Anomalous telegraph noise in small-area silicon metal-oxide-semiconductor field-effect transistors

Observation of Rich Defect Dynamics in Monolayer MoS2

Defects play a pivotal role in limiting the performance and reliability of nanoscale devices. Field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors such as monolayer MoS₂ are no exception. Probing defect dynamics in 2D FETs is therefore of significant interest. Here, we present a comprehensive insight into various defect dynamics observed in monolayer MoS₂ FETs at varying gate biases and temperatures. The measured source-to-drain currents exhibit random telegraph signals (RTS) owing to the transfer of charges between the semiconducting channel and individual defects. Based on the modeled temperature and gate bias dependence, oxygen vacancies or aluminum interstitials are probable defect candidates. Several types of RTSs are observed including anomalous RTS and giant RTS indicating local current crowding effects and rich defect dynamics in monolayer MoS₂ FETs. This study explores defect dynamics in large area-grown monolayer MoS₂ with ALD-grown Al₂O₃ as the gate dielectric.

Perspective of 2D Integrated Electronic Circuits: Scientific Pipe Dream or Disruptive Technology?

Within the last decade, considerable efforts have been devoted to fabricating transistors utilizing 2D semiconductors. Also, small circuits consisting of a few transistors have been demonstrated, including inverters, ring oscillators, and static random access memory cells. However, for industrial applications, both time-zero and time-dependent variability in the performance of the transistors appear critical. While time-zero variability is primarily related to immature processing, time-dependent drifts are dominated by charge trapping at defects located at the channel/insulator interface and in the insulator itself, which can substantially degrade the stability of circuits. At the current state of the art, 2D transistors typically exhibit a few orders of magnitude higher trap densities than silicon devices, which considerably increases their time-dependent variability, resulting in stability and yield issues. Here, the stability of currently available 2D electronics is carefully evaluated using circuit simulations to determine the impact of transistor-related issues on the overall circuit performance. The results suggest that while the performance parameters of transistors based on certain material combinations are already getting close to being competitive with Si technologies, a reduction in variability and defect densities is required. Overall, the criteria for parameter variability serve as guidance for evaluating the future development of 2D technologies.

Hardware implementation of Bayesian network based on two-dimensional memtransistors

Bayesian networks (BNs) find widespread application in many real-world probabilistic problems including diagnostics, forecasting, computer vision, etc. The basic computing primitive for BNs is a stochastic bit (s-bit) generator that can control the probability of obtaining '1' in a binary bit-stream. While silicon-based complementary metal-oxide-semiconductor (CMOS) technology can be used for hardware implementation of BNs, the lack of inherent stochasticity makes it area and energy inefficient. On the other hand, memristors and spintronic devices offer inherent stochasticity but lack computing ability beyond simple vector matrix multiplication due to their two-terminal nature and rely on extensive CMOS peripherals for BN implementation, which limits area and energy efficiency. Here, we circumvent these challenges by introducing a hardware platform based on 2D memtransistors. First, we experimentally demonstrate a low-power and compact s-bit generator circuit that exploits cycle-to-cycle fluctuation in the post-programmed conductance state of 2D memtransistors. Next, the s-bit generators are monolithically integrated with 2D memtransistor-based logic gates to implement BNs. Our findings highlight the potential for 2D memtransistor-based integrated circuits for non-von Neumann computing applications.

Random Telegraph Noise in 3D NAND Flash Memories

In this paper, we review the phenomenology of random telegraph noise (RTN) in 3D NAND Flash arrays. The main features of such arrays resulting from their mainstream integration scheme are first discussed, pointing out the relevant role played by the polycrystalline nature of the string silicon channels on current transport. Starting from that, experimental data for RTN in 3D arrays are presented and explained via theoretical and simulation models. The attention is drawn, in particular, to the changes in the RTN dependences on the array working conditions that resulted from the transition from planar to 3D architectures. Such changes are explained by considering the impact of highly-defective grain boundaries on percolative current transport in cell channels in combination with the localized nature of the RTN traps.

