2DEG Channel Research Articles

Novel double channel reverse conducting GaN HEMT with an integrated MOS-channel diode

A novel normally-off double channel reverse conducting (DCRC) HEMT with an integrated MOS-channel diode (MCD) is proposed and investigated by TCAD simulation. The proposed structure has two features: one is double heterojunctions to form dual 2DEG channels named the 1st path and the 2nd path for reverse conduction, and the other is the MCD forming by the trench source metal, source dielectric, and GaN. At the initial reverse conduction stage, the MCD acts as a switch to control the 1st path which would be turned on prior to the 2nd path. Because of the introduction of the 1st path, the DCRC-HEMT has an additional reverse conducting channel to help enhance the reverse conduction performance. Compared with the conventional HEMT (Conv. HEMT), the DCRC-HEMT can obtain a low reverse turn-on voltage (V RT) and its V RT is independent of the gate–source bias (V GS) at the same time. The DCRC-HEMT achieves the V RT of 0.62 V, which is 59.7% and 75.9% lower than that of the Conv. HEMT at V GS = 0 V and −1 V, respectively. In addition, the forward conduction capability and blocking characteristics almost remain unchanged. In the end, the key fabrication flows of DCRC-HEMT are presented.

AlGaN solar-blind phototransistor capable of directly detecting sub-fW signals: self-depletion and photorecovery of full-channel 2DEG enabled by a quasi-pseudomorphic structure

Heterojunction field-effect phototransistors using two-dimensional electron gas (2DEG) for carrier transport have great potential in photodetection owing to its large internal gain. A vital factor in this device architecture is the depletion and recovery of the 2DEG under darkness and illumination. This is usually achieved by adding an external gate, which not only increases the complexity of the fabrication and the electrical connection but also has difficulty ensuring low dark current (I dark ). Herein, a quasi-pseudomorphic AlGaN heterostructure is proposed to realize the self-depletion and photorecovery of the 2DEG, in which both the barrier and the channel layers are compressively strained, making the piezoelectric and spontaneous polarization reverse, thus depleting the 2DEG and tilting the entire barrier and channel band to form two built-in photogates. The fabricated solar-blind phototransistors exhibit a very low I dark below 7.1×10−10 mA/mm, a superhigh responsivity (R) of 2.9×109 A/W, a record high detectivity (D*) of 4.5×1021 Jones, and an ultrafast response speed at the nanosecond level. The high performance is attributed to the efficient depletion and recovery of the full 2DEG channel by the two photogates, enabling direct detection of the sub-fW signal. This work provides a simple, effective, and easily integrated architecture for carrier control and supersensitive photodetection based on polarization semiconductors.

Enhanced quantum oscillations and scattering effect in quaternary InAlGaN/GaN two-dimensional electron gas

Quantum transport properties of a large bandgap In0.15Al0.79Ga0.06N/GaN quaternary GaN high electron mobility transistor (HEMT) heterostructure are studied at low temperatures up to 2 K. Herein, we report the first evidence of weak localization in a quaternary GaN two-dimensional electron gas (2DEG) system. We observe negative magnetoresistance behavior and extracted dephasing time (τΦ) using a Hikami–Larkin–Nagaoka model at 2.2 K. Linear dependency of dephasing rate with temperature (τΦ−1∝T) is established below 20 K. Furthermore, Shubnikov–de Haas quantum oscillation induced by 2DEG is observed using perpendicular magnetic (B⊥) field strengths up to 14 T. From the temperature-dependent oscillation amplitude, we extracted an effective mass m*≈0.237me. The dominance of small-angle scattering in the 2DEG channel is identified from less than unit ratio (τq/τt≪1) of quantum lifetime (τq) to the Hall transport lifetime (τt). In our study, we have demonstrated that the In0.15Al0.79Ga0.06N/GaN quaternary heterostructure possesses high dephasing time (τΦ=5.4 ps) and larger quantum lifetime (τq=0.102 ps) indicating better suitability and a way forward to high-power–high-frequency GaN HEMT development.

