Fowler-Nordheim Tunneling Current Research Articles

Investigation of the forward gate leakage current in pGaN/AlGaN/GaN HEMTs through TCAD simulations

In this study, we examined the gate leakage characteristics of normally off pGaN/AlGaN/GaN HEMTs through a simulation study. The Fowler Nordheim Tunneling (FNT) mechanism mainly contributes to the gate leakage process as indicated by the Technology Computer-Aided Design (TCAD) simulation. However, at low bias, the FNT undercalculates the leakage current since the electric field is low in this region. This extra leakage current component at this low bias region can be attributed to the presence of surface traps. Trap-assisted tunneling current along with the FNT current can explain forward leakage characteristics of the pGaN HEMTs. Our TCAD simulations were matched with the experimental data for five devices from four different research groups to support this claim. Using TCAD simulations, we have been able to analyze several device parameters including the various potential drops inside the gate stack structure. We were able to identify some of the trap levels and compare them to the dominant defects expected to be present in the pGaN cap layer. Furthermore, we studied the effects of different device parameters on the gate leakage process in the pGaN HEMT.

Exploring the Surface Oxidation and Environmental Instability of 2H-/1T'-MoTe2 Using Field Emission-Based Scanning Probe Lithography.

An unconventional approach for the resistless nanopatterning 2H- and 1T'-MoTe2 by means of scanning probe lithography is presented. A Fowler-Nordheim tunneling current of low energetic electrons (E = 30-60eV) emitted from the tip of an atomic force microscopy (AFM) cantilever is utilized to induce a nanoscale oxidation on a MoTe2 nanosheet surface under ambient conditions. Due to the water solubility of the generated oxide, a direct pattern transfer into the MoTe2 surface can be achieved by a simple immersion of the sample in deionized water. The tip-grown oxide is characterized using Auger electron and Raman spectroscopy, revealing it consists of amorphous MoO3 /MoOx as well as TeO2 /TeOx . With the presented technology in combination with subsequent AFM imaging it is possible to demonstrate a strong anisotropic sensitivity of 1T'-/(Td )-MoTe2 to aqueous environments. Finally the discussed approach is used to structure a nanoribbon field effect transistor out of a few-layer 2H-MoTe2 nanosheet.

Evolution of Reverse‐Biased Current of a Barristor Junction by Varying Temperature and Barrier Height of the Junction

AbstractThe reverse‐biased current of a Schottky junction comprises the thermionic emission current and the Fowler–Nordheim tunneling current, depending on the barrier height and thickness. Because both are fixed at traditional Schottky junctions, so is the current mechanism. However, a barristor junction has a tunable barrier height and the current mechanism changes accordingly. Here, the evolution of the current mechanism of the graphene–WS2 junction is investigated by varying the gate voltage, drain voltage, and temperature. As the height decreases with the accumulation of electrons on graphene, the dominant transport mechanism of the junction changes from thermionic to field emission. As the temperature increases, the extrinsic carrier concentration of WS2 increases and thus the thickness decreases, resulting in the same evolution. In addition, a kink in the ID–VD curves over a specific range of gate voltages and temperatures is observed. This may originate from the alignment of the Fermi level of graphene to the Dirac point, which dramatically reduces the Fowler–Nordheim tunneling current.

Analog Tunnel Memory Based on Programmable Metallization for Passive Neuromorphic Circuits.

Although experimental implementations of memristive crossbar arrays have indicated the potential of these networks for in-memory computing, their performance is generally limited by an intrinsic variability on the device level as a result of the stochastic formation of conducting filaments. A tunnel-type memristive device typically exhibits small switching variations, owing to the relatively uniform interface effect. However, the low mobility of oxygen ions and large depolarization field result in slow operation speed and poor retention. Here, we demonstrate a quantum-tunneling memory with Ag-doped percolating systems, which possesses desired characteristics for large-scale artificial neural networks. The percolating layer suppresses the random formation of conductive filaments, and the nonvolatile modulation of the Fowler-Nordheim tunneling current is enabled by the collective movement of active Ag nanocrystals with high mobility and a minimal depolarization field. Such devices simultaneously possess electroforming-free characteristics, record low switching variabilities (temporal and spatial variation down to 1.6 and 2.1%, respectively), nanosecond operation speed, and long data retention (>104 s at 85 °C). Simulations prove that passive arrays with our analog memory of large current-voltage nonlinearity achieve a high write and recognition accuracy. Thus, our discovery of the unique tunnel memory contributes to an important step toward realizing neuromorphic circuits.

