Two-dimensional Electron Gas Density Research Articles

Adaptive impedance matching in microwave and terahertz metamaterial absorbers using PIN diodes and GaN HEMTs

Metamaterial absorbers allow electromagnetic waves to be converted into heat energy based on impedance matching. However, passive metamaterial absorbers exhibit fixed absorption characteristics, limiting their flexibility. This work demonstrates tunable microwave and terahertz absorbers by integrating adjustable resistors into the metamaterial units. First, a microwave absorber from 1 to 5 GHz was designed by embedding PIN diodes with voltage-controlled resistance. Calculations, simulations, and measurements verified two separate absorption peaks over 90% when optimized to a resistance of 250 Ω. The absorption frequencies shifted based on the resistor tuning. Building on this, a terahertz absorber was modeled by substituting gallium nitride high electron mobility transistors (GaN HEMTs) as the adjustable resistor component. The GaN HEMTs were controlled by an integrated gate electrode to modify the two-dimensional electron gas density, allowing resistance changes without external voltage terminals. Simulations revealed two absorption peaks exceeding 90% absorption at 0.34 THz and 1.06 THz by adjusting the equivalent resistance from 180 Ω to 380 Ω, and the tunable resistance is verified by DC measurement of single GaN HEMT in the unit. This work demonstrates how integrating adjustable resistors enables dynamic control over the absorption frequencies and bandwidths of metamaterial absorbers. The proposed geometries provide blueprints for tunable microwave and terahertz absorbers.

GaN-HEMT Performance Enhancement

In this work, a simulation analysis and calibration are carried out to improve the performance of AlGaN/GaN- MOSHEMTs (Metal-Oxide Semiconductor High Electron Mobility Transistors). The effect of the AlGaN layer thickness, gate length, Al mole fraction, and the interface traps on the electrical performance of the device has been presented. Device simulations have been done using Sentaurus technology computer-aided design (TCAD). The simulations and analysis show better drain current, transconductance, and cut-off frequency performance. The maximum cut-off frequency shown by the proposed HEMT device is 45.7 GHz at 100-nm gate length. Good transcoductance has been obtained by scaling down the gate length of the device, which is ascribed to the present two-dimensional electron gas (2DEG) density that supports upgrading the output current. Higher drain current is achieved without using acceptor-like traps in the Al2O3/AlGaN interface. Results show that the Al2O3/AlGaN/GaN-based MOSHEMT is a promising device for high-frequency and power electronic applications.

Electron mobility enhancement by electric field engineering of AlN/GaN/AlN quantum-well HEMTs on single-crystal AlN substrates

To enhance the electron mobility in quantum-well high-electron-mobility transistors (QW HEMTs), we investigate the transport properties in AlN/GaN/AlN heterostructures on Al-polar single-crystal AlN substrates. Theoretical modeling combined with experiment shows that interface roughness scattering due to high electric field in the quantum well limits mobility. Increasing the width of the quantum well to its relaxed form reduces the internal electric field and scattering, resulting in a binary QW HEMT with a high two-dimensional electron gas (2DEG) density of 3.68×1013 cm–2, a mobility of 823 cm2/Vs, and a record-low room temperature (RT) sheet resistance of 206 Ω/□. Further reduction of the quantum well electric field yields a 2DEG density of 2.53×1013 cm–2 and RT mobility > 1000 cm2/V s. These findings will enable future developments in high-voltage and high-power microwave applications on the ultrawide bandgap AlN substrate platform.

Effect of annealing on the electrical performance of N-polarity GaN Schottky barrier diodes

A nitrogen-polarity (N-polarity) GaN-based high electron mobility transistor (HEMT) shows great potential for high-frequency solid-state power amplifier applications because its two-dimensional electron gas (2DEG) density and mobility are minimally affected by device scaling. However, the Schottky barrier height (SBH) of N-polarity GaN is low. This leads to a large gate leakage in N-polarity GaN-based HEMTs. In this work, we investigate the effect of annealing on the electrical characteristics of N-polarity GaN-based Schottky barrier diodes (SBDs) with Ni/Au electrodes. Our results show that the annealing time and temperature have a large influence on the electrical properties of N-polarity GaN SBDs. Compared to the N-polarity SBD without annealing, the SBH and rectification ratio at ±5 V of the SBD are increased from 0.51 eV and 30 to 0.77 eV and 7700, respectively, and the ideal factor of the SBD is decreased from 1.66 to 1.54 after an optimized annealing process. Our analysis results suggest that the improvement of the electrical properties of SBDs after annealing is mainly due to the reduction of the interface state density between Schottky contact metals and N-polarity GaN and the increase of barrier height for the electron emission from the trap state at low reverse bias.

