High Threshold Voltage Research Articles

A fin-gate p-GaN HEMT with high threshold voltage and improved dynamic performance

A novel fin-gate p-GaN (FPG) HEMT is proposed to simultaneously increase threshold voltage (Vth) and improve dynamic performance of the p-GaN HEMT. The fin-gate structure works as a normally-on p-channel MESFET between gate and source by forming a Schottky-type contact on sidewall and a source-connected Ohmic-type contact on top of the fin. Thus, the Vth can change with the shutdown voltage of the p-channel MESFET, which can be modulated by the doping concentration and width of the fin-p-GaN. By optimizing the fin structure, a high positive Vth of 4V is achieved without transconductance and breakdown voltage degradation in this work. It breaks the restriction between Vth and on-resistance for conventional p-GaN HEMT. The dynamic characteristics of the FPG HEMT are investigated by SPICE simulations. Owing to the well-grounded p-GaN through the normally-on MESFET, the recovery process of the dynamic shift in Vth (ΔVth) after on/off-state stress can be accelerated by two orders of magnitude. It means an imperceptible dynamic degradation and a great potential in high frequency application for the FPG HEMT.

Charge trapping gate stack enabled non-recessed normally off AlGaN/GaN HEMT with high threshold voltage stability

This paper reports a non-recessed normally off AlGaN/GaN high-electron-mobility transistor (HEMT) with a charge trapping gate dielectric stack. The charge trapping gate dielectric stack featuring a low density of fixed positive charges consists of an O3-based Al2O3 tunneling layer and an O3-based HfO2 blocking layer, both deposited via atomic layer deposition. For HEMT with a 15 nm AlGaN barrier layer, a threshold voltage of 2.57 V and an on-resistance of 8.50 Ω × mm are achieved. For positive bias temperature instability (PBTI) testing, the electric field provided by the 15 nm AlGaN layer serves as a barrier to channel carriers, significantly enhancing the PBTI test stability. The optimized gate dielectric stack enables the fabrication of the non-recessed normally off metal–insulator–semiconductor-HEMT with enhanced threshold voltage (Vth) stability.

MOS-structured MoS2/GaN Schottky barrier diodes with high on/off current ratio and low threshold voltage

GaN Schottky barrier diodes (SBDs) have been investigated for a variety of power applications. However, the problems of low on/off current ratio and high threshold voltage caused by the difficult high-quality doping restrict its utilization in power devices. In this work, quasi-vertical MoS2/GaN SBDs with Metal-Oxide-Semiconductor (MOS) structure have been proposed. The MOS structure is formed by the part of anode (Ni/Au) over the Al2O3, Al2O3, and monolayer MoS2. Monolayer MoS2 exhibits n-type doping and p-type doping under forward bias and reverse bias, respectively, which realizes an adjustment in the Fermi level of the monolayer MoS2, resulting in a change in the resistance of the SBDs. The as-prepared SBDs present a high on/off current ratio of 2.40 × 1011, a low threshold voltage of 0.55 V at 1 A/cm2, and a low reverse leakage current of 2.50 × 10−10 A/cm2 at −1 V. This work shows that the MOS-structured MoS2/GaN SBDs are promising for next-generation power electronics.

Demonstration of normally-off p-channel GaN transistor with high threshold voltage and low subthreshold swing based on single p-GaN

Demonstration of normally-off p-channel GaN transistor with high threshold voltage and low subthreshold swing based on single p-GaN

Linearity and noise evaluation based analysis of extended source heterojunction double gate tunnel FET

This research work evaluates a performance analysis of heterostructure (SiGe/Si) double gate extended source Tunnel FET (Hetero-ES-TFET) to enhance the analog performance, linearity and noise performance. At the source-channel junction, a Hetero-ES-TFET's source is extended into the channel to increase point and line tunneling in the device. The Hetero-ES-TFET exhibits a high ION/IOFF of 3.57 × 1012 and a maximum cut off frequency fT of 54.19 GHz for optimization of device structural parameters. This analysis is conducted using a calibrated SILVACO, technology computer-aided design (TCAD) simulator. The proposed structure includes evaluation of linearity and noise performance characteristics. Furthermore, a linearity analysis as a figure of merit was conducted for the proposed device under study, including different parameters such as 3rd order intermodulation distortion point (IMD3), 3rd order intermodulation intercept point (IIP3), 2nd and 3rd order voltage intercept point (VIP2 and VIP3). The proposed Hetero-ES-TFET has achieved an incredibly high ON current and low threshold voltage. The effect of increasing source width has been examined in this work while sub-threshold swing (SS) remains unchanged during the analysis. There is an improvement in threshold voltage and ION/IOFF value by using silicon-germanium (SiGe) as a source material.

