Analog/RF performance assessment of ferroelectric junctionless carbon nanotube FETs: A quantum simulation study
Introduction
Energy-efficient nanoscale field-effect transistors (FETs) are considered promising devices for the futuristic nanoelectronics including the internet-of-things, medical engineering, bioelectronics, integrated circuits, and wearable systems, where the long-term energy management is a primary concern [[1], [2], [3]]. The steep nanotransistors with subthreshold swing (SS) below the Boltzmann limit of 60 mV/dec form an intriguing solution to scaling the power supply voltage (VDD) while keeping the required transistor performance [[4], [5], [6]]. In the last decade, employing the negative capacitance (NC) feature of the ferroelectric (FE) materials in boosting the SS performance of different emerging nanoscale FETs has acquired much attention [1,7]. Moreover, NCFETs are included in many low-power applications, such as modern NCFET-based sensors and biosensors [8] and NCFET-based flexible and wearable devices [1]. NCFET devices can be realized either in the metal-ferroelectric-insulator-semiconductor (MFIS) or the metal-ferroelectric-metal-insulator-semiconductor (MFMIS) fashions [[7], [8], [9], [10], [11]]. Note that the MFMIS configuration is the structure commonly adopted for the experimental configuration of emerging NCFET [10]. Actually, the MFM stack can be externally connected to the baseline FET while providing a kind of flexibility in terms of testing and calibration for a perfect capacitance matching [10]. This feature is beneficial for the fabrication of nanoscale NC FET. In addition, From the simulation and modeling viewpoint, the MFMIS structure is found computationally simple and sound, which allows the computational study of different cutting edge negative capacitance nanotransistors [1,7,11]. On the other hand, in the modern nanoelectronic devices, the intelligent combination of emerging nanomaterials (e.g. carbon nanoribbon and nanotube), improvement techniques, innovative concepts, and new paradigms, is considered as a promising strategy for the multi-objective improvements of different nanodevices [[12], [13], [14], [15], [16]]. In this context, the amazing benefit of carbon nanotube (CNT) and the ferroelectric-based NC have been recently combined to boost the performance of the CNTFETs, where promising finding have been reported through experimental [17] and theoretical works [18]. In the literature, the high analog/RF performance of different CNTFETs have already been reported [14], however with some manufacturing constraints that limit significantly the targeted performance. In this context, new designs, approaches, and improvement strategies that can improve the analog device performance while facilitating the manufacturing processes should be provided in order to give substantial impulses to the progress of analog CNTFETs. To the best of our knowledge, no efforts have been focused on the analog/RF performance of nanoscale NC CNTFETs nor on the role of NC in improving the functionality of CNTFETs with junctionless paradigm, which is very profitable in terms of simplifying the fabrication processes by avoiding the difficulty of realizing abrupt junctions at ultrascaled regime [5,12].
For the first time, this paper investigates the analog/RF performance of a new nanoscale analog NCFET which combines the spectacular transport of CNT-based channel, the benefits of junctionless (JL) concept in terms of fabrication simplicity, the NC-based performance booster, and the gate-all-around for good electrostatic gating. The simulation study utilizes an accurate numerical model based on the non-equilibrium Green's function (NEGF) formalism, which is solved self-consistently with the two-dimensional Poisson's equation and one-dimensional steady-state Landau–Khalatnikov equation considering ballistic limits [18]. The NCJL-CNTFET is endowed with a MFMIS-based electrostatic gating system. The computational assessment includes the transfer and output characteristics, threshold voltage behavior, gate charge, FE-induced electrostatic modulation, the transconductance, drain conductance, transconductance efficiency, gate capacitance, and cut-off frequency. Several distinctive phenomena have been recorded, and some figures of merit have been investigated. The obtained results indicate that the NC junctionless CNTFETs are promising devices for low-power, cost-effective, and high-performance analog/RF applications.
The remaining parts of this paper are as follows. Section 2 presents the device structure. The simulation approach is summarized in Section 3. The computational results and discussions are presented in Section 4. Finally, Section 5 summarizes the main achievements.
Section snippets
Device structure
The GAA structure is a promising asset to well controlling the carrier transport in emerging ultrascaled FETs, where the direct source-to-drain tunneling is the main issue [15]. Therefore, the integration of the ferroelectric material with GAA configuration while combining the coaxial electrostatic gating and the negative capacitance feature of the FE forms an interesting improvement design [18]. Fig. 1 shows a detailed structural description of the NCJL-CNTFET under study considering the 3D
Simulation approach
The nanodevices under investigation are simulated by combining the self-consistent solutions of the computational couple NEGF-Poisson with the one dimensional (1D) steady-state (S–S) Landau–Khalatnikov (L-K) equation [18,[22], [23], [24], [25], [26], [27]]. As shown in Fig. 2, the simulation procedure can be divided into two parts. First, the baseline CNTFET is simulated via the resolution of NEGF-Poisson couple. The so-called retarded Green's function, G(E), in the matrix form is computed
Results and discussions
Firstly, the adopted simulation approach should be calibrated with some experimental data in order to check its accuracy, predictive performance, and generalization. In the one dimensional steady-state Landau–Khalatnikov equation, we have adopted the measured FE parameters used in Ref. [19] that result the ''S'' curve shown in Fig. 3(a). This latter shows the P-E (polarization versus electric field) curve from these FE parameters (α = −3 × 109 m/F, β = 6 × 1011 m5/F/C2, γ = 0) [19]. Inspecting
Conclusion
In this paper, the analog/RF performance of negative capacitance gate-all-around junctionless carbon nanotube field-effect transistors has been investigated using a quantum simulation approach. The latter includes three computational solvers coupled self-consistently, namely, the non-equilibrium green's function formalism for the CNT charge density calculation, the Poisson solver for evaluating the coaxial device electrostatics, and the Landau–Khalatnikov equation of the ferroelectric. The
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The work of Dr. Khalil Tamersit was supported by the General Directorate for the Scientific Research and Technological Development (DGRSDT—MESRS), Algeria. The first author would like to thank the Laboratory of Inverse Problems, Modeling, Information, and Systems (PIMIS), Guelma University, Guelma, Algeria, for providing computational support.
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