2DEG Channel Research Articles

Insight into gain and transient response characteristics of AlGaN/GaN hetero-junction based UV photodetectors: Case study on the role of incident light intensity

Gain and response speed are two key performance parameters for high sensitivity photodetectors (PDs). However, the effect of incident light intensity on gain and transient response properties of AlGaN/GaN hetero-junction based PDs are still not fully understood. Here, we design and fabricate an AlGaN/GaN hetero-junction based ultraviolet (UV) PD with interdigitated electrodes formed by a conductive two-dimensional electron gas (2DEG) channel, which exhibits a low dark current of 2.92 × 10−11 A and a high responsivity of 3060 A/W at 10 V bias. The high-gain AlGaN/GaN 2DEG PD has a similar working mechanism to those of traditional phototransistors, but its device architecture is evidently simplified. By investigating the variation of gain and transient response characteristics of the 2DEG PD as a function of incident UV light intensity, it has been concluded that the gain of the PD in a low-light intensity region is dominated by hole accumulation-induced electron escape from the 2DEG, while in a high-light intensity region, the gain is dominated by photoconductivity effect and limited by carrier recombination. This study provides guidance for future practical applications of AlGaN/GaN-based PDs in complex UV illumination environments.

Network of artificial olfactory receptors for spatiotemporal monitoring of toxic gas.

Excessive human exposure to toxic gases can lead to chronic lung and cardiovascular diseases. Thus, precise in situ monitoring of toxic gases in the atmosphere is crucial. Here, we present an artificial olfactory system for spatiotemporal recognition of NO2 gas flow by integrating a network of chemical receptors with a near-sensor computing. The artificial olfactory receptor features nano-islands of metal-based catalysts that cover the graphene surface on the heterostructure of an AlGaN/GaN two-dimensional electron gas (2DEG) channel. Catalytically dissociated NO2 molecules bind to graphene, thereby modulating the conductivity of the 2DEG channel. For the energy/resource-efficient gas flow monitoring, trust-region Bayesian optimization algorithm allocates many sensors optimally in a complex space. Integrated artificial neural networks on a compact microprocessor with a network of sensors provide in situ gas flow predictions. This system enhances protective measures against toxic environments through spatiotemporal monitoring of toxic gases.

High-speed independent polarization modulation based on dual channel 2DEG anisotropic terahertz metasurfaces.

In communication and imaging systems, anisotropic metasurfaces bring new degrees of freedom for the independent control of electromagnetic waves with different polarizations. This paper proposes a dual-polarization modulation metasurface (DPMM) operating in the terahertz (THz) frequency range, which can effectively and independently modulate two orthogonally linearly polarized electromagnetic waves. The metasurface unit adopts an anisotropic structure, and by integrating multiple GaN-based high electron mobility transistors (GaN-HEMTs) into the anisotropic structure, independent control of the two-dimension electron gas (2DEG) carrier concentration in the dual-polarization channel is achieved, enabling selective modulation of x-polarization and y-polarization waves. Simulation and static experimental results demonstrate the excellent modulation isolation performance of the DPMM, with a maximum isolation exceeding 45 dB. Dynamic experiments further highlight the high-speed modulation effect, with modulation speeds exceeding 1 GHz for a single channel and an overall modulation speed of 2 GHz for dual polarization, highlighting its potential for enhancing channel capacity and information throughput in THz communication and imaging systems.

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.

Optical and Electrical Properties of AlGaN-Based High Electron Mobility Transistors and Photodetectors with AlGaN/AlN/GaN Channel-Stacking Structure.

High channel current of the high electron mobility transistors (HEMTs) and high relative responsivity of the photodetectors (PDs) were demonstrated in the AlGaN/AlN/GaN channel-stacking epitaxial structures. The interference properties of the X-ray curves indicated high-quality interfaces of the conductive channels. The AlGaN/AlN/GaN interfaces were observed clearly in the transmission electron microscope micrograph. The saturation I ds currents of the HEMT structures were increased by adding a number of channels. The conductive properties of the channel-stacking structures corresponded to the peaks of the transconductance (g m) spectra in the HEMT structures. The depletion-mode one- and two-channel HEMT structures can be operated at the cutoff region by increasing the reverse V gs bias voltages. Higher I ds current in the active state and lower current in the cutoff state were observed in the two-channel HEMT structure compared with one- and three-channel HEMT structures. For the channel-stacking metal-semiconductor-metal photodetector structures, the peak responsivity was observed at almost 300 nm incident monochromic light, which was increased by adding a number of channel layers. The channel current of the HEMT devices and the photocurrent in the PD devices were increased by adding a number of two-dimensional electron gas (2DEG) channels. By using a flat gate metal layer, the two-channel AlGaN/AlN/GaN HEMT structures exhibited a high I ds current, a low cutoff current, and a high peak g m value and have the potential for GaN-based power devices, fast portable chargers, and ultraviolet PD applications.