Low-Frequency Noise in Graphene Tunnel Junctions

Graphene tunnel junctions are a promising experimental platform for single molecule electronics and biosensing. Ultimately their noise properties will play a critical role in developing these applications. Here we report a study of electrical noise in graphene tunnel junctions fabricated through feedback-controlled electroburning. We observe random telegraph signals characterized by a Lorentzian noise spectrum at cryogenic temperatures (77 K) and a 1/ f noise spectrum at room temperature. To gain insight into the origin of these noise features, we introduce a theoretical model that couples a quantum mechanical tunnel barrier to one or more classical fluctuators. The fluctuators are identified as charge traps in the underlying dielectric, which through random fluctuations in their occupation introduce time-dependent modulations in the electrostatic environment that shift the potential barrier of the junction. Analysis of the experimental results and the tight-binding model indicate that the random trap occupation is governed by Poisson statistics. In the 35 devices measured at room temperature, we observe a 20-60% time-dependent variance of the current, which can be attributed to a relative potential barrier shift of between 6% and 10%. In 10 devices measured at 77 K, we observe a 10% time-dependent variance of the current, which can be attributed to a relative potential barrier shift of between 3% and 4%. Our measurements reveal a high sensitivity of the graphene tunnel junctions to their local electrostatic environment, with observable features of intertrap Coulomb interactions in the distribution of current switching amplitudes.

Characterization of Single Defects in Ultrascaled MoS2 Field-Effect Transistors

MoS₂ has received a lot of attention lately as a semiconducting channel material for electronic devices, in part due to its large band gap as compared to that of other 2D materials. Yet, the performance and reliability of these devices are still severely limited by defects which act as traps for charge carriers, causing severely reduced mobilities, hysteresis, and long-term drift. Despite their importance, these defects are only poorly understood. One fundamental problem in defect characterization is that due to the large defect concentration only the average response to bias changes can be measured. On the basis of such averaged data, a detailed analysis of their properties and identification of particular defect types are difficult. To overcome this limitation, we here characterize single defects on MoS₂ devices by performing measurements on ultrascaled transistors (∼65 × 50 nm) which contain only a few defects. These single defects are characterized electrically at varying gate biases and temperatures. The measured currents contain random telegraph noise, which is due to the transfer of charge between the channel of the transistors and individual defects, visible only due to the large impact of a single elementary charge on the local electrostatics in these small devices. Using hidden Markov models for statistical analysis, we extract the charge capture and emission times of a number of defects. By comparing the bias-dependence of the measured capture and emission times to the prediction of theoretical models, we provide simple rules to distinguish oxide traps from adsorbates on these back-gated devices. In addition, we give simple expressions to estimate the vertical and energetic positions of the defects. Using the methods presented in this work, it is possible to locate the sources of performance and reliability limitations in 2D devices and to probe defect distributions in oxide materials with 2D channel materials.

Local field effect on charge-capture/emission dynamics

Charge-capture/emission is ubiquitous in electron devices. Its dynamics often play critical roles in device operation and reliability. Treatment of this basic process is found in many text books and is considered well understood. As in many electron device models, the individuality of immobile charge is commonly replaced with the average quantity of charge density. This has worked remarkably well when large numbers of individual charges (ensemble) are involved. As device geometries become very small, the ensemble "averaging" becomes far less accurate. In this work, the charge-capture/emission dynamic of Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET) is re-examined with full consideration of individual charges and the local field in their immediate vicinity. A dramatic modification of the local band diagram resulted, forcing a drastic change in emission mechanism. The implication is that many well-understood phenomena involving charge capture/emission will need to be reconsidered. As an example, this new picture is applied to the random telegraph noise (RTN) phenomenon. When the screening of a trapped charge by a polar medium such as SiO₂ is quantitatively accounted for in this local field picture, a new physically sound RTN emission mechanism emerges. Similarly, the dynamics of post-stress recovery of Negative-Bias-Instability of p-channel MOSFET can be more rationally explained.

