Progress in the development of tunnel field-effect transistors (TFETs) is reviewed by comparing experimental results and theoretical predictions against 16-nm FinFET CMOS technology. Experiments lag the projections, but sub-threshold swings less than 60 mV/decade are now reported in 14 TFETs. The lowest measured sub-threshold swings approaches 20 mV/decade, however, the measurements at these lowest values are not based on many points. The highest current at which sub-threshold swing below 60 mV/decade is observed is in the range 1–10 nA/ ${{\mu }}$ m. A common approach to TFET characterization is proposed to facilitate future comparisons.

Tunnel Field-Effect Transistors: State-of-the-Art

The rapid development of information technology has led to urgent requirements for high efficiency and ultralow power consumption. In the past few decades, neuromorphic computing has drawn extensive attention due to its promising capability in processing massive data with extremely low power consumption. Here, we offer a comprehensive review on emerging artificial neuromorphic devices and their applications. In light of the inner physical processes, we classify the devices into nine major categories and discuss their respective strengths and weaknesses. We will show that anion/cation migration-based memristive devices, phase change, and spintronic synapses have been quite mature and possess excellent stability as a memory device, yet they still suffer from challenges in weight updating linearity and symmetry. Meanwhile, the recently developed electrolyte-gated synaptic transistors have demonstrated outstanding energy efficiency, linearity, and symmetry, but their stability and scalability still need to be optimized. Other emerging synaptic structures, such as ferroelectric, metal–insulator transition based, photonic, and purely electronic devices also have limitations in some aspects, therefore leading to the need for further developing high-performance synaptic devices. Additional efforts are also demanded to enhance the functionality of artificial neurons while maintaining a relatively low cost in area and power, and it will be of significance to explore the intrinsic neuronal stochasticity in computing and optimize their driving capability, etc. Finally, by looking into the correlations between the operation mechanisms, material systems, device structures, and performance, we provide clues to future material selections, device designs, and integrations for artificial synapses and neurons.

A comprehensive review on emerging artificial neuromorphic devices

The tunnel field-effect transistor (TFET) is considered a future transistor option due to its steep-slope prospects and the resulting advantages in operating at low supply voltage ( $\mathrm{V}_{\rm DD}$ ). In this paper, using atomistic quantum models that are in agreement with experimental TFET devices, we are reviewing TFETs prospects at $\mathrm{L}_{\rm G}= 13$ nm node together with the main challenges and benefits of its implementation. Significant power savings at iso-performance to CMOS are shown for GaSb/InAs TFET, but only for performance targets which use lower than conventional $\mathrm{V}_{\rm DD}$ . Also, P-TFET current-drive is between $1\times $ to $0.5\times $ of N-TFET, depending on choice of $\mathrm{I}_{\rm OFF}$ and $\mathrm{V}_{\rm DD}$ . There are many challenges to realizing TFETs in products, such as the requirement of high quality III–V materials and oxides with very thin body dimensions, and the TFET’s layout density and reliability issues due to its source/drain asymmetry. Yet, extremely parallelizable products, such as graphics cores, show the prospect of longer battery life at a cost of some chip area.

Tunnel Field-Effect Transistors: Prospects and Challenges

The discovery of ferroelectricity in oxides that are compatible with modern semiconductor manufacturing processes, such as hafnium oxide, has led to a re-emergence of the ferroelectric field-effect transistor in advanced microelectronics. A ferroelectric field-effect transistor combines a ferroelectric material with a semiconductor in a transistor structure. In doing so, it merges logic and memory functionalities at the single-device level, delivering some of the most pressing hardware-level demands for emerging computing paradigms. Here, we examine the potential of the ferroelectric field-effect transistor technologies in current embedded non-volatile memory applications and future in-memory, biomimetic and alternative computing models. We highlight the material- and device-level challenges involved in high-volume manufacturing in advanced technology nodes (≤10 nm), which are reminiscent of those encountered in the early days of high-K-metal-gate transistor development. We argue that the ferroelectric field-effect transistors can be a key hardware component in the future of computing, providing a new approach to electronics that we term ferroelectronics. This Perspective examines the use of ferroelectric field-effect transistor technologies in current embedded non-volatile memory applications and future in-memory, biomimetic and alternative computing models, arguing that the devices will be a key component in the development of data-centric computing.

The future of ferroelectric field-effect transistor technology

In this letter, a double-gate junctionless tunnel field effect transistor (JL-TFET) is proposed and investigated. The JL-TFET is a Si-channel heavily n-type-doped junctionless field effect transistor (JLFET), which uses two isolated gates (Control-Gate, P-Gate) with two different metal work-functions to behave like a tunnel field effect transistor (TFET). In this structure, the advantages of JLFET and TFET are combined together. The simulation results of JL-TFET with high- $k$ dielectric material (TiO2) of 20-nm gate length shows excellent characteristics with high $I_{{\rm ON}}/I_{{\rm OFF}}$ ratio $(\sim 6\times 10^{8})$ , a point subthreshold slope (SS) of ${\sim}{\rm 38}~{\rm mV}$ /decade, and an average SS of ${\sim}{\rm 70}~{\rm mV}$ /decade at room temperature, which indicates that JL-TFET is a promising candidate for a switching performance.

