Two-dimensional Electron Gas Channel Research Articles

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.

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.

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.

An artificial neuromuscular junction for enhanced reflexes and oculomotor dynamics based on a ferroelectric CuInP2S6/GaN HEMT.

A neuromuscular junction (NMJ) is a particularized synapse that activates muscle fibers for macro-motions, requiring more energy than computation. Emulating the NMJ is thus challenging owing to the need for both synaptic plasticity and high driving power to trigger motions. Here, we present an artificial NMJ using CuInP2S6 (CIPS) as a gate dielectric integrated with an AlGaN/GaN-based high-electron mobility transistor (HEMT). The ferroelectricity of the CIPS is coupled with the two-dimensional electron gas channel in the HEMT, providing a wide programmable current range of 6 picoampere per millimeter to 5 milliampere per millimeter. The large output current window of the CIPS/GaN ferroelectric HEMT (FeHEMT) allows for amplifier-less actuation, emulating the biological NMJ functions of actuation and synaptic plasticity. We also demonstrate the emulation of biological oculomotor dynamics, including in situ object tracking and enhanced stimulus responses, using the fabricated artificial NMJ. We believe that the CIPS/GaN FeHEMT offers a promising pathway for bioinspired robotics and neuromorphic vision.

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.

Admittance frequency dispersion in lateral AlGaN/GaN Schottky barrier diodes: Other origins of two Gp/ω peaks

The frequency dispersion in admittance measurements in AlGaN/GaN high-electron-mobility transistors, which is typically interpreted to result from an interface trap density Dit, is also known to be caused by effects other than Dit. To study the origin of two peaks of conductance component “Gp/ω” as a function of frequency in lateral gated diodes using AlGaN/GaN structures on Si wafers grown by metal–organic chemical vapor deposition, we measured capacitance C and conductance Gp/ω–voltage V as a function of angular frequency ω (C–ω–V and Gp/ω–ω–V, respectively) of long-gate field-effect transistors with a varied gate length. We also simulated the C–ω–V and Gp/ω–ω–V curves using an equivalent circuit that consisted of actually measured component parameters without Dit. We confirmed that the Gp/ω–ω curves show two peaks caused by the two-dimensional electron gas channel resistance and the gate current leakage and quantitatively determined the two ωpeak positions and intensities. We also discussed the effect of acceptor concentration in the GaN channel-layer on lowering of ωpeak at weak inversion.

Epitaxial κ-Ga2O3/GaN heterostructure for high electron-mobility transistors

GaN power technology, especially AlGaN/GaN high electron-mobility transistors (HEMTs), has made significant progress in recent years. However, the performance of HEMTs is still limited due to a trade-off between on-resistance and off-state breakdown voltage (BV). Integrating a polar gate dielectric with GaN HEMTs can potentially improve both sheet resistance of the two-dimensional electron gas channel and the field distribution between gate and drain. In this regard, orthorhombic κ-Ga2O3 has attractive properties since it is predicted to be strongly polar and highly dielectric while it grows epitaxially on GaN. By successfully integrating crystalline κ-Ga2O3 on GaN HEMTs, the channel sheet resistance is reduced by 20% from a reference device with amorphous Al2O3 gate dielectric. As a result, the cut-off frequency increases from 4.8 to 9.1 GHz. The dielectric property of κ-Ga2O3 also improves BV from 354 to 380 V by reducing the peak electric field in the gate-drain region.

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.

Selective area epitaxy of degenerate n-GaN for HEMT ohmic contact by MOCVD

This Letter reports low-temperature (700 °C) growth of heavily Si-doped GaN (n++GaN) by metal-organic chemical vapor deposition with a resistivity as low as 1.9 × 10−4 Ω·cm and an atomically smooth surface. Indium adatoms added during the growth of n++GaN play an important role in improving both the surface morphology and free electron concentration. On the one hand, acting as surfactant, they greatly boost the adatoms surface mobility at low growth temperature and mitigate Si-induced anti-surfactant effect. On the other hand, they can effectively suppress the formation of compensating defects, thus contributing to an extremely high electron concentration of 2.8 × 1020 cm−3. This high-quality n++GaN was further applied to the realization of Ohmic contacts with an ultra-low contact resistance for AlGaN/GaN high electron mobility transistors. The carrier gas was carefully modulated for the selective area epitaxy (SAE) of n++GaN to facilitate the nucleation of GaN on the dielectric mask, which effectively suppressed the undesired mass transport and resulted in a uniform SAE of n++GaN in the recessed source/drain regions. A nearly defect-free interface between the n++GaN and two-dimensional electron gas channel has been also realized, and the resistance induced by the interface was only 0.03 Ω·mm. As a result, an ultra-low contact resistance of 0.07 Ω·mm has been realized. This work lays a solid foundation for further improving the performance of GaN-based RF and power devices.