Comparative analysis of metal diffusion effects in polymer films coated with spin coating and floating film transfer techniques

In this work a comparative analysis between the extent of gold diffusion of the top coated gold layer into the conducting polymer matrix of spin coated polymer films and polymer films coated via floating film transfer method (FTM) has been investigated. For the sake of comparison two different device configurations i.e. ITO/rr-P3HT (spin coated)/Au and ITO/rr-P3HT (spin coated)/rr-P3HT (FTM coated)/Au have been compared. The J-V characteristics of these devices revealed a non-Ohmic injection occurring near the top gold electrode/polymer interface. Furthermore, the current flowing through the device consisting of FTM layer/Au interface was observed to be lower as compared to the one consisting of spin coated layer/Au interface, indicating higher gold diffusion in well-ordered FTM thin films. These observations have been validated by optical and structural characterization methods viz. absorption, photoluminescence, Raman spectroscopy and X-Ray Diffraction (XRD) measurement. Finally Fowler-Nordheim (FN) tunneling currents were studied under high electric field applications for the two devices which confirmed presence of higher barrier at the interface of gold and FTM coated polymer film.

Accuracy of activation energy from Arrhenius plots and temperature-dependent internal photoemission spectroscopy

In this work, the activation energy obtained from the temperature dependent internal photoemission spectroscopy (TDIPS) and thermionic dark currents using GaAs/AlGaAs photodetectors are compared. Different barrier heights within the p-type GaAs/AlGaAs heterostructures are studied. The temperature dependent spectral response shows the red-shifting of the detector threshold wavelength for increasing temperature due to the decreasing band-offset. The activation energy extracted from Arrhenius plot of the dark current-voltage-temperature (I-V-T), and measured spectral response show the carrier activation energy increases with increasing Al mole fraction and decreases with increasing doping density. For Infrared detectors with ≤6.5 μm, the Arrhenius analysis yields the values of activation energy with less than 5% deviation from the actual or TDIPS fitting values. However, for detectors with longer threshold wavelengths (≫9.3 μm), activation energy extracted from the Arrhenius plot leads to energy values which deviate more than ~10% from the corresponding TDIPS values. The higher percentage deviation (≫10%) of activation energy determined by Arrhenius plot from the corresponding TDIPS values attribute to the temperature dependent Fermi distribution tailing effect and Fowler–Nordheim tunneling current.

The effect of hydrogen on programmed threshold-voltage distribution in NAND flash memories

This paper, for the first time, presents a comprehensive study into the effect of hydrogen concentration in the forming gas on the reliability of NAND Flash memories. It is newly observed that a higher hydrogen concentration leads to broaden the threshold-voltage (VT) distribution after programming. This suggests that a higher hydrogen concentration makes a larger amount of ionized hydrogens (H+) exist in the shallow trench isolation (STI), which results in the fluctuation of the Fowler–Nordheim tunneling current during programming. It is also found that the VT distribution width difference between higher and lower hydrogen concentration turns out to be smaller after the program/erase cycling. This could be explained by the neutralization of H+ due to the capture of the tunneling electrons at the STI edge. On the other hand, the higher hydrogen concentration proves to be able to provide better retention characteristics. Thus, the programming and retention characteristics are the tradeoff relationship. Therefore, optimization of the hydrogen concentration is crucial for the NAND Flash process integration.

