FET Threshold Voltage Research Articles

고전계 스트레스상에서 Cascode-GaN-FET 소자의 노화 특성 연구

This paper investigated changes in threshold voltage, on-resistance, turn-on delay, and turn-off delay of cascode GaN FETs under high electric field stress. As a result, the threshold voltage decreased, and the on-resistance increased under the stress of applying a positive high voltage. Also, the turn-on delay was reduced, and the turn-off delay increased. On the other hand, under the stress of applying a high negative voltage, the threshold voltage, on-resistance, and turn-on delay decreased, and the turn-off delay increased. These results imply that the conduction loss of the cascode GaN FET increases, and the dead time of the voltage source converter composed of the cascode GaN FET increases under positive high voltage stress. Also, the fact that the threshold voltage decreased under both positive and negative high voltage stress means that if the cascode GaN FET is subjected to high voltage stress, the reduced threshold voltage is more likely to turn on undesirably even at low gate voltages.

Modeling Non-Equilibrium Ion-Transport in Ion-Selective-Membrane/Electrolyte Interfaces for Electrochemical Potentiometric Sensors

We present an approach to simulate ion’s drift and diffusion in chemical sensors based on ion-selective-membranes (ISMs) in either ion-selective electrode structures or coupled to field-effect transistors. The model is used to analyze the sensitivity, selectivity, and transient response of ISMs in non-equilibrium conditions upon real-time concentration changes. Our simple implementation combines advanced features from semiconductor theory and analytical electrochemistry, such as the Schafetter-Gummel discretization scheme and Chang-Jaffé boundary conditions for ions at the interfaces, thus allowing to perform simulations beyond sensing. The results are in agreement with experimental transient responses reported in the literature. As relevant case studies, we examine the ISM preconditioning in miniaturized sensors and the electrostatic interaction between the FET channel and ISMs. In the first case, simulations reveal that calibration curves performed on incomplete ISM conditioning can lead to hysteretical responses when the ion affinity in the electrolyte and ISM is similar (e.g., with organic ions). In the second case, we find that the gate oxide field in contact with the ISM affects the device characteristics such that the ion concentrations not only change the FET threshold voltage but also the slope of its IV curve. This effect can be minimized by working in the subthreshold regime or using extended gates.

Sensitivity Enhancement of Dual Gate FET Based Biosensor Using Modulated Dielectric for Covid Detection

This paper presents a dual gate dielectric modulated FET (DGDMFET) biosensor with enhanced sensitivity for covid detection. In earlier literature, the biosensors are operated using the surface interaction with the virus biomolecules that are reflected through a channel or gate. The downside of these types of sensors has limited sensitivity. In this paper, we have considered that the change in the dielectric constant due to virus proteins results in a significant shift in the threshold voltage of FET. Enhancement of sensitivity is done by using the novel dual metal gate arrangement with different work functions (higher at the source end and lower at the drain end) and the chromic oxide (Cr2O3) layer, which is carved out vertically to form nanogap. At the same time, interface charge density is maintained nearly equal to 1.0 × 1011 cm−2 at the Si-SiO2 layer. To demonstrate the proposed biosensor, electrical parameters (electron concentration, surface potential, energy band distribution, and electric field) and the absolute percentage sensitivity of threshold voltage, subthreshold slope, ON current, and transconductance are evaluated and compared with related literature. The ATLAS device simulator is used for the simulation of the proposed device.