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.

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.

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.

AlGaN/GaN Heterostructure Schottky Barrier Diodes with Graded Barrier Layer

The AlGaN/GaN Schottky barrier diodes (SBDs) working as high-power mixer and multiplier show great potential in millimeter wave (MMW) field owing to their high breakdown voltage. Nevertheless, its further application is severely limited by large reverse leakage current (Jr) since the two-dimensional electron gas (2DEG) channel is hard to be pinched off at low voltage. To address this limitation, a graded AlGaN/GaN heterostructure is introduced to extend the 2DEG channel into a quasi-three-dimensional electron slab. By comparing the fixed Al composition AlGaN/GaN SBD, Jr of the graded AlGaN/GaN SBD is significantly reduced due to the extension of channel carriers, confirming the effective Jr suppression effect of this structure. Furthermore, on this basis, a recessed anode structure is utilized to expect a smaller Jr. The results indicated that the graded AlGaN/GaN SBDs with air-bridge structure have achieved a pretty low Jr value (1.6 × 10−13 A at -15 V), and its cutoff frequency is as high as 60.6 GHz. It is expected that such SBDs with low Jr have significant advantages in future applications.

Room-Temperature Nonreciprocal Charge Transport in an InAs-Based Rashba Channel

Nonreciprocal charge transport is observed in a non-centrosymmetric system without a ferromagnetic layer. To observe the nonreciprocity of the Rashba system, an InAs-based two-dimensional electron gas channel is utilized and the angular dependent harmonic Hall measurement is performed. From the amplitude of the curve, a nonreciprocal coefficient of 1.36 A−1T−1 is extracted at 1.9 K. While the extracted value of the nonreciprocal coefficient decreases down to 0.44 A−1T−1 at 300 K, we can clearly observe the nonreciprocal charge transport at room temperature. In addition, the independent transport measurements clarify that the amplitude of nonreciprocal coefficient is closely connected with the strength of the Rashba effect.

Electron Heat Source Driven Heat Transport in GaN at Nanoscale: Electron-Phonon Monte Carlo Simulations and a Two Temperature Model.

The thermal energy transport in semiconductors is mostly determined by phonon transport. However in polar semiconductors like GaN electronic contribution to the thermal transport is non-negligible. In this paper, we use an electron–phonon Monte Carlo (MC) method to study temperature distribution and thermal properties in a two-dimensional GaN computational domain with a localized, steady and continuous electron heat source at one end. Overall, the domain mimics the two-dimensional electron gas (2DEG) channel of a typical GaN high electron mobility transistor (HEMT). High energy electrons entering the domain from the source interact with the phonons, and drift under the influence of an external electric field. Cases of the electric field being uniform and non-uniform are investigated separately. A two step/temperature analytical model is proposed to describe the electron as well as phonon temperature profiles and solved using the finite difference method (FDM). The FDM results are compared with the MC results and found to be in good agreement.

Reverse Blocking GaN High Electron Mobility Transistors with Stepped P-GaN Drain

In this work, a reverse-blocking high electron mobility transistor with stepped p-type GaN drain (SPD RB-HEMT) has been proposed and studied in TCAD Sentaurus. In SPD RB-HEMT, the reverse-blocking capability is achieved by employing a stepped p-type GaN (P-GaN) layer connected with the Schottky metal drain. The stepped P-GaN layer is constituted by four P-GaN steps, and the thicknesses of the first to the fourth P-GaN step are descending from the source to the drain side. Due to the P-GaN/AlGaN/GaN structure, the conduction band of the AlGaN/GaN interface is lifted up. Therefore, the two-dimensional electron gas (2-DEG) channel is depleted, and the reverse current is blocked. On the other hand, when the drain is forward biased, the 2-DEG channel is recovered, and the drain induced barrier lowering (DIBL) effect in stepped P-GaN drain structure can lead to a lower on-resistance of the device. The SPD RB-HEMT exhibits a reverse breakdown voltage of −2209 V and on-resistance of 3.01 mΩ·cm2 in TCAD Sentaurus simulation. Compared with the conventional Schottky barrier drain and planar P-GaN drain RB-HEMTs, the SPD RB-HEMTs have shown the balanced and improved forward and reverse-blocking characteristics.