Modification of FN tunneling provoking gate-leakage current in ZTO (zinc-tin oxide) TFT by regulating the ZTO/SiO2 area ratio

This study addresses the variation in gate-leakage current due to the Fowler-Nordheim (FN) tunneling of electrons through a SiO2 dielectric layer in zinc-tin oxide (ZTO) thin film transistors. It is shown that the gate-leakage current is not related to the absolute area of the ZTO active layer, but it is reduced by reducing the ZTO/SiO2 area ratio. The ZTO/SiO2 area ratio modulates the ZTO-SiO2 interface dipole strength as well as the ZTO-SiO2 conduction band offset and subsequently affects the FN tunneling current through the SiO2 layer, which provides a route that modifies the gate-leakage current.

Temperature-dependent analysis of conduction mechanism of leakage current in thermally grown oxide on 4H-SiC

The conduction mechanism of the leakage current of a thermally grown oxide on 4H silicon carbide (4H-SiC) was investigated. The dominant carriers of the leakage current were found to be electrons by the carrier-separation current-voltage method. The current-voltage and capacitance-voltage characteristics, which were measured over a wide temperature range, revealed that the leakage current in SiO2/4H-SiC on the Si-face can be explained as the sum of the Fowler-Nordheim (FN) tunneling and Poole-Frenkel (PF) emission leakage currents. A rigorous FN analysis provided the true barrier height for the SiO2/4H-SiC interface. On the basis of Arrhenius plots of the PF current separated from the total leakage current, the existence of carbon-related defects and/or oxygen vacancy defects was suggested in thermally grown SiO2 films on the Si-face of 4H-SiC.

New Method to Analyze the Shift of Floating Gate Charge and Generated Tunnel Oxide Trapped Charge Profile in NAND Flash Memory by Program/Erase Endurance

A new test system was devised and used to separate the amount of floating gate (FG) charge (QFG) from the oxide trapped charge (Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OX</sub> ) generated by program-and-erase (P/E) cycles. We also extracted the pure Vmid shift caused by the generation of Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OX</sub> , which is separated from the part of Vt shift coming from Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FG</sub> deviation. The identification of Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FG</sub> and Vmid shift makes it possible to analyze the detailed the oxide trapped charge profile. The Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FG</sub> shift generated by P/E cycles displays asymmetry between the programmed and erased states: the absolute value of Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FG</sub> exhibits a maximum at ~100 cycles in the programmed states, while Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FG</sub> monotonically decreases in the erased one. Considering that the Fowler-Nordheim tunneling current is sensitive to the oxide trap near the cathode, itself the source of electron tunneling current, our results indicate that the hole trap is dominant near to Si, whereas the electron trap is dominant near FG.

Study on nanoparticles embedded multilayer gate dielectric MOS non-volatile memory devices

Here, we present a computational study on stacked multilayer nanoparticles embedded gate dielectric MOS non-volatile memory devices. Two device structures, one with a pure SiO2 tunnel oxide and other with a stacked HfO2-SiO2 tunnel oxide were compared. The Au nanocrystals were assumed embedded in a Si3N4 layer. The electrical parameters of the composite multilayer were evaluated using Maxwell-Garnett theory and virtual crystal approximation. From the WKB approximation, the direct and the Fowler-Nordheim tunnelling currents were evaluated, and subsequently the I-V characteristics and the flatband voltage shifts were also simulated. The flatband shift simulations were compared with recent experimental results.

Polycrystalline silicon multi-functional memory (MFM) with a recessed channel structure

A channel recessed multi-functional memory using a polycrystalline-silicon thin-film floating body and SiO2/Si3N4/SiO2 (ONO) stacked gate dielectrics was investigated for the high speed dynamic random access memory (DRAM) and nonvolatile flash memory (NVM) operations. In the DRAM mode, clearly identified bi-stable current levels owing to the impact ionization effect were achieved as on- and off-states. In the NVM mode, a satisfactory memory window was obtained from the programming and the erasing operations by using the Fowler-Nordheim tunneling current. Additionally, the stable retention and endurance characteristics were achieved from the solid-phase crystallized poly-Si channel and the ONO stacked memory storage node.