Controlling Threshold Voltage of CMOS SOI Nanowire FETs With Sub-1 nm Dipole Layers Formed by Atomic Layer Deposition

In this article, bidirectional control of threshold voltage (<inline-formula> <tex-math notation="LaTeX">${V}_{T}$ </tex-math></inline-formula>) is realized in both n- and p-silicon-on-insulator (SOI) nanowire FETs (NWFETs) by using sub-1 nm atomic-layer-deposited (ALD) dipole layers (Y₂O₃ and Al₂O₃) for the first time. A 0.7 nm Y₂O₃ inserted between bottom native SiO_<i>x</i> (&#x003C; 1 nm) and top HfO₂ (3 nm) can shift the <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> by &#x2212;138 and &#x2212;58 mV for n- and p-NWFET, respectively, while 0.7 nm Al₂O₃ can shift the <inline-formula> <tex-math notation="LaTeX">${V}_{T}$ </tex-math></inline-formula> of n-NWFET by &#x002B;219 mV and p-NWFET by &#x002B;134 mV. The tunability of such a high<i>-k</i> superstructure for the flat band voltage (<inline-formula> <tex-math notation="LaTeX">${V}_{\text {FB}}$ </tex-math></inline-formula>) shift of capacitors and <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> shift of planar n-SOI FETs are also investigated. Furthermore, to concisely control the <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${V}_{\text {FB}}$ </tex-math></inline-formula> as design, capacitors fabricated with quadra-layer (SiO_<i>x</i>/HfO₂/Al₂O₃/Y₂O₃) high<i>-k</i> superstructure were fabricated and 3 mV <inline-formula> <tex-math notation="LaTeX">${V}_{\text {FB}}$ </tex-math></inline-formula> shift is achieved by carefully adjusting the composition of intermixed-dipole layers. This work points out the route to concisely tune the threshold voltage of complementary metal-oxide-semiconductor (CMOS) FETs with the desired direction and strength.

Highly stable threshold voltage in GaN nanowire FETs: The advantages of p-GaN channel/Al2O3 gate insulator

We present an extensive investigation of the charge-trapping processes in vertical GaN nanowire FETs with a gate-all-around structure. Two sets of devices were investigated: Gen1 samples have unipolar (n-type) epitaxy, whereas Gen2 samples have a p-doped channel and an n-p-n gate stack. From experimental results, we demonstrate the superior performance of the transistor structure with a p-GaN channel/Al2O3 gate insulator in terms of dc performance. In addition, we demonstrate that Gen2 devices have highly stable threshold voltage, thus representing ideal devices for power electronic applications. Insight into the trapping processes in the two generations of devices was obtained by modeling the threshold voltage variations via differential rate equations.

A Reliable BioFET Immunosensor for Detection of p53 Tumour Suppressor in Physiological-Like Environment.

The concentration of wild-type tumour suppressor p53wt in cells and blood has a clinical significance for early diagnosis of some types of cancer. We developed a disposable, label-free, field-effect transistor-based immunosensor (BioFET), able to detect p53wt in physiological buffer solutions, over a wide concentration range. Microfabricated, high-purity gold electrodes were used as single-use extended gates (EG), which avoid direct interaction between the transistor gate and the biological solution. Debye screening, which normally hampers target charge effect on the FET gate potential and, consequently, on the registered FET drain-source current, at physiological ionic strength, was overcome by incorporating a biomolecule-permeable polymer layer on the EG electrode surface. Determination of an unknown p53wt concentration was obtained by calibrating the variation of the FET threshold voltage versus the target molecule concentration in buffer solution, with a sensitivity of 1.5 ± 0.2 mV/decade. The BioFET specificity was assessed by control experiments with proteins that may unspecifically bind at the EG surface, while 100pM p53wt concentration was established as limit of detection. This work paves the way for fast and highly sensitive tools for p53wt detection in physiological fluids, which deserve much interest in early cancer diagnosis and prognosis.