AlGaN-based metal–semiconductor–metal photodiodes fabricated with nonplanar structure geometry for ultraviolet light detection

Based upon GaN-on-Si wafers with a Al0.24Ga0.76N/AlN/GaN heterostructure, metal–semiconductor–metal photodetectors (MSM PDs) with nonplanar structure geometry are proposed for ultraviolet (UV) light detection. Although the presence of a highly conductive two-dimensional electron gas (2DEG) channel in the Al0.24Ga0.76N/AlN/GaN heterostructure is beneficial for realizing a high-electron-mobility transistor with a drain current of 600 mA/mm and a transconductance of 96 mS/mm, a high dark current is also observed in Schottky-barrier based MSM PDs. Using the nonplanar structure design for the MSM PDs (i.e., one Schottky contact formed on the top surface of the AlGaN mesa while the other is on the GaN buffer) achieves a significant decrease in PD’s dark current (<6 × 10−10 A). In addition, an anomalous increase in dark current of the proposed MSM PDs was found at forward bias, which could be attributed to the reduced potential barrier of the electrons in 2DEG and/or the lowering of the Schottky barrier height by hole trapping at the metal/semiconductor interface. Due to the improved signal to noise ratio and increased dissociation of the photogenerated carriers at the reverse junction (i.e., the reversely biased Schottky contact), the proposed MSM PDs operating at reverse bias exhibit enhanced UV photoresponsivity at λ < 360 nm. In addition to being used for UV light detection, these MSM PDs with a 3-dB bandwidth of 29.6 Hz are also useful for potential optical communications applications.

MOCVD-grown β-Ga2O3 as a Gate Dielectric on AlGaN/GaN-Based Heterojunction Field Effect Transistor

We report the electrical properties of Al0.3Ga0.7N/GaN heterojunction field effect transistor (HFET) structures with a Ga2O3 passivation layer grown by metal–organic chemical vapor deposition (MOCVD). In this study, three different thicknesses of β-Ga2O3 dielectric layers were grown on Al0.3Ga0.7N/GaN structures leading to metal-oxide-semiconductor-HFET or MOSHFET structures. X-ray diffraction (XRD) showed the (2¯01) orientation peaks of β-Ga2O3 in the device structure. The van der Pauw and Hall measurements yield the electron density of ~ 4 × 1018 cm−3 and mobility of ~770 cm2V−1s−1 in the 2-dimensional electron gas (2DEG) channel at room temperature. Capacitance–voltage (C-V) measurement for the on-state 2DEG density for the MOSHFET structure was found to be of the order of ~1.5 × 1013 cm−2. The thickness of the Ga2O3 layer was inversely related to the threshold voltage and the on-state capacitance. The interface charge density between the oxide and Al0.3Ga0.7N barrier layer was found to be of the order of ~1012 cm2eV−1. A significant reduction in leakage current from ~10−4 A/cm2 for HFET to ~10−6 A/cm2 for MOSHFET was observed well beyond pinch-off in the off-stage at -20 V applied gate voltage. The annealing at 900° C of the MOSHFET structures revealed that the Ga2O3 layer was thermally stable at high temperatures resulting in insignificant threshold voltage shifts for annealed samples with respect to as-deposited (unannealed) structures. Our results show that the MOCVD-gown Ga2O3 dielectric layers can be a strong candidate for stable high-power devices.

InAlN/GaN HEMT With n+GaN Contact Ledge Structure for Millimeter-Wave Low Voltage Applications