Self‐Doped Mixed Ionic‐Electronic Conductors to Tune the Threshold Voltage and the Mode of Operation in Organic Electrochemical Transistors

AbstractOrganic mixed ionic‐electronic conductors with tunable doping, low threshold voltages, and air stability are crucial for bioelectronic applications. A homopolymer based on an alkoxy thiophene monomer and its copolymer with a thiophene carrying ethylene glycol side chains are synthesized and converted to self‐doped conjugated polyelectrolytes, P3HOTS‐TMA+, and P3HOTS‐TMA+‐co‐P3MEEET. The self‐doping occurs during the conversion to polyelectrolytes. Both polyelectrolytes show high electrical conductivity without any external dopants. UV–Vis–NIR spectroscopy and spectroelectrochemistry confirm excellent air stability of the doped state. In an organic electrochemical transistor (OECT), the P3HOTS‐TMA+ operates in depletion mode, while P3HOTS‐TMA+‐co‐P3MEEET exhibits accumulation mode of operation with low threshold voltage, both showing fast response times. On the other hand, the non‐doped homopolymer, P3MEEET, shows a high negative threshold voltage in accumulation mode. Thus, copolymerization with the self‐dopable monomer changes the mode of operation as well as the threshold voltage substantially. Ultraviolet photoelectron spectroscopy reveals a considerable reduction of the hole injection barrier for the self‐doped system P3HOTS‐TMA+. Mott‐Schottky analysis shows reduction in charge carrier concentration in the copolymer compared to the homopolymer. Thus, the copolymerization strategy with a self‐dopable monomer is an efficient tool for tuning the degree of doping leading to low threshold voltage in OECTs.

Analytical modeling of threshold voltage and on-resistance in multi-barrier E-mode MISHEMT with gate-recess and field-plates

This work presents an analytical model for threshold voltage and On-resistance of multi barrier Metal–Insulator–Semiconductor High-Electron-Mobility-Transistor (MISHEMT) with gate-recess and field-plates. The device featuring a high two-dimensional electron-gas (2DEG) density in the channel region. The primary objectives of this device are to achieve a high threshold voltage (Vth) and enhance electron mobility with specific low ON-resistance (Ron_sp) by mitigating the degradation effects arising from scattering and interface-charges. Also, a physics based analytical model for Vth and 2DEG charge density at upper and lower channels is presented. This model is validated by comparing with TCAD numerical simulations and are well matched. The proposed MISHEMT demonstrates improved electron mobility in the lower channel of 1260 cm2/V.s, Vth of ∼2.6 V and Ron_sp is minimized by 33 % in contrast with a conventional MISHEMT. Additionally, the proposed MISHEMT becomes a promising device for achieving both high threshold voltage and mobility which are required for power semiconductor devices.

Charge trapping layer enabled high-performance E-mode GaN HEMTs and monolithic integration GaN inverters

In this work, high threshold voltage and breakdown voltage E-mode GaN HEMTs using an Al:HfOx-based charge trapping layer (CTL) are presented. The developed GaN HEMTs exhibit a wide threshold modulation range of ΔVTH ∼ 17.8 V, which enables the achievement of enhancement-mode (E-mode) operation after initialization process owing to the high charge storage capacity of the Al:HfOx layer. The E-mode GaN HEMTs exhibit a high positive VTH of 8.4 V, a high IDS,max of 466 mA/mm, a low RON of 10.49 Ω mm, and a high on/off ratio of ∼109. Moreover, the off-state breakdown voltage reaches up to 1100 V, which is primarily attributed to in situ O3 pretreatment effectively suppressing and blocking leakage current. Furthermore, thanks to the VTH of GaN HEMTs being tunable by initialization voltage using the proposed CTL scheme, we prove that the direct-coupled FET logic-integrated GaN inverters can operate under a variety of conditions (β = 10–40 and VDD = 3–15 V) with commendable output swing and noise margins. These results present a promising approach toward realizing the monolithic integration of GaN devices for power IC applications.