Effect analysis and magnetoelectric properties of hydrogen in Co-doped MoSe&lt;sub&gt;2&lt;/sub&gt; Co-growth

In this paper, Co&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;、MoO&lt;sub&gt;3&lt;/sub&gt; and Se powders were used as precursors in in-situ co-growth chemical vapor deposition method. Cobalt-doped MoSe&lt;sub&gt;2&lt;/sub&gt; nanosheets were grown on SiO&lt;sub&gt;2&lt;/sub&gt; substrate at 710 ℃. The influence of hydrogen content on its growth and regulation mechanism was discussed. Surface morphology analysis showed that the introduction of hydrogen promoted the formation of oxy-selenium metal compounds required for nucleation and the CoMoSe compound molecules required for lateral growth. AFM(atomic force microscope) results show that hydrogen is beneficial to the growth of single-layer two-dimensional cobalt-doped MoSe&lt;sub&gt;2&lt;/sub&gt;. With the increase of the amount of Co&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; precursor, the Raman and PL(photoluminescence) spectra of the sample showed red shift and blue shift, respectively, and the bandgap was modulated from 1.52 eV to 1.57 eV. The XPS(X-ray photoelectron spectroscopy) results analysis showed that the elemental composition ratio of Co was 4.4%. The magneto and electric properties of the samples were analyzed by SQUID-VSM(superconducting quantum interference device) and semiconductor parameter analyzer for electrical testing. The results show that MoSe&lt;sub&gt;2&lt;/sub&gt; changes from diamagnetic to soft magnetic after Co incorporation; the threshold voltage of back gate FETs is shifted by 5 V from pure MoSe&lt;sub&gt;2&lt;/sub&gt;, and the off-state current is lower. This research provides a basis for the research and application development of ultra-thin two-dimensional materials.

Interfacial energy band bending and carrier trapping at the vacuum-deposited MAPbI3 perovskite/gate dielectric interface

We report energy band bending of methylammonium lead halide (MAPbI3) perovskite film in contact with indium-tin-oxide (ITO) surface using photoelectron spectroscopy in air (PESA) and ultraviolet photoelectron spectroscopy (UPS) measurements. MAPbI3 perovskite films were vacuum-deposited using co-evaporation of methylammonium iodide (CH3NH3I, MAI) and lead iodide (PbI2) powders. Using PESA and UPS, the highest occupied molecular orbital levels were measured varying the thickness of the perovskite films. Substantial energy band bending close to the ITO surface was not observed. The vacuum-deposited perovskite films feature ambipolar carrier transport from field effect current measurements. Particularly, the threshold voltage for electron conduction was −20 and 25 V for the forward and reverse gate scans, exhibiting significant current hysteresis. The magnitude of the threshold voltage, far away from zero gate voltage, implies that interface trapping rather than energy band bending dominate the threshold voltage. In other words, perovskite/gate dielectric interface trap states for electrons modulate the magnitude of the threshold voltage. Our work provides insights into the origin of gate voltage dependent threshold voltage in perovskite FETs minimizing the contribution from ion migration based on the observation of ambipolar carrier transport at room temperature for vacuum deposited perovskite films.

Self-Heating Assessment on Bulk FinFET Devices Through Characterization and Predictive Simulation

This paper describes three different measurement methodologies for the electrical characterization of FinFET self-heating at wafer-level. Finite element simulations of heat transport are used to interpret heater-sensor temperature gradients and validate the measurements. The different sensor types were designed to use the threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) of an adjacent FET, the forward bias (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> ) of an adjacent pn-junction or the gate resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> ) of the device itself. We report that self-heating is underestimated by 35% when sensed at a neighboring device. We also confirm that heat from local and surrounding sources are additive.

Bootstrap Voltage and Dead Time Behavior in GaN DC–DC Buck Converter With a Negative Gate Voltage

The low threshold voltage of Gallium Nitride enhancement mode FETs is a concern in synchronous dc–dc buck converters. This paper presents a converter using a gate drive with a negative biased gate voltage on the low-side FET to improve the dV / dt robustness. The resulting much larger voltage drop during the dead time conduction is a concern as it can quickly overcharge the bootstrap capacitor voltage. This paper demonstrates that with a negative biased gate voltage, the rise in the bootstrap capacitor voltage is predominantly caused by the change in effective dead time conduction due to a variation in output load. A spice model to estimate the change is presented and verified. This paper also found that a change in the bootstrap capacitor voltage must be prevented as it leads to a significant change in turn-on dynamics, resulting in higher dV / dt stress with an increase in output load. To prevent a change in bootstrap capacitor voltage, different techniques such as Zener diode clamping and an external free-wheeling Schottky diode are tested and discussed. Finally, a modification of a standard bootstrap circuit using a series inductor is proposed and tested. It has been found to be the most effective method in preventing a variation in the bootstrap capacitor voltage and should be included when using a negative gate drive in a GaN dc–dc buck converter.