In this work, high performance InAlN/GaN HEMT based on the n+GaN regrown ohmic contact with n+GaN contact ledge structure is proposed. The regrown ohmic contact of InAlN/GaN HEMT is formed by MBE n+GaN regrowth and selfstopping etching, which makes the total ohmic contact resistance between the 2DEG channel and the ohmic metal decrease to 0.12 Ωmm and forms n+GaN contact ledge structure. Owing to the n+GaN contact ledge on the InAlN barrier, with the increasing of drain-source voltage (VDS), an additional current path comes into being between the n+GaN contact ledge on the InAlN barrier and the 2DEG channel, which can “shorten” the device effective drainsource distance, thus further reducing the parasitic resistance. Compared with regrown InAlN/GaN HEMT without n+GaN contact ledge structure, the peak transconductance (Gm.max) of regrown InAlN/GaN HEMT with n+GaN contact ledge structure increases from 747 mS/mm to 874 mS/mm, and the saturation current density (ID.max) increases from 2.6 A/mm to 2.9 A/mm. Besides, the self-stopping etching on the access region does not induce extra defects, and negligible current collapse is obtained. As the results of low parasitic resistance, high output current density, low knee voltage and negligible current collapse, poweradded-efficiency (PAE) of 44% together with output power density (Pout) of 2.5 W/mm is achieved at 30 GHz and VDS of 10 V, which indicates regrown InAlN/GaN HEMT with n+GaN contact ledge structure has great potential for millimeter-wave low voltage applications. Additionally, the transfer and schottky gate characteristics show negligible degradation after OFF/ON-state electrical stress tests.

Recessed-gate AlGaN/GaN MIS-FETs with dual 2DEG channels

In this study, a normally-on AlGaN/GaN metal–insulator-semiconductor high electron mobility transistor (MIS-HEMT) and a normally-off AlGaN/GaN metal–insulator-semiconductor field-effect transistor (MIS-FET) with recessed MIS gate are fabricated by dual-channel epitaxy structure. The recessed region is cleaned by dipping it into diluted buffered oxide etch (BOE), HCl, and tetramethylammonium hydroxide (TMAH) solutions. After cleaning, a 20 nm Al2O3 is deposited by atomic layer deposition (ALD). Then, the post-deposition annealing is executed. Non-recessed MIS-HEMT demonstrates a threshold voltage of −9.81 V, a high drain current of 660.76 mA mm−1, and a small on-resistance of 6.47 Ω mm. Recessed gate MIS-FET shows a positive threshold voltage of 1.46 V with a drain current of 144.81 mA mm−1. In addition, the devices present a high gate breakdown voltage of 26.5 V by non-recessed MIS-HEMT and 18.5 V by recessed MIS-FET. Both devices show a low I–V hysteresis under double sweep I D–V GS measurement.

Normally-OFF AlGaN/GaN-based HEMTs with decreasingly graded AlGaN cap layer

In this work, an enhancement-mode (E-mode) AlGaN/GaN-based high-electron-mobility transistor (HEMT) with a graded AlGaN cap layer (GACL) is proposed and numerically studied by Silvaco technology computer-aided design. The GACL is designed with a decreasingly graded Al composition x along [0001] direction and the initial x is smaller than the Al composition of the Al0.2Ga0.8N barrier layer (BL). This GACL scheme can simultaneously produce high-concentration polarization-induced holes and negative net polarization charges at the GACL/BL interface. This can facilitate the separation of the conduction band (E C) and Fermi level (E F) at the 2DEG channel and therefore benefit the normally-OFF operation of the device. The optimized graded-AlGaN-gated metal-semiconductor HEMT can achieve a large threshold voltage of 4 V. Furthermore, we demonstrated that shortening the gate length on the GACL and inserting an oxide layer between the gate and GACL can be both effective to suppress gate leakage current, enhance gate voltage swing, and improve on-state drain current of the device. These numerical investigations can provide insights into the physical mechanisms and structural innovations of the E-mode GaN-based HEMTs in the future.

Determination of hot-electron drift velocity in (Be)ZnMgO/ZnO 2DEG channels

Recent experimental study of electron transport in ZnO/ZnMgO and BeZnMgO/ZnO heterostructures containing two-dimensional electron gas (2DEG) channels of two polarities is reported where electrons are accelerated and become hot by a pulsed electric field. The measurements with electrical pulses ranging from 2 ns to 10 ns in duration ensure the control of self-heating effect. Electron transport in the ZnO 2DEG channels located in ZnO layers at the ZnMgO or BeZnMgO barrier or in ZnO 3DEG channels is treated mainly in terms of drift velocity. The highest values of 1.3 × 107 cm/s at 360 kV/cm, 2.0 × 107 cm/s at 270 kV/cm, and 2.5 × 107 cm/s and 320 kV/cm, respectively, are attained and explained by emphasizing the effect of hot phonons.