Design of multi-channel heterostructures for GaN devices

The emerging multi-channel GaN architecture offers great opportunities in high-performance devices, whose performance is fundamentally determined by the density and distribution of electrons among the parallel multi-channels. In this work, we present approaches to design multi-channel GaN heterostructures based on a proposed physical-based model and experimental results. The model presents excellent accuracy based on comparisons with TCAD and experimental results. Impacts of key designing parameters upon the two-dimensional electron gas density and the distribution of electrons were systematically investigated within undoped and doped multi-channels, presenting a criterion to determine the maximum channel thickness and evaluate the multi-channel design, and offering a design guideline to design and optimize a multi-channel-GaN heterostructure for a given targeted application.

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 systematic study on the efficacy of low-temperature GaN regrown on p-GaN to suppress Mg out-diffusion

Embedding p-type gallium nitride (p-GaN) in AlxGa1-xN-based thin films has garnered significant interest as a versatile structure for bandgap engineering such as tunnel/super-junctions or current blocking/guiding functions in electronic devices. However, Mg, a p-GaN dopant, has an undesirable diffusive capacity into the nearby thin films at a high growth temperature (>1,000°C), resulting in structural challenges in device design. This study systematically investigated the low-temperature GaN (LT-GaN) layer regrown on p-GaN that suppresses Mg diffusion according to metal–organic chemical vapor deposition growth conditions. Prototype Al0.3Ga0.7N (40 nm)/GaN (140 nm) high-electron-mobility transistors (HEMTs) were regrown with LT-GaN on p-GaN (300 nm), and a high two-dimensional electron gas (2DEG) density of 3.13E12 cm−2 was achieved by inserting a 100-nm-thick LT-GaN layer grown at 750°C; in contrast, only 1.76E10 cm−2 2DEG density was obtained from Al0.3Ga0.7N/GaN HEMTs regrown directly on p-GaN (Mg: 4.0E19 cm−3). The fabricated Al0.3Ga0.7N/GaN HEMTs with 100-nm-thick LT-GaN demonstrated a high drain current density of 84.5 mA/mm with a low on-state resistance of 31 Ω·mm. The AlxGa1-xN/LT-GaN/p-GaN platform demonstrated here paves the way for various III-nitride-based structures with embedded p-GaN.

Thermal stability study of gallium nitride based magnetic field sensor

We investigated the thermal stability and performance of AlGaN/AlN/GaN Hall-effect sensors under industry-relevant atmospheric conditions. The thermal stability and performance of Hall sensors are evaluated by monitoring Hall sensitivity, two-dimensional electron gas density, and Ohmic contact resistance during aging at 200 °C for up to 2800 h under atmospheric conditions. This was accomplished by characterizing AlGaN/AlN/GaN micro-Hall sensors, with and without contacts, and before and after being placed under different thermal aging times. Observed electrical performance was correlated with the micro-structural evolution of AlGaN/AlN/GaN Hall sensor heterostructures. Results indicate that the AlGaN/AlN/GaN Hall sensor provides stable performance for as long as 2800 h aging at 200 °C without any significant degradation of (i) Hall sensitivity, (ii) two-dimensional electron gas, and (iii) Ohmic contacts. However, there was a small change in sheet density and mobility, which is due to a decrease in polarization, resulting from local inhomogeneous strain relief at the barrier layer. During the early stage of thermal aging, a decrease in contact resistance was also observed and attributed to (i) out-diffusion of “Ga” at the vicinity of the contact interface, and (ii) a reduction in oxygen concentration and formation of Al–Ti intermediate alloy at the GaN/Ti interface, resulting in a reduced barrier and enhanced electron transport at the contacts. However, despite these small changes, results indicate that the AlGaN/AlN/GaN Hall sensor provides stable performance for as long as 2800 h thermal aging at 200 °C.

