Heterostructure Field-effect Transistors Research Articles

Proton Irradiation Effects on the Time-Dependent Dielectric Breakdown Characteristics of Normally-Off AlGaN/GaN Gate-Recessed Metal-Insulator-Semiconductor Heterostructure Field Effect Transistors.

In this work, we investigated the time-dependent dielectric breakdown (TDDB) characteristics of normally-off AlGaN/GaN gate-recessed metal–insulator–semiconductor (MIS) heterostructure field effect transistors (HFETs) submitted to proton irradiation. TDDB characteristics of normally-off AlGaN/GaN gate-recessed MISHFETs exhibited a gate voltage (VGS) dependence as expected and showed negligible degradation even after proton irradiation. However, a capture emission time (CET) map and cathodoluminescence (CL) measurements revealed that the MIS structure was degraded with increasing trap states. A technology computer aided design (TCAD) simulation indicated the decrease of the vertical field beneath the gate due to the increase of the trap concentration. Negligible degradation of TDDB can be attributed to this mitigation of the vertical field by proton irradiation.

Nonlinearity in piezotransistive GaN microcantilevers

We have investigated nonlinear dynamic characteristics of piezotransistive GaN microcantilevers, with integrated AlGaN/GaN heterostructure field effect transistor as a highly sensitive deflection transducer. When excited with a piezochip actuator, both softening and hardening type of nonlinear behavior were exhibited by the GaN microcantilevers in their first flexural mode. The nonlinear behavior was found to strongly depend on the dimensions of these microcantilevers. While the hardening behavior was found to be enhanced with increase in length of the cantilever for a fixed width, the nonlinear behavior changed much more sharply with width, exhibiting a clear switchover from hardening to softening type, with an increase in width of the cantilever for a constant length. The nonlinear characteristics of the cantilevers were modeled using a Duffing equation with excellent agreement observed between the theoretical model and experimental data for both nonlinearity types and frequency sweep directions.

Light-Enhanced Ion Migration in Two-Dimensional Perovskite Single Crystals Revealed in Carbon Nanotubes/Two-Dimensional Perovskite Heterostructure and Its Photomemory Application.

Two-dimensional (2D) hybrid perovskite sandwiched between two long-chain organic layers is an emerging class of low-cost semiconductor materials with unique optical properties and improved moisture stability. Unlike conventional semiconductors, ion migration in perovskite is a unique phenomenon possibly responsible for long carrier lifetime, current–voltage hysteresis, and low-frequency giant dielectric response. While there are many studies of ion migration in bulk hybrid perovskite, not much is known for its 2D counterparts, especially for ion migration induced by light excitation. Here, we construct an exfoliated 2D perovskite/carbon nanotube (CNT) heterostructure field effect transistor (FET), not only to demonstrate its potential in photomemory applications, but also to study the light induced ion migration mechanisms. We show that the FET I–V characteristic curve can be regulated by light and shows two opposite trends under different CNT oxygen doping conditions. Our temperature-dependent study indicates that the change in the I–V curve is probably caused by ion redistribution in the 2D hybrid perovskite. The first principle calculation shows the reduction of the migration barrier of I vacancy under light excitation. The device simulation shows that the increase of 2D hybrid perovskite dielectric constant (enabled by the increased ion migration) can change the I–V curve in the trends observed experimentally. Finally, the so synthesized FET shows the multilevel photomemory function. Our work shows that not only we could understand the unique ion migration behavior in 2D hybrid perovskite, it might also be used for many future memory function related applications not realizable in traditional semiconductors.

Full Energy Spectra of Interface State Densities for n‐ and p‐type MoS2 Field‐Effect Transistors

Abstract2D materials are promising to overcome the scaling limit of Si field‐effect transistors (FETs). However, the insulator/2D channel interface severely degrades the performance of 2D FETs, and the origin of the degradation remains largely unexplored. Here, the full energy spectra of the interface state densities (Dit) are presented for both n‐ and p‐ MoS2 FETs, based on the comprehensive and systematic studies, i.e., full rage of channel thickness and various gate stack structures with h‐BN as well as high‐k oxides. For n‐MoS2, Dit around the mid‐gap is drastically reduced to 5 × 1011 cm−2 eV−1 for the heterostructure FET with h‐BN from 5 × 1012 cm−2 eV−1 for the high‐k top‐gate. On the other hand, Dit remains high, ≈1013 cm−2 eV−1, even for the heterostructure FET for p‐MoS2. The systematic study elucidates that the strain induced externally through the substrate surface roughness and high‐k deposition process is the origin for the interface degradation on conduction band side, while sulfur‐vacancy‐induced defect states dominate the interface degradation on valance band side. The present understanding of the interface properties provides the key to further improving the performance of 2D FETs.