Recess-free enhancement-mode AlGaN/GaN RF HEMTs on Si substrate

Enhancement-mode (E-mode) GaN-on-Si radio-frequency (RF) high-electron-mobility transistors (HEMTs) were fabricated on an ultrathin-barrier (UTB) AlGaN (<6 nm)/GaN heterostructure featuring a naturally depleted 2-D electron gas (2DEG) channel. The fabricated E-mode HEMTs exhibit a relatively high threshold voltage (V TH) of +1.1 V with good uniformity. A maximum current/power gain cut-off frequency (f T/f MAX) of 31.3/99.6 GHz with a power added efficiency (PAE) of 52.47% and an output power density (P out) of 1.0 W/mm at 3.5 GHz were achieved on the fabricated E-mode HEMTs with 1-µm gate and Au-free ohmic contact.

Enhanced field-effect mobility (>250 cm2/V·s) in GaN MOSFETs with deposited gate oxides via mist CVD

We report an enhanced field-effect mobility (>250 cm2·V−1·s−1) in GaN MOSFETs. High mobility was achieved by reducing the oxidation of the GaN surface, which was a major factor affecting channel mobility in GaN MOSFETs. Among various gate oxide deposition methods, mist CVD using O3 suppressed GaN surface oxidation. The best field-effect mobility was observed using mist CVD-deposited gate oxides, achieving a peak mobility of 266 cm2·V−1·s−1 with a high threshold voltage of 4.8 V.

Threshold Voltage Modulation and Gate Leakage Suppression of Enhancement‐Mode GaN HEMT by Metal/Insulator/p‐GaN Gate Structure

Herein, a metal/insulator/p‐GaN gate HEMT (MIP‐HEMT) with Si3N4 gate dielectric layer is fabricated. Compared to the conventional p‐GaN HEMT, the MIP‐HEMT has a higher threshold voltage (Vth) of 4.8 V and better gate leakage suppression. The mechanism of threshold voltage increase in MIP‐HEMT is elucidated through an analysis of the electric field distribution in the gate region and the proposed gate capacitance model. Furthermore, MIP‐HEMTs with gate dielectric layers of varying dielectric constants and thicknesses are designed and simulated. The obtained results lead to the derivation of an empirical formula for the threshold voltage of the MIP‐HEMT under ideal dielectric/p‐GaN interface conditions, in relation to the dielectric constant and thickness of the gate dielectric layer. Additionally, simulations are conducted on the MIP‐HEMT incorporating fixed charges at the dielectric/p‐GaN interface, and the impact of interface charges is investigated. This refinement enables a more accurate prediction of the MIP‐HEMT's threshold voltage. Conclusively, MIP‐HEMTs can effectively modulate the threshold voltage by manipulating the parameters of the gate dielectric layer, thereby extending the threshold voltage range of conventional enhancement‐mode devices.

First integration of Ni barrier layer for enhanced threshold switching characteristics in Ag/HfO2-based TS device

Utilizing Ag/HfO2 with nickel (Ni) as a barrier layer, a novel threshold switching (TS) device is devised to overcome challenges such as low reliability, high threshold voltage, and high leakage current. Compared against an Ag/Ti/HfO2-based TS device, the Ag/Ni/HfO2-based TS device exhibits improved electrical characteristics: yield enhancement from 31.7 % to 40.0 %, enhanced endurance from ∼10 cycles to ∼300 cycles, and suppression in off-state current (IOFF) from 1.2 × 10−7 A to 5.2 × 10−10 A under a high compliance current (e.g., 10−4 A). The results obtained through transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM) support the evidence of those accomplishments. Reducing the effective area of the TS device improves control over erratically generated filaments and the electric field within the switching layer, resulting in enhanced performance such as a reduced threshold voltage (VTH ∼0.35 V), minimized VTH variability (∼0.01 V), decreased a threshold current (ITH, i.e., the leakage current in the off-state before activation, ∼5.2 × 10−10 A), and maximum conductance (∼5.0 × 10−5 S) of low-resistance state. These findings suggest that the optimized Ag/Ni/HfO2-based TS device can serve as a practical solution for low-power applications.

Effect of nitrogen doping on characteristics of SiTe Ovonic threshold switch for selectors

In this study, the effect of nitrogen doping on silicon telluride (SiTe) chalcogenide materials has been investigated to enhance the performance of the Ovonic threshold switch (OTS) selector devices especially on the off-current density to reduce sneak current. The nitrogen doped SiTe (N–SiTe) device demonstrates significant improvement in selector performance including low off-current (18 nA) and low off-current density (3.4 × 10−4 kA/cm2) resulting in high selectivity (5.5 × 104), low power operation at threshold voltage of 0.96 V, enhanced voltage window (0.24 V), fast switching with slope of < 2.97 mV/dec and potentially fast pulse response/switching speed. Besides, analyses by Raman and X–ray photoelectron spectroscopies indicate that the nitrogen doping changes predominant orbital and bonding configuration and decreases Te lone pairs that act as traps, thus suppressing subthreshold conduction. Increased optical bandgap by nitrogen doping also results in reduced leakage current as well as higher threshold voltage. First-principles calculations reveal that localized electrons with high density near nitrogen are responsible for reduced subthreshold conduction of the chalcogenide. Our results provide new guidance in material selection by demonstrating the effect of nitrogen doping to improve OTS selector performance on an arsenic-free binary chalcogenide.