Hybrid GaO/AlGaN/GaN Ultraviolet Detector With Gate Metal in the Grooved AlGaN Layer for Obtaining Low Dark Current and Large Detectivity

In this work, a metal/Ga2O3/AlGaN/GaN hybrid-structured metal–semiconductor–metal ultraviolet photodetector (MSM UV PD) with low dark current has been proposed and fabricated. In the dark condition, the depletion region formed by the metal gate and the AlGaN layer pinches off the two-dimensional electron gas (2DEG) channel, and we can obtain a dark current even lower than 10−10 A/cm2. In the illumination condition, due to the electric field formed by the metal and the Ga2O3 layer, the photogenerated electrons will move to the AlGaN/GaN channel to form the 2DEG. We then get a photo-to-dark current ratio of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.77\times10$ </tex-math></inline-formula> 8. Furthermore, the detectivity of the device is higher than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.30\times10$ </tex-math></inline-formula> 12 Jones when a 254-nm UV illumination signal is applied.

Self-Heating of Annealed Ti/Al/Ni/Au Contacts to Two-Dimensional Electron Gas in AlGaN/GaN Heterostructures

In this work, we investigated the self-heating effects of annealed Ti/Al/Ni/Au ohmic contacts and two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures under strong electric field by using the short pulse current–voltage and microwave noise measurement techniques. The experimental results demonstrated that the self-heating phenomena in ohmic contacts with the increase of current may dominate over the electrical performances of 2DEG channel. Moreover, the excess noise temperature of contact resistance was found to be more than four times higher than that of 2DEG channel at the same high current density values. The results reveal the importance of self-heating of the ohmic contacts on the device performance under short-duration (only 100 ns) and low-duty-cycle (only 10−5) pulse regime.

Optimizing breakdown voltage and on-state resistance by modulating the barrier height along 2DEG channel for power p-GaN HEMTs

This article proposes a novel step-type gate p-GaN HEMT (STG-HEMT) to optimize breakdown voltage (BV) and on-state resistance (R ON) by modulating the barrier height along the two-dimensional electron gas (2DEG) channel. The step-type gate consists of thicker and thinner p-GaN layers. At off-state, the barrier height is higher due to the clamping potential effect induced by the thinner p-GaN layer, which contributes to improving BV. At on-state, the barrier height under the thinner p-GaN layer is lower, which contributes to improving 2DEG density under the gate (namely reducing R ON). Verified by the calibrated simulation, the results show STG-HEMT’s BV is increased by 55% and STG-HEMT’s R ON is decreased by 20% compared with the conventional power p-GaN HEMT (C-HEMT). At transient behavior, the total switching loss keeps nearly unchanged, while the gate driver loss is increased by about 19%. Furthermore, the impact of the gate length and p-GaN layer’s parameters (including thickness, length, activated Mg doping density) on R ON, BV, and threshold voltage are discussed.

Ultra‐High Speed, High‐Sensitivity Spin‐Cast MXene‐Semiconductor‐MXene Photodetectors

AbstractA simple room‐temperature process of depositing MXene on a III‐V structure with embedded 2D electron gas (2DEG) is used, which results in a large area, , photodetector (PD) device that greatly outperforms vacuum deposited Ti/Au metal‐semiconductor‐metal (MSM) PD's. By co‐optimizing properties of 2D MXene contacts and the III‐V material heterojunctions, this device sets new operating records with responsivity up to 1.04 A W‐1 at low optical powers, corresponding to &gt;230% internal quantum efficiency, dark current of 50 , &gt;105.6‐dB dynamic range, and 25–150 ps response time, which improves the previous MXene‐Semiconductor‐MXene responsivity by &gt;2.7 times and is 7 × 103 –−106 times faster compared to other MXene‐based PDs. This is achieved by enhancing the Schottky barrier height by forming a Van der Waals (vdW) heterojunction between a wide bandgap AlGaAs surface layer and spin coated Ti3C2Tz electrodes. A layered architecture transports the optically generated electrons to a 2DEG channel at the GaAs/AlGaAs heterointerface, where they are rapidly collected. The landscaped electric field pushes the slow holes to an underlying low temperature‐grown GaAs (LT‐GaAs) region where they recombine. The proposed Schottky‐2DEG Photoconductor‐Schottky model for device operation shows how this device circumvents the canonical limitations of gain‐bandwidth product, and carrier transit time, while replacing the need for vacuum deposition of gold or other precious metals.