Impact of eliminating ungated access regions on DC and thermal performance of GaN-based MIS-HEMT

The impacts of eliminating access regions (AR) on DC and thermal performance of GaN-based MIS-HEMT have been studied using a device of 4 µm gate length. Nominal absence of ARs was achieved by a metal-organic chemical vapor deposition-regrown heavily n-doped GaN contact region extended to the two-dimensional electron gas (2DEG) channel (with sheet resistance as low as 14 Ω/ ◻ and ohmic contact resistance of 0.20 Ω⋅ mm) and 22-nm-thick AlN/Al2O3/HfO2 insulator layers acting as both gate dielectrics and sidewall spacers. As a result, a low knee voltage (2.5 V @ VGS = +2 V) comparable to deeply-scaled devices was attained, revealing the dominant role of ARs in knee voltages. High linearity at lower supply voltages (gate voltage swing of 7.3 V @ VDS = 5 V) and a faster GVS saturation trend with V DS increasing were observed, benefiting from the improved utility of applied lateral voltage. Moreover, a much lower thermal resistance compared to that of the conventional MIS-HEMT structure (146 vs. 202 K W−1) was extracted by a static-pulsed I-V measurement method. Simplified TCAD simulations were conducted to explain the underlying mechanisms, demonstrating that the enhanced surface heat flow covered by the gate metal as well as the more uniform electric field along the 2DEG channel accounts for the better capability of heat management in the device free of ARs. Our results indicate the size of DC and thermal performance enhancements obtained by eliminating ARs in a GaN-based MIS-HEMT structure.

Recess-free thin-barrier AlGaN/GaN Schottky barrier diodes with ultra-low leakage current: Experiment and simulation study

In GaN Schottky barrier diodes (SBD), there is a trade-off between the turn-on voltage and the leakage current. In this study, recess-free 4 nm-thin-barrier AlGaN/GaN SBDs with minimal leakage current as well as excellent turn-on voltage homogeneity are developed, which enable better electrical control to pinch off the 2DEG channel under the anode region and avoids trap introduced in the barrier etching process. In detail, the effect of reverse stress on the SBD performance is initially explored by using differential conductance, and then, the anode-to-cathode distances and temperatures dependence of SBDs output characteristics are systematically studied. The fabricated thin-barrier GaN SBDs show a turn-on voltage of ∼0.75 V and a low level leakage current of 9.66 × 10−10 (1.91 × 10−8) A/mm at 300 (423) K, which is among the lowest reported values at the comparable reverse bias voltage and temperature. Moreover, structure parameters of the thin-barrier AlGaN/GaN SBD are systematically modeled and optimized by the TCAD simulations, including anode metal work function, Al mole fraction, the anode-to-cathode distance, and slanted anode angle. The introduction of slanted anode is found to have significant effect on the improvement of reverse breakdown voltage and leakage current characteristics.

Investigation of carrier transport and recombination at type-II band aligned p-NiO/AlGaN interface in p-NiO gate AlGaN/GaN HEMTs under forward bias

In this work, fine carrier transport and recombination processes in p-NiO gate AlGaN/GaN high electron mobility transistors were investigated by analyzing their electroluminescence under forward gate bias, with photoluminescence spectrum as a reference. Red luminescence with a peak of 1.9 eV was captured when the gate bias voltage exceeded 4 V, which was verified to originate from the tunneling enhanced interface recombination of injected holes from the gate metal and spilled electrons from the 2DEG channel at the type-II band aligned p-NiO/AlGaN heterostructure interface. Under higher gate bias voltage, holes were further injected into the GaN buffer layer, producing ultraviolet luminescence and yellow luminescence, corresponding respectively to the band edge emission and defect-assisted radiative recombination of GaN. Threshold voltage shift measurements under forward gate bias were conducted to further investigate the carrier transport and recombination processes.