Surface Potential Based Analytical Modeling of Double Gate MOSFET with Si and Au Nano-Dots Embedded Gate Dielectric for Non-Volatile Memory Applications

In this paper we present an analytical study of Double Gate (DG) MOSFET memory devices with Si and Au nanocrystal embedded gate dielectric stacks. We considered an undoped long channel DG MOSFET, having a multilayer SiO2 (5 nm)–Si/Au nc embedded Si3N4 (6 nm)–SiO2 (7 nm) gate dielectric. From a quasi-1-D analytic solution of the Poisson equation, the potential and the electric fields in the substrate and the different layers of the gate oxide stack were derived. Thereafter following a trap-like model using the WKB approximation, we have investigated the Fowler Nordheim tunneling currents from the Si inversion layer to the embedded nanocrystal states in such devices. We evaluated the write-erase characteristics, gate tunneling currents, threshold voltage shifts and the output characteristics of the NVM devices from our analytical model. The performance of the nanoparticle embedded DGMOS memory device was compared with that of a DG SONOS NVM of similar dimensions. From the studies, the nc embedded devices showed better performance than the conventional SONOS DGMOS NVM. Among the two ncs compared the nc-Au embedded device emerged as the better performer in terms of higher charge density, faster charging, higher threshold voltage shift and better charge retention.

Investigation of Crystal Size Impacts on the Tunneling Current in Germanium Nanocrystal Metal-Oxide-Semiconductor Transistors

Abstract: The crystal size of nano-crystalline germanium dependence depends on the gate current in memory devices has been theoreti-cally investigated when the mean diameter is uniquely controlled by the nominal thickness of the deposition of Ge layer. Because the band gap and dielectric constant of nano-crystalline germanium will be largely affected by crystal size in the regime of nano-scale, the calculated gate currents in nano-crystalline germanium strongly depends on the crystal size, especially on a few nanometers, when the equivalent oxide thickness memoirs the same. The numerical calculations demonstrate that dielectric constant of nano-crystalline germa-nium impacts on the Fowler-Nordheim tunneling current can be neglected when the oxide field is higher than 3 MV/cm. The results also demonstrate that the leakage current can be reduced by fabricating thinner nc-Ge layer in memory devices. Keywords: Tunneling, nano-crysta lline germanium, gate current, leakage current. 1. INTRODUCTION

Investigation of Crystal Size Impacts on the Tunneling Current in Germanium Nanocrystal Metal-Oxide-Semiconductor Transistors

The crystal size of nano-crystalline germanium dependence depends on the gate current in memory devices has been theoretically investigated when the mean diameter is uniquely controlled by the nominal thickness of the deposition of Ge layer. Because the band gap and dielectric constant of nano-crystalline germanium will be largely affected by crystal size in the regime of nano-scale, the calculated gate currents in nano-crystalline germanium strongly depends on the crystal size, especially on a few nanometers, when the equivalent oxide thickness memoirs the same. The numerical calculations demonstrate that dielectric constant of nano-crystalline germanium impacts on the Fowler-Nordheim tunneling current can be neglected when the oxide field is higher than 3 MV/cm. The results also demonstrate that the leakage current can be reduced by fabricating thinner nc-Ge layer in memory devices. Keywords: Tunneling, nano-crystalline germanium, gate current, leakage current

Effect of Oxygen Partial Pressure on the Electrical and Optical Properties of DC Magnetron Sputtered AmorphousTiO2Films

Titanium dioxide (TiO2) thin films were deposited on p-Si (100) and Corning glass substrates held at room temperature by DC magnetron sputtering at different oxygen partial pressures in the range9×10−3–9×10−2 Pa. The influence of oxygen partial pressure on the structural, electrical, and optical properties of the deposited films was systematically studied. XPS studies confirmed that the film formed at an oxygen partial pressure of6×10−2 Pa was nearly stoichiometric. TiO2films formed at all oxygen partial pressures were X-ray amorphous. The optical transmittance gradually increased and the absorption edge shifted towards shorter wavelengths with the increase of oxygen partial pressure. Thin film capacitors with configuration of Al/TiO2/p-Si have been fabricated. The results showed that the leakage current density of films formed decreased with the increase of oxygen partial pressure to6×10−2Pa owing to the decrease in the oxygen defects in the films thereafter it was increased. The current transport mechanism in the TiO2thin films is shown to be Schottky effect and Fowler-Nordheim tunnelling currents.