Enhancement-mode n-GaN gate p-channel heterostructure field effect transistors based on GaN/AlGaN 2D hole gas

In this letter, E-mode n-GaN gate heterostructure field effect transistors (HFETs) utilizing polarization induced 2D hole gas (2DHG) have been proposed. It is found that the introduction of the n-GaN cap between the GaN channel layer and the gate metal can effectively deplete the 2DHG in the gate region, resulting in E-mode operation. The simulation results indicate that by adjusting the GaN channel thickness and n-GaN cap doping concentration, threshold voltage of the n-GaN gate p-channel HFETs can be more than |−1.5| V without sacrificing the ON-state current and the ON/OFF ratio, which is enabled to overcome the trade-off observed from conventional p-channel devices.

Band Offsets of Insulating & Semiconducting Oxides on (AlxGa1-x)O3

β-Ga2O3 is a wide bandgap semiconductor (~4.8 eV) with properties suited to power electronics, truly solar-blind UV detection, and extreme environment applications. Additional bandgap tunability can be achieved through incorporation of Al into β-Ga2O3, leading to β-(Al,Ga)2O3 monoclinic phase alloys with the bandgap tuned from 4.8- 6 eV. This enables heterostructure designs such as modulation-doped electron channels, quantum wells, and superlattices. Out of these possible structures, β-(AlxGa1-x)2O3/Ga2O3 heterostructure field-effect transistors (HFETs) have shown impressive and improving performance compared to Ga2O3 based metal semiconductor field effect transistor (MESFETs). For these heterostructures, other wide bandgap oxides can be used as gates dielectrics, and narrow energy bandgap conductive oxides can be utilized to reduce contact resistance between the ohmic contacts and semiconductor surface. For gate dielectrics on (AlxGa1-x)2O3, one must utilize oxide materials with a bandgap of 7 eV of higher to have adequate offsets in both the conduction and valence bands. In this study, we utilize x-ray photoelectron spectroscopy (XPS) to report on the determination of the band alignment of various insulating and semiconducting dielectrics onto (AlxGa1-x)2O3 at various aluminum concentrations. The dielectrics were deposited using atomic layer deposition (ALD) or sputtering. For SiO2 and Al2O3, there are differences of up to 1 eV in band alignments on single crystal (AlxGa1-x)2O3, depending on whether they are deposited by sputtering or ALD. For the conductive oxides, AZO/ β-(AlxGa1-x)2O3 and ITO/ β-(AlxGa1-x)2O3 heterojunctions have a nested gap (type I) band offset. The valence and conduction band offsets are negative in both cases and can be used to enhance carrier transport across the heterointerface.

A Computational Study of a Heterostructure Tunneling Carbon Nanotube Field-Effect Transistor

In this paper a heterostructure tunneling carbon nanotube field-effect transistor, namely H-T-CNTFET, is introduced. In the proposed structure, a carbon nanotube with a different chirality is used for the drain side compared to the source and the channel region. Characteristics of the proposed structure are numerically simulated by a mode-space non-equilibrium Green’s function formulism in the ballistic limit. Simulation results show that, compared to a conventional tunneling carbon nanotube field-effect transistor, the proposed structure has higher ON-current, better ambipolar behavior and better switching characteristics. In addition, the analog characteristics of the device, such as the transconductance (gm) and unity-gain frequency (fT), are also improved.

High-breakdown-voltage AlGaN-channel metal-insulator-semiconductor heterostructure field-effect transistors employing a quaternary AlGaInN barrier layer and an Al2O3 gate insulator

In this study, Al2O3-gate-insulated metal-insulator-semiconductor (MIS) heterostructure field-effect transistors (HFETs) were fabricated using an Al0.61Ga0.37In0.02N/Al0.18Ga0.82N two-dimensional-electron-gas heterostructure, and their electrical properties were characterized. It was confirmed that the thermally stable quaternary AlGaInN barrier layer contributed to a good ohmic contact resistance of 10.5 Ω mm. This value seemed to be considerably small as an AlGaN-channel heterostructure. The fabricated MIS-HFETs showed good pinch-off characteristics and exhibited a maximum drain current (IDSmax) of approximately 180 mA/mm at the gate bias of +2 V. A high off-state breakdown voltage of 2.5 kV was obtained for the device with a gate-to-drain length of 20 μm.