Breast-Cancer-Stem-Cell Sensor Using Igzo Nomfet Implemented with Au Nano-Particles

Recently, many researchers have studied circulating-cancer-stem-cell (CCSC) to pre-recognize the presence of a cancer. Since, it has been that cancer cells include negative or positive charges, several electronic sensors have been attractively proposed. In our study, we designed an In-Ga-Zn-oxide (IGZO) based FET implemented with Au nano-particles on IGZO channel, as shown in Fig. 1(a). The average size and density of the Au nano-particles on the channel were 12.05 nm and 2.66×1011 cm-2, respectively. In particular, this FET has a unique time-dependent current characteristic such as depressing at a positive gate bias or facilitating at a negative bias, which determines whether or not the cancers present a fixed charge or mobile charges. In our experiment, breast-cancer-stem-cell (BCSC) connected with the cancer-anti-body (CD44) was dropped on the IGZO FET with Au nano-particles where a linker (8-Mercaptooctanoic acid) was attached on Au nano-particles, as shown in Fig. 1(b). The linkers were well uniformly absorbed and attached on Au nano-particles, confirmed by FT-IR, as shown in Fig. 2. First of all, we investigated the dependency of threshold voltage on the presence of Au nano-particles, linkers, and breast-cancer-stem-cells with anti-bodies shown in Fig.3. The threshold voltage of the IGZO FET without Au nano-particles was +2.4 V. The implementation of Au-nano-particles on the IGZO channel increased from +2.4 to +12.0 V because of the presence of positive fixed charges at the interface between the IGZO channel and Au nano-particles. The attachment of the linker on Au nano-particles decreased the threshold voltage from +12.0 V to +6.9 V since the functional group of the linker was carboxyl. Two kinds of breast-cancer-stem-cells were dropped on the linker connected with Au-nano-particles; i.e., the cells without or with anti-body. The cells without anti-body did not shift threshold while the cells with anti-body reduced -4.3 V where the concentration of BCSC was 8000ea/10ul. The threshold-voltage shift amount was well correlated with the concentration of BCSC, as shown in Fig. 4. The threshold-voltage shift amount increased linearly with the concentration of BCSC, having the slope of the threshold-voltage shift amount of 0.44V/(kea/ul). In addition, we confirmed that BCSC showed a negative fixed charge rather than mobile charge by testing time dependent drain-current characteristic. In our presentation, we will review the detailed mechanism by which BCSCs shift threshold voltage and show a negative fixed charge characteristic. Reference [1] F. Alibart et al. Adv. Funct. Mater. 20 330. (2010)[2] K.-C. Kwon et al. Appl. Phys. Exp. 6, 067001 (2013).[3] XiuHua Zhang et al. Sensors, 3, 61-68 (2003) Acknowledgment This research was supported by the Converging Research Center Program funded by the Ministry of Science, ICT & Future Planning (Project No. 2015054348) and supported by Brain Korea 21 PLUS Program in 2014 Figure 1

A Compact 2-D Analytical Model for Electrical Characteristics of Double-Gate Tunnel Field-Effect Transistors With a SiO2/High-&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$k$ &lt;/tex-math&gt; &lt;/inline-formula&gt; Stacked Gate-Oxide Structure