Effect of Fe Doping Profile on Current Collapse in GaN-based RF HEMTs.

Using computer-aided design (TCAD) simulation, the impact of the Fe doping profile, including concentration, decay rate, and depth of the doping region on current-collapse magnitude (▵CC) in 0.5-μm gated GaN-based high electron mobility transistors (HEMTs) is systematically investigated. Accurate simulation models are established and developed to facilitate the fabrication of electronics. It is elucidated that the intricate interplay between trapping and de-trapping of Fe-related traps at the gate-drain edge is responsible for current collapse. The concentration and decay rate of the doping region have a more significant impact on current collapse than the depth. Increased trap state density near two-dimensional electron gas (2DEG) channel caused by deep-level acceptors would boost ▵CC. However, a minor dynamic reduction in 2DEG density (nT) induces a relatively small ▵CC. By adjusting the concentration, decay rate, and depth of the doping region, ▵CC of GaN-based Radio Frequency (RF) HEMTs can be reduced by approximately 50.3 %. The optimized distribution of Fe doping discussed in this work helps to prepare GaN-based RF HEMTs with a limited current collapse effect.

Current-Driven Terahertz Oscillations in Diamond TeraFET

We report on sub-terahertz plasmonic wave generation in the 2DEG channel of diamond TeraFET when biased by a DC current at the drain. Our numerical results demonstrated that p-diamond can support resonant oscillation of 300 GHz at room temperature, allowing it to function as a sub-THz emitter. We investigated the impact of different channel lengths, gate biases, drift velocities, and temperatures on the fundamental mode oscillation. The model incorporated the decay factors owing to electron scattering and electron fluid viscosity.

Enhanced robustness against hot-electron-induced degradation in active-passivation p-GaN gate HEMT

The hot-electron-related reliability is an important issue for GaN power devices under harsh operation condition or environment. These high-energy electrons can scatter toward the device surface or buffer layer, introducing newly generated traps/defects and resulting in the degradation of dynamic ON-resistance (RON). This work investigates the dynamic characteristics in active-passivation p-GaN gate HEMTs (AP-HEMTs) after hot-electron stress (HES). Unlike the dielectric passivation whose dynamic RON performance is often reported to severely worsen as hot-electron-induced defects/traps accumulate, the active passivation is found to have a superior robustness against hot-electron stress. In this study, after an HES of 30 min with VD = 200 V and IS = 10 mA/mm, the dynamic RON/static RON of a conventional HEMT increases dramatically from 3.63 to 9.35 for VDS-OFF = 650 V, whereas that of AP-HEMT only shows a slight increase from 1.51 to 1.85. Two mechanisms have been experimentally proved for the improved hot-electron robustness in AP-HEMT. (i) The mobile holes in active passivation layer can effectively screen the preexisting and/or newly generated surface defects/traps from affecting the 2DEG channel. (ii) The recovery of buffer trapping is accelerated by hole injection from gate and active passivation.

Review of the AlGaN/GaN High-Electron-Mobility Transistor-Based Biosensors: Structure, Mechanisms, and Applications.

Due to its excellent material performance, the AlGaN/GaN high-electron-mobility transistor (HEMT) provides a wide platform for biosensing. The high density and mobility of two-dimensional electron gas (2DEG) at the AlGaN/GaN interface induced by the polarization effect and the short distance between the 2DEG channel and the surface can improve the sensitivity of the biosensors. The high thermal and chemical stability can also benefit HEMT-based biosensors' operation under, for example, high temperatures and chemically harsh environments. This makes creating biosensors with excellent sensitivity, selectivity, reliability, and repeatability achievable using commercialized semiconductor materials. To synthesize the recent developments and advantages in this research field, we review the various AlGaN/GaN HEMT-based biosensors' structures, operations mechanisms, and applications. This review will help new researchers to learn the basic information about the topic and aid in the development of next-generation of AlGaN/GaN HEMT-based biosensors.