Semi-analytical modeling of Ag and Au nanoparticles and fullerene (C60) embedded gate oxide compound semiconductor MOSFET memory devices

In this paper we present an analytical simulation study of Non-volatile MOSFET memory devices with Ag/Au nanoparticles/fullerene (C60) embedded gate dielectric stacks. We considered a long channel planar MOSFET, having a multilayer SiO2---HfO2 (7.5 nm)---Ag/Au nc/C60 embedded HfO2 (6 nm)---HfO2 (30 nm) gate dielectric stack. We considered three substrate materials GaN, InP and the conventional Si substrate, for use in such MOSFET NVM devices. From a semi-analytic solution of the Poisson equation, the potential and the electric fields in the substrate and the different layers of the gate oxide stack were derived. Thereafter using the WKB approximation, we have investigated the Fowler-Nordheim tunneling currents from the Si inversion layer to the embedded nanocrystal states in such devices. From our model, we simulated the write-erase characteristics, gate tunneling currents, and the transient threshold voltage shifts of the MOSFET NVM devices. The results from our model were compared with recent experimental results for Au nc and Ag nc embedded gate dielectric MOSFET memories. From the studies, the C60 embedded devices showed faster charging performance and higher charge storage, than both the metallic nc embedded devices. The nc Au embedded device displayed superior characteristics compared to the nc Ag embedded device. From the model GaN emerged as the overall better substrate material than Si and InP in terms of higher threshold voltage shift, lesser write programming voltage and better charge retention capabilities.

Analyzing trap generation in silicon-nanocrystal memory devices using capacitance and current measurement

The combination of capacitance- and current-voltage (CV/IV) measurements is used to analyze trap generation in silicon-nanocrystal memory devices during Fowler-Nordheim (FN) programming/erasing cycling. CV and IV curves are measured after certain P/E cycles. The flatband voltage (Vfb) and the threshold voltage (Vth) are extracted from CV curves by solving one-dimensional Schrodinger and Poisson equations. Both hole and electron trappings are observed in the tunneling SiO2. They show up in the accumulation and the inversion, respectively. By fitting FN tunneling current, the area densities of cycling-induced electron traps in the blocking oxide and in the tunneling oxide are finally determined.

Impact of Edge Encroachment on Programming and Erasing Gate Current in nand -Type Flash Memory

The edge encroachment of tunnel oxide is experimentally found to degrade the Fowler–Nordheim (FN) tunneling gate current of nand-type Flash cells. This work elucidates the impact of edge encroachment on FN tunneling current for use in programming and erasing operations. The fringing field effect and tunnel oxide with trapezoidal edge are considered in the determination of physical gate current in which a conformal-mapping method is used to estimate the contribution of the fringing fields. These analytical results are confirmed using 2-D device simulations and experimental measurements. The results show that the overlapped encroachment causes an exponential degradation of intrinsic FN tunneling current. Preventing the encroachment of lateral edges resulting from overall tunnel-oxide enlargement is critical to ensuring normal programming and erasing speeds in future nand-type Flash cells.

A Program Disturb Model and Channel Leakage Current Study for Sub-20 nm nand Flash Cells

We have developed a program-disturb model to characterize the channel potential of the program-inhibited string during NAND flash cell programming. This model includes cell-to-cell capacitances from 3-D technology computer-aided design simulation and leakage currents associated with the boosted channel. We studied the program-disturb characteristics of sub-30-nm NAND cells using a delayed programming pulse method. The simulation results agree with the experimental data very well and show quantitative impacts of junction leakage current, band-to-band tunneling (BTBT) current, Fowler-Nordheim tunneling current, and channel capacitance on the program disturb. We further discuss the cell-scaling trend and identify that the BTBT current can be a dominant mechanism for the program disturb of sub-20-nm NAND cells.