A High Reverse Breakdown Voltage p-GaN Gate HEMT with Field Control Drain

In this work, a high reverse blocking voltage (BV R) p-GaN gate high electron mobility transistor with field control drain (FCD-HEMT) has been proposed and fabricated. The FCD-HEMT features the field control drain (FCD), consisting of electrically shorted Ohmic contact structure and p-GaN cap. In the OFF-state, the 2-Dimensional Electron Gas (2DEG) channel is cut off due to the p-GaN cap introduced field control, which provides FCD-HEMT with reverse blocking capability. In the ON-state, the re-formed 2DEG channel offers a non-potential barrier pathway for electrons transfer from source to drain and ensures a low resistance of the FCD-HEMT. The fabricated device exhibits 1400 V forward breakdown voltage (BV F) and −1240 V reverse breakdown voltage (BV R), 12.7 mΩ·cm2 low specific ON-resistance, and 188 mA mm−1 max drain current while maintaining normally-OFF capability. These results demonstrate the great potential of FCD-HEMTs in 1200 V-class power applications.

Demonstration of Temperature-Dependent Analysis of GAA – β-(AlGa)2O3/Ga2O3 High Electron Mobility Transistor

In this work, we have, designed and demonstrated Gate all around (GAA)-β-AGO/GO HEMT to achieve large current density, electron mobility and electron velocity at cryogenic temperatures. Because the controlling of the gate is better in GAA-based structures the performance of the proposed device is magnified for a different temperature range. The 2DEG channel mobility of the proposed device is boosted from 130 cm2/Vs at 300 K to 4100 cm2/Vs at 50 K. Mobility of the proposed device is boosted from the previous value by the factor of 2.5 at room temperature. The higher electron mobility at lower temperatures made it suitable to reach a larger saturation velocity center of the channel. Analog, RF and linearity parameters are analyzed with different temperature ranges at a gate length of 0.9 µm. The drain current and cutoff frequency of the device is 750 mA/mm and 18 GHz, respectively at 50 K and it reduces for a larger temperature. The breakdown voltage of the proposed device at 300 K is 139 V. The noise analysis is also taken for the investigation of its random nature. This type of device is used in power-switching applications.

Polarization modulation of 2DEG toward plasma-damage-free GaN HEMT isolation

GaN electronics have hinged on invasive isolation such as mesa etching and ion implantation to define device geometry, which, however, suffer from damages, hence potential leakage paths. In this study, we propose a new paradigm of polarization isolation utilizing intrinsic electronic properties, realizing in situ isolation during device epitaxy without the need of post-growth processing. Specifically, adjacent III- and N-polar AlGaN/GaN heterojunctions were grown simultaneously on the patterned AlN nucleation layer on c-plane sapphire substrates. The two-dimensional electron gas (2DEG) was formed at III-polar regions but completely depleted in N-polar regions, thereby isolating the 2DEG channels with a large 3.5 eV barrier. Structures of polarization-isolated high electron mobility transistors (PI-HEMTs) exhibit significantly reduced isolation leakage currents by up to nearly two orders of magnitude at 50 V voltage bias compared to the state-of-the-art results. Aside from that, a high isolation breakdown voltage of 2628 V is demonstrated for the PI-HEMT structure with 3 μm isolation spacing, which is two-times higher than a conventional mesa-isolation HEMT. Moreover, the PI-HEMT device shows a low off-state leakage current of 2 × 10−8 mA/mm with a high Ion/Ioff ratio of 109 and a nearly ideal subthreshold slope of 61 mV/dec. This work demonstrates that polarization isolation is a promising alternative toward the plasma-damage-free isolation for GaN electronics.