Ultrahigh Gauge Factor in Graphene/MoS2 Heterojunction Field Effect Transistor with Variable Schottky Barrier.

Piezoelectricity of transition metal dichalcogenides (TMDs) under mechanical strain has been theoretically and experimentally studied. Powerful strain sensors using Schottky barrier variation in TMD/metal junctions as a result of the strain-induced lattice distortion and associated ion-charge polarization were demonstrated. However, the nearly fixed work function of metal electrodes limits the variation range of a Schottky barrier. We demonstrate a highly sensitive strain sensor using a variable Schottky barrier in a MoS2/graphene heterostructure field effect transistor (FET). The low density of states near the Dirac point in graphene allows large modulation of the graphene Fermi level and corresponding Schottky barrier in a MoS2/graphene junction by strain-induced polarized charges of MoS2. Our theoretical simulations and temperature-dependent electrical measurements show that the Schottky barrier change is maximized by placing the Fermi level of the graphene at the charge neutral (Dirac) point by applying gate voltage. As a result, the maximum Schottky barrier change (ΔΦSB) and corresponding current change ratio under 0.17% strain reach 118 meV and 978, respectively, resulting in an ultrahigh gauge factor of 575 294, which is approximately 500 times higher than that of metal/TMD junction strain sensors (1160) and 140 times higher than the conventional strain sensors (4036). The ultrahigh sensitivity of graphene/MoS2 heterostructure FETs can be developed for next-generation electronic and mechanical-electronic devices.

Current collapse in high-Al channel AlGaN HFETs

We report the first study of current collapse in ultra-wide bandgap high-Al AlGaN channel heterostructure field-effect transistors (HFETs) using short-pulse biasing. Our results show that, under applied pulsed gate and drain voltages, the current collapse results from increased resistance of the source- gate and gate-drain regions but not from the channel under the gate. We also show that passivation of access regions of the high-Al channel HFET with SiNx results in significant reduction in current collapse.

Design optimization of the graded AlGaN/GaN HEMT device performance based on material and physical dimensions

PurposeTo design and optimize the traditional aluminum gallium nitride/gallium nitride high electron mobility transistor (HEMT) device in achieving improved performance and current handling capability using the Synopsys’ Sentaurus TCAD tool.Design/methodology/approachVarying material and physical considerations, specifically investigating the effects of graded barriers, spacer interlayer, material selection for the channel, as well as study of the effects in the physical dimensions of the HEMT, have been extensively carried out.FindingsCritical figure-of-merits, specifically the DC characteristics, 2DEG concentrations and mobility of the heterostructure device, have been evaluated. Significant observations include enhancement of maximum current density by 63 per cent, whereas the electron concentration was found to propagate by 1,020 cm−3 in the channel.Practical implicationsThis work aims to provide tactical optimization to traditional heterostructure field effect transistors, rendering its application as power amplifiers, Monolithic Microwave Integrated Circuit (MMICs) and Radar, which requires low noise performance and very high radio frequency design operations.Originality/valueAnalysis in covering the breadth and complexity of heterostructure devices has been carefully executed through extensive TCAD modeling, and the end structure obtained has been optimized to provide best performance.

Al0.75Ga0.25N/Al0.6Ga0.4N heterojunction field effect transistor with fT of 40 GHz

An Al0.75Ga0.25N/Al0.6Ga0.4N heterostructure field effect transistor with graded MBE-regrown contacts is designed, grown, and fabricated on AlN/sapphire substrate. Maximum drain current density (ID,max) of 460 mA mm−1 was obtained on transistors with gate length of 130 nm. The small signal measurement shows current/power gain cutoff frequency (fT/fmax) of 40 GHz/58 GHz, respectively. Parasitic resistance is found to be a significant factor limiting the frequency performance of the devices. The high frequency performance of devices with high Al concentration AlGaN channel demonstrate potential for high power and high frequency applications.