A Novel Enhancement-Mode Gallium Nitride p-Channel Metal Insulator Semiconductor Field-Effect Transistor with a Buried Back Gate for Gallium Nitride Single-Chip Complementary Logic Circuits

In this work, a novel enhancement-mode GaN p-MISFET with a buried back gate (BBG) is proposed to improve the gate-to-channel modulation capability of a high drain current. By using the p-GaN/AlN/AlGaN/AlN double heterostructure, the buried 2DEG channel is tailored and connected to the top metal gate, which acts as a local back gate. Benefiting from the dual-gate structure (i.e., top metal gate and 2DEG BBG), the drain current of the p-MISFET is significantly improved from −2.1 (in the conv. device) to −9.1 mA/mm (in the BBG device). Moreover, the dual-gate design also bodes well for the gate to p-channel control; the subthreshold slope (SS) is substantially reduced from 148 to ~60 mV/dec, and such a low SS can be sustained for more than 3 decades. The back gate effect and the inherent hole compensation mechanism of the dual-gate structure are thoroughly studied by TCAD simulation, revealing their profound impact on enhancing the subthreshold and on-state characteristics in the BBG p-MISFET. Furthermore, the decent device performance of the proposed BBG p-MISFET is projected to the complementary logic inverters by mixed-mode simulation, showcasing excellent voltage transfer characteristics (VTCs) and dynamic switching behavior. The proposed BBG p-MISFET is promising for developing GaN-on-Si monolithically integrated complementary logic and power devices for high efficiency and compact GaN power IC.

P-GaN gate power HEMT heterostructure as a versatile platform for extremely wide-temperature-range (X-WTR) applications

In this work, we manifest that the epitaxial structure for p-GaN gate high-electron-mobility transistor is a versatile platform to develop electronics for operating in an extremely wide temperature range (X-WTR) from 2 to 675 K, with comprehensive X-WTR studies on device operation and circuit behaviors. The key enabler for the high-temperature operation is the wide bandgap that substantially suppresses the thermal excitation of the intrinsic carrier. However, for the low-temperature side, the two-dimensional electron and hole gas (2DEG and 2DHG) channels at the heterojunctions are formed by the temperature-insensitive polarization fields, which free the carriers from freezing out. The monolithically integrated GaN n-FET, p-FET, and the resultant complementary circuits are, therefore, shown to operate in X-WTR.

Spin-filter device using the Zeeman effect with realistic channel and structure parameters

We have theoretically calculated the performance of a Zeeman-type spin polarizer, which consists of ferromagnetic (FM) and metal–insulator–semiconductor (MIS) gate nanostructures on top of an InAs two-dimensional electron gas (2DEG) channel. For the calculations, we have taken a realistic electron concentration, electron effective mass and the effective g-factor of the InAs 2DEG into account. In addition, we have assumed realistic FM and MIS structure sizes by conventional electron beam lithography. In the calculations, we have demonstrated clear oscillation of spin polarization over ±85%. Furthermore, we have shown that it works not only as a spin-polarized current generator but also a detector. In addition, we have proposed a novel spin-filter device utilizing the Zeeman-type spin polarizers. We have found that its conductance characterization allows us to evaluate the operation of Zeeman-type spin polarizers. We expect the Zeeman-type spin polarizers and spin-filter devices to open up a new field of spintronics.

Effect of electron-electron collisions on entrance potential drop in high-mobility 2DEG channels

Usually, the evidence of viscous hydrodynamic transport of frequently colliding conduction electrons is considered to be the “Gurzhi effect,” which requires the scattering of electrons on the channel walls with momentum loss. However, we demonstrate that expansion of Sharvin’s potential drop directly related to frequent electron-electron collisions can be detected even in channels with ideally smooth walls in a two-dimensional degenerate electron gas (2DEG). In theory, this effect could be experimental evidence of the predicted earlier difference between the relaxation times of the antisymmetric and symmetric momentum distributions attributed to the Pauli principle and topological limitations in 2DEG.