A compact 2-D analytical model for electrical characteristics such as surface potential, drain current, and threshold voltage of double-gate tunnel FET (DG TFETs) with a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /High-k stacked gate-oxide structure is proposed in this paper. Poisson's equation has been solved using parabolic approximation method to model the channel potential. The band-to-band tunneling generation rate has been expressed as a function of channel electric field derived from the channel potential and then integrated analytically over the channel thickness to derive the drain current of the stacked-gate DG TFETs using the shortest tunneling path (L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sup> ) concept. The effect of source/drain depletion regions has been included for the better accuracy of the proposed model. The maximum transconductance method has finally been used to extract the threshold voltage from the drain current of the device. The effects of various device parameters on the channel potential, drain current, and threshold voltage have been investigated. The model results have been compared with the simulation data obtained using the commercially available ATLAS 2-D device simulator from SILVACO for the validity of the proposed model.

Analytical Modeling of Channel Potential and Threshold Voltage of Double-Gate Junctionless FETs With a Vertical Gaussian-Like Doping Profile

This paper proposes an analytical 2-D model for the channel potential and threshold voltage of double-gate junctionless FETs with a vertical Gaussian-like doping profile. The 2-D Poisson equation has been solved by using the evanescent-mode analysis to obtain the potential distribution function in the channel. The position of the conduction path also has been modeled to calculate the potential at different positions of the conduction path. The validity of the proposed 2-D potential and threshold voltage models is shown by comparing the results with the simulation data obtained by a 2-D TCAD ATLAS device simulator.

Control of the threshold voltage in ZnO nanobelt field-effect transistors by using MoOx thin film

We report on the feasible control of the threshold voltage (Vth) in ultra-thin ZnO nanobelt FETs by using substoichiometric molybdenum trioxide (MoOx, x < 3) either as a modification layer on the surface of ZnO nanobelts or as electrodes instead of the widely used Ti/Au. ZnO nanobelt FETs using Ti/Au as the electrodes usually exhibit a negative threshold voltage, indicating n-channel depletion mode behavior, whereas ZnO FETs with MoOx/Au electrodes instead of Ti/Au show a positive shift of threshold voltage, exhibiting an n-channel type enhancement mode, which can be explained by a high Schottky barrier created at the interface of MoOx and the ZnO channel. In contrast, the decoration on the surface of ZnO channel by MoOx significantly increases the zero-bias conductivity and electron carrier concentration, and then negatively shifts the threshold voltage. We propose that MoOx thin film may play a passivation effect role, much more so than the doping effect role, due to the large amount of adsorbed species on as-grown ZnO nanobelts, especially oxygen species.

Tuning the threshold voltage from depletion to enhancement mode in a multilayer MoS2 transistor via oxygen adsorption and desorption.

Selective chemical doping in two-dimensional (2D) molybdenum disulfide (MoS2) is attractive for tailoring electrical properties according to device requirements. However, the ultra-thin 2D nature of MoS2 makes it difficult to realize effective doping by conventional ion implantation. Here, a simple method based on low-temperature (150 °C) annealing in air is developed for effective chemical doping in MoS2. We have demonstrated that the threshold voltage (V(th)) of multilayer MoS2 FET can be effectively tuned from depletion mode (V(th) = -1.8 V) to enhancement mode (V(th) = 1.1 V) by annealing in air at 150 °C. An energy band model based on electron trapping/detrapping due to oxygen adsorption on the MoS2 surface is proposed to explain the underlying mechanism. The model is consistent with an oxygen adsorption-desorption process evidenced by vacuum annealing that recovers the V(th) to its original value. These results can provide a simple approach for V(th) engineering and make a significant step toward 2D nanoelectronic device applications.

Functionalization of transition metal dichalcogenides with metallic nanoparticles: implications for doping and gas-sensing.