Room-temperature discrete-charge-fluctuation dynamics of a single molecule adsorbed on a carbon nanotube

Detection and use of physical noise fluctuations in a signal provides significant advantages in the development of bio- and neuro-sensing and functional mimicking devices. Low-dimensional carbon nanomaterials are a good candidate for use in noise generation due to the high surface sensitivity of these materials, which may themselves serve as the main building blocks of these devices. Here, we demonstrate that the addition of a molecule with high redox activity to a carbon nanotube (CNT) field-effect transistor provides tunable current fluctuation noise. A unique charge-trap state in the vicinity of the CNT surface due to the presence of the single molecule is the origin of the noise, which generates a prominent and unique slow discrete random telegraph signal in the device current. The power spectral density reveals the peculiar frequency limit of the fluctuation for different types of molecules depending on their redox activity and adsorption configuration. These results indicate that the detected noise will provide new opportunities to obtain electronic information for a single molecule combined with a nanotube surface, and that controllability of the noise may contribute to the expansion of noise utilization in future bio-inspired devices.

Anomalous random telegraph noise in nanoscale transistors as direct evidence of two metastable states of oxide traps

In this paper, a new pattern of anomalous random telegraph noise (RTN), named “reversal RTN” (rRTN) induced by single oxide trap, is observed in the drain current of nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs) with high-k gate dielectrics. Under each gate voltage, the rRTN data exhibit two zones with identical amplitudes but reversal time constants. This abnormal switching behavior can be explained by the theory of complete 4-state trap model (with two stable states and two metastable states), rather than the simple 2-state or improved 3-state trap model. The results provide a direct experimental evidence of the existence of two metastable states in a single oxide trap, contributing to the comprehensive understanding of trap-related reliability and variability issues in nanoscale transistors.

Role of hydrogen in volatile behaviour of defects in SiO2-based electronic devices

Charge capture and emission by point defects in gate oxides of metal-oxide-semiconductor field-effect transistors (MOSFETs) strongly affect reliability and performance of electronic devices. Recent advances in experimental techniques used for probing defect properties have led to new insights into their characteristics. In particular, these experimental data show a repeated dis- and reappearance (the so-called volatility) of the defect-related signals. We use multiscale modelling to explain the charge capture and emission as well as defect volatility in amorphous SiO2 gate dielectrics. We first briefly discuss the recent experimental results and use a multiphonon charge capture model to describe the charge-trapping behaviour of defects in silicon-based MOSFETs. We then link this model to ab initio calculations that investigate the three most promising defect candidates. Statistical distributions of defect characteristics obtained from ab initio calculations in amorphous SiO2 are compared with the experimentally measured statistical properties of charge traps. This allows us to suggest an atomistic mechanism to explain the experimentally observed volatile behaviour of defects. We conclude that the hydroxyl-E' centre is a promising candidate to explain all the observed features, including defect volatility.

Charge Blinking Statistics of Semiconductor Nanocrystals Revealed by Carbon Nanotube Single Charge Sensors

We demonstrate the relation between the optical blinking of colloidal semiconductor nanocrystals (NCs) and their electrical charge blinking for which we provide the first experimental observation of power-law statistics. To show this, we harness the performance of CdSe/ZnS NCs coupled with carbon nanotube field-effect transistors (CNTFETs), which act as single charge-sensitive electrometers with submillisecond time resolution, at room temperature. A random telegraph signal (RTS) associated with the NC single-trap charging is observed and exhibits power-law temporal statistics (τ(-α), with α in the range of ∼1-3), and a Lorentzian current noise power spectrum with a well-defined 1/f(2) corner. The spectroscopic analysis of the NC-CNTFET devices is consistent with the charging of NC defect states with a charging energy of Ec ≥ 200 meV. These results pave the way for a deeper understanding of the physics and technology of nanocrystal-based optoelectronic devices.

Analysis of photogenerated random telegraph signal in single electron detector (photo-SET)

In this paper, we analyzed slow single traps, situated inside the tunnel oxide of small area single electron photo-detector (photo-SET or nanopixel). The relationship between excitation signal (photons) and random-telegraph-signal (RTS) was evidenced. We demonstrated that photoinduced RTS observed on a photo-detector is due to the interaction between single photogenerated charges that tunnel from dot to dot and current path. Based on RTS analysis for various temperatures, gate bias and optical power we determined the characteristics of these single photogenerated traps: the energy position within the silicon bandgap, capture cross section and the position within the Si/SiO(x = 1.5) interfaces.