Capacitance spectroscopy of InAs quantum dots inserted in an AlGaAs/GaAs HEMT for photodetector applications

We have investigated the electrical characteristics of an AlGaAs/GaAs high electron mobility transistor, in which a layer of InAs self-assembled Quantum Dots (QDs) was inserted below the 2DEG channel. A Negative Differential Capacitance (NDC) appeared in the capacitance–voltage characteristics at a bias of 1 V and at low temperatures (even at room temperature) under different illumination powers using white light bulbs. This results in an increase in negative differential conductance with the increase in frequency and optical power. This also applies to the NDC except that it decreases with increasing frequency. The numerical simulation of the energy band structure of the device confirmed that the conduction band lowers to its minimum at a special bias value of 1 V. The numerical analysis of the evolution of the energy levels in the QD-HEMT follows the appearance of multiple capacitance peaks and their behavior with the gate voltage.

Investigation of the leakage mechanism in multi-channel GaN-on-Si SBDs

This work presented the reverse leakage current (I R) mechanisms in multi-channel GaN-on-Si Schottky barrier diodes (SBDs). The device showed excellent performances in both ON and OFF-states thanks to the advanced multi-channel tri-gate architecture. The I R is dominated by thermal field emission at 25 °C–75 °C before pinch-off of the 2-dimensional electron gas (2DEG) channel in the Schottky region, due to the thinned Schottky barrier, which turned to be thermal emission (TE)-dominated under further elevated temperatures resulted from the small Schottky barrier height. Once the 2DEG channel is pinched off by the tri-anode, the I R is firstly dominated by Poole–Frenkel emission and then is trap-assisted tunneling after the full depletion of the channels beneath the field plates. These results are supported by excellent consistency between experimental results and theoretical models, offering a key understanding of multi-channel SBDs and shedding lights on this promising multi-channel technology for future energy conversion.

Terahertz Radiation from High Electron Mobility Avalanche Transit Time Sources Prospective for Biomedical Spectroscopy

A Schottky barrier high-electron-mobility avalanche transit time (HEM-ATT) structure is proposed for terahertz (THz) wave generation. The structure is laterally oriented and based on AlGaN/GaN two-dimensional electron gas (2-DEG). Trenches are introduced at different positions of the top AlGaN barrier layer for realizing different sheet carrier density profiles at the 2-DEG channel; the resulting devices are equivalent to high–low, low–high and low-high–low quasi-Read structures. The DC, large-signal and noise simulations of the HEM-ATTs were carried out using the Silvaco ATLAS platform, non-sinusoidal-voltage-excited large-signal and double-iterative field-maximum small-signal simulation models, respectively. The breakdown voltages of the devices estimated via simulation were validated by using experimental measurements; they were found to be around 17–18 V. Under large-signal conditions, the series resistance of the device is estimated to be around 20 Ω. The large-signal simulation shows that the HEM-ATT source is capable of delivering nearly 300 mW of continuous-wave peak power with 11% conversion efficiency at 1.0 THz, which is a significant improvement over the achievable THz power output and efficiency from the conventional vertical GaN double-drift region (DDR) IMPATT THz source. The noise performance of the THz source was found to be significantly improved by using the quasi-Read HEM-ATT structures compared to the conventional vertical Schottky barrier IMPATT structure. These devices are compatible with the state-of-the-art medium-scale semiconductor device fabrication processes, with scope for further miniaturization, and may have significant potential for application in compact biomedical spectroscopy systems as THz solid-state sources.

Modeling and simulation of an insulated-gate HEMT using p-SnO2 gate for high VTH design

In this paper, we proposed a novel recess-free insulated-gate High Electron Mobility Transistor (HEMT) with p-SnO2 gate realizing high threshold voltage (VTH) over 2 V. p-SnO2 is used as the gate material instead of traditional Ni/Au gate. Owing to the high work function of the p-SnO2 (both wide bandgap and large electron affinity (χ)), the 2DEG channel under the gate region can be deeply depleted realizing high VTH even with thick AlGaN barrier over 10 nm, which also ensures high channel mobility. An analytical model is presented and verified by experimentally calibrated TCAD simulation. Results show that for a 25 nm Al2O3, VTH of the proposed HEMT reaches 3.1 V with p-SnO2 doping NA = 5 × 1019 cm−3, which is ∼2.5 V higher than that of a conventional MIS-HEMT with the same parameters. Moreover, the specific on-resistance (Ron,sp) of the proposed HEMT is only 1.31 mΩ cm2, which is reduced by 54.8% compared with that of a recessed-gate MIS-HEMT with the same VTH of ∼2.2 V.