Improving Breakdown Voltage and Threshold Voltage Stability by Clamping Channel Potential for Short-Channel Power p-GaN HEMTs.

This paper proposes a novel p-GaN HEMT (P-HEMT) by clamping channel potential to improve breakdown voltage (BV) and threshold voltage (VTH) stability. The clamping channel potential for P-HEMT is achieved by a partially-recessed p-GaN layer (PR p-GaN layer). At high drain bias, the two-dimensional electron gas (2DEG) channel under the PR p-GaN layer is depleted to withstand the drain bias. Therefore, the channel potential at the drain-side of the p-GaN layer is clamped to improve BV and VTH stability. Compared with the conventional p-GaN HEMT (C-HEMT), simulation results show that the BV is improved by 120%, and the VTH stability induced by high drain bias is increased by 490% for the same on-resistance. In addition, the influence of the PR p-GaN layers’ length, thickness, doping density on BV and VTH stability is analyzed. The proposed device can be a good reference to improve breakdown voltage and threshold voltage stability for short-channel power p-GaN HEMTs.

Highly transparent conducting Two-Dimensional electron gas channel in ultrathin heterostructures for flexible optoelectronic device applications

The fabrication of a two-dimensional electron gas (2DEG) channel between amorphous thin film hetero-oxide-based heterostructures provides promising opportunities for the development of flexible oxide electronics. Herein, we investigated the 2DEG phenomenon at the interface of an amorphous-Al2O3-In2O3-amorphous-Al2O3 (AIA) heterostructure on a flexible substrate, where the Al2O3 top layer provided passivation and facilitated 2DEG formation, whereas the buffer Al2O3 bottom layer provided an atomically smooth surface and isolation between the PI substrate and the film. The AIA device exhibited a sheet resistance of ∼ 0.65 kΩ/□ and a carrier concentration of ∼ 2.9 × 1020 cm−3 owing to the formation of the 2DEG channel. Interestingly, the sheet resistance decreased by 74% after the deposition of an Al2O3 top layer as maintaining the optical transparency > 90% in the visible range. In addition, in the bending test, the device was subjected to > 10,000 cycles, which led to an 8.9% decrease in device sheet resistance, thus revealing the high stability of the device. Furthermore, at the nanoscale, the current decreased with increasing bending, as confirmed by current maps and local I-V measurements. The outstanding performance of the AIA device presents a possibility for the application of 2DEG-based TCOs to fabricate flexible optoelectronic devices.

NiO/AlGaN interface reconstruction and transport manipulation of p-NiO gated AlGaN/GaN HEMTs

Normally off AlGaN/GaN high electron mobility transistors with a p-type gate are promising for power switching applications, with advantages of low energy consumption and safe operation. In this work, p-NiO is employed as a gate stack, and the interfacial reconstruction and band structure modification at the p-NiO/AlGaN interface have been demonstrated to manipulate channel transport of AlGaN/GaN high electron mobility transistors by post-annealing. In addition to achieving a positive threshold voltage of 0.6 V and a large saturation output current of 520 mA/mm, we found that the gate leakage and On/Off drain current ratio can be improved significantly by more than 104 due to the p-NiO/AlGaN interfacial reconstruction. However, high annealing temperature also results in an increasing ON-resistance and a dramatically increased knee voltage (VK), which can be attributed to the formation of an ultra-thin γ-Al2O3 layer and the substitution of O on N site as a shallow donor at the p-NiO/AlGaN interface confirmed by experimental analyses. Theoretical calculations indicate that such interface reconstruction facilitates an additional potential well at the p-NiO/AlGaN interface to which electrons are spilled out from a two-dimensional electron gas channel under high forward gate voltage, resulting in the increased VK. Finally, an optimized annealing condition was confirmed that can eliminate this increased VK phenomenon and simultaneously remain these significantly improved device performances. These findings provide deep understanding of the performance manipulation of AlGaN high electron mobility transistors, which is very important for engineering the p-NiO/AlGaN interface toward high-performance and stable devices.