2, the breakdown field of the grown oxides was about 9.8 MV/cm, the median peak field- effect mobility is about 32.5 cm2.V-1 .s-1, and the maximum peak field-effect mobility of 38 cm2-V-1 .s-1 was achieved in fabricated lateral 4H-SiC MOSFFETs.

Vacuum relaxation and annealing-induced enhancement of mobility of regioregular poly(3-hexylthiophene) field-effect transistors

In order to enhance the performance of regioregular poly（3-hexylthiophene） （RR-P3HT） field-effect transistors （FETs）, RR-P3HT FETs are prepared by the spin-coating method followed by vacuum placement and annealing. This paper reports that the crystal structure, the molecule interconnection, the surface morphology, and the charge carrier mobility of RR-P3HT films are affected by vacuum relaxation and annealing. The results reveal that the field-effect mobility of RR-P3HT FETs can reach 4.17×10^-2m^2/（V·s） by vacuum relaxation at room temperature due to an enhanced local self-organization. Furthermore, it reports that an appropriate annealing temperature can facilitate the crystal structure, the orientation and the interconnection of polymer molecules. These results show that the field-effect mobility of device annealed at 150 ℃ for 10 minutes in vacuum at atmosphere and followed by placement for 20 hours in vacuum at room temperature is enhanced dramatically to 9.00×10^-2m^2/（V·s）.

Lateral depletion-mode 4H-SiC n-channel junction field-effect transistors operational at 400℃

This paper presents the development of lateral depletion-mode n-channel 4 H-SiC junction field-effect transistors(LJFETs)using double-mesa process toward high-temperature integrated circuit(IC)applications.At room temperature,the fabricated LJFETs show a drain-to-source saturation current of 23.03μA/μm,which corresponds to a current density of 7678 A/cm^(2).The gate-to-source parasitic resistance of 17.56 kΩ·μm is reduced to contribute only 13.49%of the on-resistance of 130.15 kΩ·μm,which helps to improve the transconductance up to 8.61μS/μm.High temperature characteristics of LJFETs were performed from room temperature to 400℃.At temperatures up to 400℃in air,it is observed that the fabricated LJFETs still show normally-on operating characteristics.The drain-to-source saturation current,transconductance and intrinsic gain at 400℃are 7.47μA/μm,2.35μS/μm and 41.35,respectively.These results show significant improvement over state-of-the-art and make them attractive for high-temperature IC applications.

Ge/Si heterojunction L-shape tunnel field-effect transistors with hetero-gate-dielectric

A Ge/Si heterojunction L-shaped tunnel field-effect transistor combined with hetero-gate-dielectric （GHL-TFET） is proposed and investigated by TCAD simulation. Current-voltage characteristics, energy-band diagrams, and the distri- bution of the band-to-band tunneling （BTBT） generation rate of GHL-TFET are analyzed. In addition, the effect of the vertical channel width on the ON-current is studied and the thickness of the gate dielectric is optimized for better suppression of ambipolar current. Moreover, analog/RF figure-of-merits of GHL-TFET are also investigated in terms of the cut-off frequency and gain bandwidth production. Simulation results indicate that the ON-current of GHL-TFET is increased by about three orders of magnitude compared with that of the conventional L-shaped TFET. Besides, the introduction of the hetero-gate-dielectric not only suppresses the ambipolar current effectively but also improves the analog/RF performance drastically. It is demonstrated that the maximum cut-off frequency of GHL-TFET is about 160 GHz, which is 20 times higher than that of the conventional L-shaped TFET.

Improved performance of 4H-SiC metal-semiconductor field-effect transistors with step p-buffer layer

An improved 4H-SiC metal-semiconductor field-effect transistors （MESFETs） with step p-buffer layer is proposed, and the static and dynamic electrical performances are analysed in this paper. A step p-buffer layer has been applied not only to increase the channel current, but also to improve the transconductance. This is due to the fact that the variation in p-buffer layer depth leads to the decrease in parasitic series resistance resulting from the change in the active channel thickness and modulation in the electric field distribution inside the channel. Detailed numerical simulations demonstrate that the saturation drain current and the maximum theoretical output power density of the proposed structure are about 30% and 37% larger than those of the conventional structure. The cut-off frequency and the maximum oscillation frequency of the proposed MESFETs are 14.5 and 62 GHz, respectively, which are higher than that of the conventional structure. Therefore, the 4H-SiC MESFETs with step p-buffer layer have superior direct-current and radio-frequency performances compared to the similar devices based on the conventional structure.