Novel high-voltage GaN CAVET with high threshold voltage and low reverse conduction loss

A novel GaN current-aperture vertical electron transistor (CAVET) with an energy band pinning (EBP) structure (EBP-CAVET) is proposed and investigated by simulations. The EBP-CAVET is featured with hybrid contacts on the p-GaN layer, locally having Ohmic contact combined with Schottky gate metals in the longitudinal direction. The Ohmic contact is shorted to the source electrode and the Schottky gate metals are shorted to the gate electrode. The conduction band (Ec) in the gate region is modulated by the alternately arranged contacts. In the Ohmic contact region, Ec remains a fixed value, which acts as an energy band pinning and suppresses the variation of the Ec in the gate region. In the on-state (VGS > 0 V) and blocking-state with high VDS, the EBP structure inhibits the Ec shifting downwards in the gate region to achieve a high threshold voltage (Vth), a low leakage current density (Jdss), and a high breakdown voltage (BV). In the reverse conduction state (VDS < 0 V &VGS ≤ 0 V), the EBP structure suppresses the Ec shifting upwards to achieve a low and independent of gate bias reverse turn-on voltage (VRT). The proposed EBP-CAVET achieves a high Vth of 2.10 V, a VRT of 0.69 V, a low Jdss of 3.1 × 10−11 A/cm2 at VDS = 1000 V, a high BV of 1660 V. Compared with an integrated Schottky barrier diode or fin diode in a field effect transistor (FET), the EBP structure occupies a much smaller chip area. Thus, the EBP-CAVET achieves a low specific ON-resistance (Ron,sp) of 1.19 mΩ cm2 and an extremely high power-figure-of-merit (PFOM) up to 2.32 GW/cm2. Consequently, the proposed structure presents a new design concept and enhances the application potential for GaN CAVET with high Vth and low reverse conduction loss.

Novel alkyne chromophore of the isophorone derivatives: synthesis, electrochemical evaluation, DFT, and processable bottom contact/top-gate OFET applications

Fused alkyne molecules are important in organic semiconductors due to their desirable properties. Here, we report the design and synthesis of a new series of A–π–D molecules (III–VII) that can serve as mild electron acceptors to generate wide-bandgap p-type small compounds for use in organic field-effect transistors. The incorporation of donor units into fused isophorone frameworks can be used to tune the frontier molecular orbital energies. The electrochemical, optical, and thermal properties of the compounds were characterized. Compound VI, which has a fused phenyl-substituted alkyne moiety, had the highest occupied molecular orbital energy level as determined by optical and electrochemical analysis. Density functional theory calculations revealed that compounds VI and III had lower hole reorganization energy (λh) than the corresponding isophorone extended conjugated-based compounds (I–II). Conversely, compounds I and II had lower electron reorganization energy (λe) than the corresponding fused alkyne compounds. This is in line with the observed adiabatic ionization potential and electron affinity values. Consequently, devices fabricated with compound VI exhibited high mobility and low threshold voltage.

Normally-off GaON/p-GaN gate HEMTs with selective plasma oxidation: from structural characterization, performance improvement to physical mechanism

In this letter, a p-GaN gate high-electron-mobility transistor (HEMT) with a high threshold voltage and better gate reliability was demonstrated by using selective plasma oxidation (SPO) and an additional low-temperature annealing step before gate metal is evaporated. After the SPO, a gallium oxynitride (GaON) dielectric layer was formed on the surface of p-GaN under the gate metal, and was studied by x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry, high-resolution transmission electron microscopy and energy dispersion spectroscopy. In addition, the fabricated metal/GaON/p-GaN gate HEMT exhibited a large threshold voltage (V TH) improvement from 1.46 V to 2.47 V. Furthermore, the forward gate breakdown voltage (V GS,BD) increased from 7.55 V to 11.10 V, and the maximum forward gate operating voltage (V GS-max) significantly improved from 5.0 V to 7.80 V for a ten-year lifetime with a 63.2% failure rate. Kelvin probe force microscopy reveals that the surface potential increased after SPO, and the shift of valence band maximum obtained by XPS spectra was 0.7 eV lower than that of the p-GaN, which further improves the Schottky barrier height at the gate metal/GaON interfaces to holes, thereby improving V TH and reducing I GS of the device. As a barrier layer, GaON suppressed the injection of carrier into the depletion region under a high electric field and enhanced the reliability of the gate.