Junctionless Tunnel Field Effect Transistor

In this paper, a novel junction depleted-modulation design to achieve equivalently abrupt tunnel junction of Si Tunnel FET (TFET) is proposed. By changing the gate layout configuration, the new Junction-modulated TFET can reliably and effectively achieve much steeper switching behavior and higher ON current without area penalty and special fabrication compared with traditional TFET. Further junction optimization by introducing the self-depleted doping pocket with much relaxed process requirements is also experimentally demonstrated based on the bulk Si substrate. With traditional Si CMOS-compatible process, the fabricated device shows a minimum substhreshold slope of 36mV/dec within one decade of drain current.

A novel Si tunnel FET with 36mV/dec subthreshold slope based on junction depleted-modulation through striped gate configuration

In this paper, the impacts of interface traps on tunneling FET (TFET) are examined in terms of different trap energies and distributions, charge neutrality level (CNL), and effects of random trap fluctuation, in comparison with MOSFET. It is found that the Vth shifts and subthreshold swing (SS) degradation induced by interface traps in TFET and MOSFET have the same trends, but the impacts on ION are different because of the novel conduction mechanism of TFETs when compared with MOSFETs. Moreover, nTFET is intrinsically more immune (or susceptible) to Vth shift induced by acceptor(or donor-) type interface traps than nMOSFET. Therefore, reducing the potential degradation induced by the interface traps can be achieved by optimizing the position of CNL. The results indicate that nTFET is more immune to the Vth shift than nMOSFET with CNL below a critical energy. In addition, the trap-induced SS degradation of TFET is severer than MOSFET in electrostatics. Moreover, it is found that the ION, Vth, and IOFF fluctuations in nMOSFET and nTFET are also dependent on the position of CNL. With CNL below the critical energy, the ION fluctuation and Vth fluctuation of nTFET are smaller than those of nMOSFET. The results are helpful for the interface optimization of TFETs.

A Comparative Study on the Impacts of Interface Traps on Tunneling FET and MOSFET

In this paper, an analytical model of the channel surface potential in the tunnel field effect transistors (TFETs) is established and verified. The dual-modulation effects in TFETs that the surface potential of the channel is alternatively controlled by the gate bias and drain bias in different operating regimes are emphasized and studied. The transition point corresponding to the switching between the two operating regimes is also analyzed quantitatively. For the first time, a closed-form analytical model of the surface potential in TFETs, including the impacts of both the gate voltage and drain voltage is proposed. Furthermore, a compact current model of the TFET-based on the derived surface potential expression is given. The model predicted tunneling current agree well with the TCAD simulation results in all operating regions of TFETs, which will be helpful for the circuit properties simulation of the TFET.

An Analytical Surface Potential Model Accounting for the Dual-Modulation Effects in Tunnel FETs

In this paper, a novel heterostacked tunnel FET (HS-TFET) is proposed for steeper average subthreshold swing (SS). Different from conventional TFETs, HS-TFETs owns a stacked source configuration consisting of an upper source layer with a relatively larger bandgap material and an underlying source layer with smaller bandgap materials. Since smaller bandgap materials exhibit much higher band-to-band tunneling efficiency, the underlying layer of HS-TFET could provide extra drain current increment with increasing gate voltage, and thus effectively improve the subthreshold characteristics for steeper average SS. The simulation results show that the proposed Si–Ge-based HS-TFET can achieve much steeper average SS (25 mV/decade) than conventional Si TFET (42 mV/decade), exhibiting more than one decade higher I60 without leakage current degradation.

A Novel Tunnel FET Design With Stacked Source Configuration for Average Subthreshold Swing Reduction

In this paper, a novel TFET design, called Pocket-mSTFET (PMS-TFET), is proposed and experimentally demonstrated by evaluating the performance from device metrics to circuit implementation for low-power SoC applications. For the first time, from circuit design perspective, TFETs performance in terms of I ON , I OFF , subthreshold slope (SS), output behavior, capacitance, delay, noise and gain are experimentally benchmarked and also compared with MOSFET. By gate and source engineering without area penalty, the compatibly-fabricated PMS-TFET on SOI substrate shows superior performance with the minimum SS of 29mV/dec at 300K, high I ON (∼20µA/µm) and large I ON /I OFF ratio (∼108) at 0.6V. Largely alleviated super-linear onset issue, reduced Miller capacitance and delay, and much lower noise level were also experimentally obtained, as well as high effective gain. Circuit-level implementation based on PMS-TFET also shows significant improvement on energy efficiency and power reduction at V DD of 0.4V, which indicates great potential of this TFET design for low-power digital and analog applications.

Qianqian Huang

Papers

A novel Si tunnel FET with 36mV/dec subthreshold slope based on junction depleted-modulation through striped gate configuration

A Comparative Study on the Impacts of Interface Traps on Tunneling FET and MOSFET

An Analytical Surface Potential Model Accounting for the Dual-Modulation Effects in Tunnel FETs

A Novel Tunnel FET Design With Stacked Source Configuration for Average Subthreshold Swing Reduction

Comprehensive performance re-assessment of TFETs with a novel design by gate and source engineering from device/circuit perspective