Transition metal dichalcogenides (TMDs), belonging to the class of two-dimensional (2D) layered materials, have instigated a lot of interest in diverse application fields due to their unique electrical, mechanical, magnetic, and optical properties. Tuning the electrical properties of TMDs through charge transfer or doping is necessary for various optoelectronic applications. This paper presents the experimental investigation of the doping effect on TMDs, mainly focusing on molybdenum disulfide (MoS2), by metallic nanoparticles (NPs), exploring noble metals such as silver (Ag), palladium (Pd), and platinum (Pt) as well as the low workfunction metals such as scandium (Sc) and yttrium (Y) for the first time. The dependence of the doping behavior of MoS2 on the metal workfunction is demonstrated and it is shown that Pt nanoparticles can lead to as large as 137 V shift in threshold voltage of a back-gated monolayered MoS2 FET. Variation of the MoS2 FET transfer curves with the increase in the dose of NPs as well as the effect of the number of MoS2 layers on the doping characteristics are also discussed for the first time. Moreover, the doping effect on WSe2 is studied with the first demonstration of p-type doping using Pt NPs. Apart from doping, the use of metallic NP functionalized TMDs for gas sensing application is also demonstrated.

Extraction of the Random Component of Time-Dependent Variability Using Matched Pairs

Based on the so-called defect-centric statistics, we propose the average impact of a single charged trap on FET threshold voltage as a physically based measure of the random component of time-dependent variability. We show that it can be extracted using matched pairs, analogously to time-zero variability. To that end, the defect-centric statistics of matched pairs are discussed and the correlation between time-zero and time-dependent variances is formalized.

2-D Analytical Model for the Threshold Voltage of a Tunneling FET With Localized Charges

In this paper, we have developed a 2-D analytical model for the surface potential and threshold voltage of a tunneling field-effect transistor (TFET) with localized charges in the oxide. These charges are generated in the oxide due to hot carrier effects in the channel. The models are derived by dividing the channel into damaged and undamaged regions and then solving the 2-D Poisson's equation in these regions. The threshold voltage is then extracted by using constant current method. The proposed models are verified by using 2-D device simulations. The model can be used to study the impact of localized charges on the threshold voltage of a TFET for varying device dimensions and charge densities and can also be utilized to design TFET-based charge trapped memory devices.

Low gate interface traps AlGaN/GaN HEMTs using a lattice matched ZrZnO transparent gate design

AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) using a radio-frequency magnetron sputtered ZrZnO transparent oxide layer as a gate insulator are investigated and compared with traditional GaN HEMTs. A negligible hysteresis voltage shift in the C–V curves is seen, from 0.09V to 0.36V, as the thickness of ZrZnO films increases. The composition of ZrZnO at different annealing temperatures is observed using X-ray photoelectron spectroscopy (XPS). The ZrZnO thin film achieves good thermal stability after 600°C, 700°C and 800°C post-deposition annealing (PDA) because of its high binding energy. Based on the interface trap density analysis, Dit has a value of 2.663×1012cm−2/eV for 10-nm-thick ZrZnO-gate HEMTs and demonstrates better interlayer characteristics, which results in a better slopes for the Ids degradation (5.75×10−1mA/mmK−1) for operation from 77K to 300K. The 10-nm-thick ZrZnO-gate device also exhibits a flat and a stable 1/f noise, as VGS–Vth, and at various operating temperatures. Therefore, ZrZnO has good potential for use as the transparent film for a gate insulator that improves the GaN-based FET threshold voltage and improves the number of surface defects at various operating temperatures.

Impact of Quantum Confinement on Gate Threshold Voltage and Subthreshold Swings in Double-Gate Tunnel FETs

We investigate how the inclusion of quantum confinement in double-gate tunneling field-effect transistors (DG-TFETs) modifies the conventional behavior of electrical parameters of utmost importance in these devices, such as subthreshold swings (point and average) and the gate threshold voltage. We make use of a simple approach that allows us to incorporate a quantum-mechanical description in which the discreteness of subband energy levels causes a significant reduction in the band-to-band tunneling probabilities. The inclusion of quantum confinement along with a nonlocal band-to-band model for tunneling is shown to greatly affect the aforementioned parameters as key issues for the characterization of these novel devices.