Suppression of High Threshold Voltage for Boron-Doped Diamond MOSFETs

Suppression of High Threshold Voltage for Boron-Doped Diamond MOSFETs

Monolithic fully-controlled HEMT bidirectional power switch with merged Schottky barrier diodes and p-GaN gate transistors for ultra-low turn-on voltage and high threshold voltage

A monolithic fully-controlled high electron mobility transistor (HEMT) bidirectional power switch (FC-HEMT BPS) is studied and implemented for ultra-low turn-on voltage and high threshold voltage. The alternately embedded recess Schottky barrier diodes with p-GaN gate HEMTs in the common active region is a distinctive feature of the proposed device. The FC-HEMT BPS is a sophisticated device with two independent gate controls, capable of operating in four distinct modes. Its flexibility in managing current and voltage in the first and third quadrants makes it versatile for a range of applications in power electronics. The proposed device offers notable advantages over conventional counterparts, including reduced turn-on voltage V ON-D of ∼0.4 V in diode modes and the elimination of turn-on voltage V ON-B (i.e. 0 V) in bidirectional conduction mode. These features make it a promising solution for efficient power electronics applications.

Dynamics of antiphase bursting modulated by the inhibitory synaptic and hyperpolarization-activated cation currents.

Antiphase bursting related to the rhythmic motor behavior exhibits complex dynamics modulated by the inhibitory synaptic current (Isyn), especially in the presence of the hyperpolarization-activated cation current (Ih). In the present paper, the dynamics of antiphase bursting modulated by the Ih and Isyn is studied in three aspects with a theoretical model. Firstly, the Isyn and the slow Ih with strong strength are the identified to be the necessary conditions for the antiphase bursting. The dependence of the antiphase bursting on the two currents is different for low (escape mode) and high (release mode) threshold voltages (Vth) of the inhibitory synapse. Secondly, more detailed co-regulations of the two currents to induce opposite changes of the bursting period are obtained. For the escape mode, increase of the Ih induces elevated membrane potential of the silence inhibited by a strong Isyn and shortened silence duration to go beyond Vth, resulting in reduced bursting period. For the release mode, increase of the Ih induces elevated tough value of the former part of the burst modulated by a nearly zero Isyn and lengthen burst duration to fall below Vth, resulting in prolonged bursting period. Finally, the fast-slow dynamics of the antiphase bursting are acquired. Using one-and two-parameter bifurcations of the fast subsystem of a single neuron, the burst of the antiphase bursting is related to the stable limit cycle, and the silence modulated by a strong Isyn to the stable equilibrium to a certain extent. The Ih mainly modulates the dynamics within the burst and quiescent state. Furthermore, with the fast subsystem of the coupled neurons, the silence is associated with the unstable equilibrium point. The results present theoretical explanations to the changes in the bursting period and fast-slow dynamics of the antiphase bursting modulated by the Isyn and Ih, which is helpful for understanding the antiphase bursting and modulating rhythmic motor patterns.

Enhanced performance of conjugated polymer-based ternary memory devices by embedding Pt nanoparticles

Organic ternary memory devices show higher data storage density than common inorganic binary memory, however, facing the challenge of the low ON/OFF current ratio and high threshold voltage hinders their application. Herein, a conjugated polymer, poly[2,7-(9,9-dioctyl)-fluorene-alt-12 H-benz[5,6]-isoindole[2,1-a] benzimidazole-12-one] (PFBIM), was synthesized and applied to fabricate memristors by spin coating. The PFBIM-based memristors showed unique ternary memory behavior, but their ON/OFF ratio is just 9.6 × 103 and the threshold voltage is − 1.35 V. To improve memory performance, the strategy of polymer film blends Pt NPs was implemented. Impressively, enhanced performance was achieved when the PFBIM-based memristor incorporated 5% Pt NPs, the ON/OFF ratio increased to 1.37 × 105 and the threshold voltage decreased to − 0.65 V. The ternary switching mechanism and the role of Pt NPs on enhanced memristor performance were studied. This work may provide useful information to ameliorate performance and promote the further advance of ternary memory devices.