High photosensitivity AlGaN/GaInN/GaN heterojunction field-effect transistor type visible photosensors

We realized the high-performance AlGaN/GaInN/GaN-based heterostructure field-effect transistor type visible photosensors with high photosensitivity and high rejection ratio. We designed the photosensors including two-dimentional electron gas layer at AlGaN/GaInN/GaN hetero-interface to detect visible light with high photosensitivity. Also, carrier depletion using p-type GaN gate was applied for reduction of the dark current. Furthermore, we realized photosensor with externally low dark current density by applying a C-doped GaN layer as an underlying layer. We found that inserting an unintentionally doped GaN interlayer between the GaInN active layer and the C-doped GaN underlying layer is important for realizing a high-performance photosensors. By employing these device designs, a high photosensitivity of 104 A W−1 at wavelength of 430 nm and high rejection ratio of more than 106 were realized under the irradiation of 100 μW cm−2. The absorption edge wavelength was approximately 480 nm corresponding to the bandgap energy of GaInN active layer. Therefore, this device structure is useful as the visible photosensor with high sensitivity and high rejection ratio.

Simulation of the RF Power Performance of a GaN HFET and Comparison to Experiment

The large-signal RF power performance of an AlGaN/GaN heterostructure field-effect transistor (HFET) is simulated by technology computer-aided design (TCAD) software, and compared to measurement. A clear procedure for extraction of the simulation physical parameters is described. Trapping effects are included, but temperature effects are not. Good agreement between simulation and measurement is demonstrated, paving the way for efficiency optimization of GaN HFETs using TCAD.

Simulation of a high-performance enhancement-mode HFET with back-to-back graded AlGaN layers

A novel three-dimensional hole gas (3DHG)enhancement-mode (E-mode) heterostructure field-effect transistor (HFET) isproposed and investigated. It features back-to-back graded AlGaN (BGA)barrier layers consisting of a positive-graded AlGaN layer and anegative-graded AlGaN layer, which form polarization gradient andsubsequently induce the three-dimensional electron gas (3DEG) and 3DHG inthe positive- and negative-graded AlGaN layers, respectively. The source anddrain are located at the same side of the metal-insulator-semiconductor(MIS) trench gate, and the source is in contact with the HfO$_{2}$ gateinsulator. First, the on-state current is significantly improved owing tothe high-density 3DEG in the positive-graded AlGaN. Next, the verticalconductive channel between the source and 3DEG is blocked by the 3DHG,thereby realizing the E-mode. The threshold voltage ($V_{\rm~th})$ can bemodulated by a partial doping conductive channel. Subsequently, a highbreakdown voltage (BV) is obtained, because the polarization junction formedby the polarization charges assists in depleting the drift region in theoff-state. Next, the BGA-HFET is smaller than the conventional HFET(Con-HFET) owing to the special location of the source. The BV of theproposed HFET sharply increases to 919 V from 39 V of the Con-HFET with thesame gate-drain spacing, and the saturation drain current is increased by103.5%

Fast and slow interface traps in transparent NiO gated AlGaN/GaN heterostructure field-effect transistors

In the present study, the reliability of transparent NiO gated AlGaN/GaN heterostructure field-effect transistors have been investigated. Compared with the TiN Schottky gated device, the p-NiO electrode can suppress the gate leakage and shift the threshold voltage positively. Pulse-mode and temperature-dependent current-voltage measurements demonstrate that both kinds of devices present good stability. The frequency-dependent conductance measurement confirms that only fast trap is detected for the TiN gated device while both fast and slow traps for the NiO gated one. The slow trap is ascribed to the hole trapping in the type-II staggered band configuration between NiO and AlGaN/GaN, which is also confirmed from the obvious threshold voltage instability in high frequency loop scan curves.

Efficient Atomistic Simulation of Heterostructure Field-Effect Transistors

In this paper, atomistic-level quantum mechanical simulations are performed for nanoscale field-effect transistors (FETs) with lateral or vertical heterojunction, within the non-equilibrium Green’s function formalism. For efficient simulation of such heterostructure FETs, a novel approach is developed where the Green’s functions are calculated by complementarily using the two algorithms of the recursive Green’s function and the $R$ -matrix. The $R$ -matrix algorithm is extended to seamlessly combine the two methods on the open system and an algorithm for the electron correlation function based on the extended $R$ -matrix algorithm is also developed. The proposed method significantly reduces simulation time, making rigorous atomistic simulations of heterojunction FETs possible. As an application, device simulations are performed for the germanane/InSe vertical tunneling FET (VTFET) modeled through the first-principles density functional theory. Our simulation results reveal that the germanane/InSe VTFET is a promising candidate for future low power applications.