Normally-off AlGaN/GaN heterojunction field-effect transistors with in-situ AlN gate insulator

AlGaN/GaN heterojunction field-effect transistors (HFETs) with p-GaN cap layer are developed for normally-off operation, in which an in-situ grown AlN layer is utilized as the gate insulator. Compared with the SiN x gate insulator, the AlN/p-GaN interface presents a more obvious energy band bending and a wider depletion region, which helps to positively shift the threshold voltage. In addition, the relatively large conduction band offset of AlN/p-GaN is beneficial to suppress the gate leakage current and enhance the gate breakdown voltage. Owing to the introduction of AlN layer, normally-off p-GaN capped AlGaN/GaN HFET with a threshold voltage of 4 V and a gate swing of 13 V is realized. Furthermore, the field-effect mobility is approximately 1500 cm2⋅V−1⋅s−1 in the 2DEG channel, implying a good device performance.

Multichannel AlGaN/GaN Schottky Barrier Diode with Low Turn‐On Voltage and On‐Resistance

Herein, multichannel AlGaN/GaN Schottky barrier diodes (SBDs) with recess structure and tungsten (W) anode are proposed. The multichannel heterostructure improves the current drive capability of the devices and achieves a lower forward voltage (V F). W anode reduces the turn‐on voltage (V ON) due to its lower work function and direct contact with all 2D electron gas (2DEG) channels in the recessed area. Post anode annealing is utilized to improve the interface between the metal and GaN etching surface. The fabricated devices with a 6.5 μm anode–cathode distance (L AC) achieve an ultralow V ON of 0.29 V, V F of 0.69 V, and a low on‐resistance (R ON) of 2.39 Ω mm. The capacitance effect of multichannel heterostructures has been analyzed via the capacitance–voltage characteristics, showing the great potential of multichannel AlGaN/GaN SBDs for high‐frequency applications.

A perspective on multi-channel technology for the next-generation of GaN power devices

The outstanding properties of Gallium Nitride (GaN) have enabled considerable improvements in the performance of power devices compared to traditional silicon technology, resulting in more efficient and highly compact power converters. GaN power technology has rapidly developed and is expected to gain a significant market share in an increasing number of applications in the coming years. However, despite the great progress, the performance of current GaN devices is still far from what the GaN material could potentially offer, and a significant reduction of the device on-resistance for a certain blocking voltage is needed. Conventional GaN high-electron-mobility-transistors are based on a single two-dimensional electron gas (2DEG) channel, whose trade-off between electron mobility and carrier density limits the minimum achievable sheet resistance. To overcome such limitations, GaN power devices including multiple, vertically stacked 2DEG channels have recently been proposed, showing much-reduced resistances and excellent voltage blocking capabilities for a wide range of voltage classes from 1 to 10 kV. Such devices resulted in unprecedented high-power figures of merit and exceeded the SiC material limit, unveiling the full potential of lateral GaN power devices. This Letter reviews the recent progress of GaN multi-channel power devices and explores the promising perspective of the multi-channel platform for future power devices.

Modeling the Impact of Dynamic Fin-Width on the I– V, C– V and RF Characteristics of GaN Fin–HEMTs

In this article, we introduce an engineering approach to model the current and charge characteristics of an AlGaN/GaN Fin–HEMT. The model handles the effective width of the 2DEG channel by considering its depletion due to the presence of gates on the sidewalls of the fin. This width modulation of the 2DEG, and the bias-dependence associated with it, significantly affects the transconductance and the capacitance behavior of the device. In this work, we modify and couple two industry-standard compact models: 1) the advanced SPICE model for GaN HEMTs (ASM-GaN-HEMT) model and 2) the Berkeley short-channel insulated-gate FET model for common multiple gate (BSIM-CMG) model. While the former provides the physics-based calculations for electrostatics and transport in the 2DEG, the latter is incorporated to account for the width-modulation due to the sidewalls. The developed methodology is validated for dc, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> , and RF behavior against experimental data of GaN Fin–HEMTs. To the authors’ knowledge, this is the first compact model that provides an end-to-end dc–radio frequencies (RF) modeling solution for the state-of-the-art GaN Fin–HEMTs and therefore could play a role in the evolution of the GaN Fin–HEMT technology.