Improved Ultraviolet Detection and Device Performance of Al2O3-Dielectric In0.17Al0.83N/AlN/GaN MOS-HFETs

Ultraviolet (UV) detection and electrical characteristics of In0.17Al0.83N/AlN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOS-HFETs) with Al2O3 gate-dielectric and passivation formed by using ultrasonic spray pyrolysis deposition (USPD) are studied with respect to a conventional Schottky-gate HFET. The present MOS-HFET (Schottky-gate HFET) has demonstrated superior spectral responsivity (SR) of 360 (340) A/W at 350 nm at $V_{\mathrm{ GS}} = 5$ (3) V and $V_{\mathrm{ DS}} = 6$ (7) V, maximum drain–source saturation current density ( $I_{\mathrm{ DS, max}}$ ) of 810.5 (546.6) mA/mm, maximum extrinsic transconductance of ( $g_{m, \max }$ ) of 180.4 (221.2) mS/mm, gate-voltage swing (GVS) of 2.4 (0.5) V, on/off current ratio ( $I_{\mathrm{ on}}/I_{\mathrm{ off}}$ ) of $5.5 \times \,\,10^{8}$ ( $1.7 \times \,\,10^{5}$ ), two-terminal off-state gate–drain breakdown voltage (BVGD) of −158.5 (−127) V, three-terminal drain–source breakdown voltage (BVDS) of 162 (83.4) V at $V_{\mathrm{ GS}} = -10$ V, and power-added efficiency (P.A.E.) of 26.3% (16.5%) at 2.4 GHz at 300 K. In addition to the improved device performance, this paper demonstrates, for the first time, the UV sensing based on an InAlN/AlN/GaN MOS-HFET design.

Impact of oxide/barrier charge on threshold voltage instabilities in AlGaN/GaN metal-oxide-semiconductor heterostructures

Threshold voltage instabilities in AlGaN/GaN metal-oxide-semiconductor (MOS) heterostructures were investigated by means of capacitance-voltage (CV) hysteresis measurement and simulations. We focused on the impact of net charge at oxide/semiconductor interface (Qint) on the CV hysteresis. Our simulations suggest that due to different band bending at the interface, any positive or negative Qint with lower density (in the order of 1012 cm−2) results in a presence of relatively shallow unoccupied oxide/barrier interface states in equilibrium. On the other hand, high density of negative Qint (Qint/q ≈ −1013 cm−2) results in very deep unoccupied interface states, which in turn leads to incomplete electron re-emission during backward CV sweep and thus increased CV hysteresis of the MOS heterostructures compared to previous case. Impact of Qint is illustrated experimentally on MOS heterostructures with Al2O3 gate dielectric grown by metal-organic chemical vapor deposition, showing Qint of −1.2 × 1013 and +0.5 × 1012 cm−2 for structures with and without post-deposition annealing, respectively. Our results therefore suggest that normally-off AlGaN/GaN MOS heterostructure field-effect transistors featuring high density of negative Qint can be expected to be more susceptible for threshold voltage instabilities compared to normally-on counterparts.

Two-dimensional phosphorene/C3N p-n heterostructure: Effect of contact type on electronic and optical properties

p-n heterostructure (HTS) is a fundamental component for high-performance electronic and optoelectronic device. Vertical stacking through van der Waals (vdW) force is emerging as a feasible technique to construct p-n HTS. Herein, we designed a novel kind of direct-bandgap C3N monolayer, via adjusting the arrangement of C and N atoms in C3N hexagonal cell. On the basis of the density functional theory combined with the non-equilibrium Green’s function method, we built two-dimensional vdW-contact phosphorene (BP)/C3N p-n HTS, and analyzed its electronic and optical properties in comparison with the inplane-jointed ones. The strong charge transfer between BP and C3N segments results in a wide bandgap of 0.48 eV for joint-contact type BP/C3N HTS, whereas the effective interlayer coupling in vdW-contact type leads to an improved light adsorption as compared to the isolated C3N monolayer. By fabricating dual-gated BP/C3N HTS field-effect transistors (FETs), the dynamic transport behaviors demonstrated that the band bending under a lower threshold voltage makes band-to-band tunneling possible for vdW-contact type. Our work suggests that vdW-contact type is superior to joint-contact type in constructing p-n HTS for high-performance electronic and optoelectronic devices.