More than 60% RF loss reduction and improved crystal quality of GaN-on-Si achieved by in-situ doping tert-butylphosphorus

In this work, the parasitic conductive channel in a silicon substrate is compensated using in-situ doping for the first time. The experimental results show that in-situ doping of tert-butylphosphorus (TBP) has little adverse effect on the mobility and the density of two-dimensional electron gas but the compensation of the parasitic channel in the substrate is effective, and the aluminum nitride (AlN) / Si template based on the optimized conditions reduces the radio frequency loss by more than 60%. In addition, it was found that a suitable preflow of TBP can enhance the migration of Al atoms on the substrate surface and improve the crystal quality of gallium nitride (GaN), which reduces the dislocation density of GaN by 25% compared to the sample without TBP preflow. The hypothesis is that TBP preflow can affect the strain of the substrate, which makes the dislocations more likely to annihilate during the subsequent growth of AlN nucleation layer, resulting in the eventual improvement of GaN crystal quality.

Breakdown enhancement and hot electrons mitigation for p-GaN gate HEMTs by electric field modulation

In this article, an enhancement mode (E-mode) p-GaN gate AlGaN/GaN high electron mobility transistor (HEMT) with the novel strain technology (NST) has been proposed. The NST consists of GaN/AlGaN/GaN and AlN/AlGaN/GaN heterojunctions laterally from the gate to drain side. By utilizing the different polarization effect of these two heterojunctions, it laterally forms different density of two-dimensional electron gas (2DEG). Accordingly, it decreases the electric field around the gate and introduces an extra electric field peak far away from the gate edge to improve the breakdown voltage (BV) effectively. The experimental BV of the proposed NST-HEMT is increased by 52 % to 1185 V from 780 V of the conventional HEMT. Meanwhile, the output current of NST-HEMT is almost consistent with the conventional HEMT. Moreover, the NST-HEMT realizes better hot electrons mitigation than conventional HMET, and its Vth and Ron degradations are only 1.38 % and 10.72 % after 8 h stress time.

Crack-Free High-Composition (>35%) Thick-Barrier (>30 nm) AlGaN/AlN/GaN High-Electron-Mobility Transistor on Sapphire with Low Sheet Resistance (<250 Ω/□)

In this article, a high-composition (>35%) thick-barrier (>30 nm) AlGaN/AlN/GaN high-electron-mobility transistor (HEMT) structure grown on a sapphire substrate with ultra-low sheet resistivity (<250 Ω/□) is reported. The optimization of growth conditions, such as reduced deposition rate, and the thickness optimization of different epitaxial layers allowed us to deposit a crack-free high-composition and thick AlGaN barrier layer HEMT structure. A significantly high two-dimensional electron gas (2DEG) density of 1.46 × 1013 cm−2 with a room-temperature mobility of 1710 cm2/V·s was obtained via Hall measurement using the Van der Pauw method. These state-of-the-art results show great potential for high-power Ga-polar HEMT design on sapphire substrates.

Sub-bandgap photon-assisted electron trapping and detrapping in AlGaN/GaN heterostructure field-effect transistors

We have investigated photon-assisted trapping and detrapping of electrons injected from the gate under negative bias in a heterostructure field-effect transistor (HFET). The electron injection rate from the gate was found to be dramatically affected by sub-bandgap laser illumination. The trapped electrons reduced the two-dimensional electron gas (2DEG) density at the AlGaN/GaN heterointerface but could also be emitted from their trap states by sub-bandgap photons, leading to a recovery of 2DEG density. The trapping and detrapping dynamics were found to be strongly dependent on the wavelength and focal position of the laser, as well as the gate bias stress time prior to illumination of the HFET. Applying this phenomenon of trapping and detrapping assisted by sub-bandgap photons, red, green, and purple lasers were used to demonstrate photo-assisted dynamic switching operations by manipulation of trapped carriers at the surface of an AlGaN/GaN HFET. A physical model based on band diagrams, explaining the trapping and detrapping behavior of electrons, has been presented.

A novel p-GaN HEMT with AlInN/AlN/GaN double heterostructure and InAlGaN back-barrier

In this paper, a novel p-type gallium nitride (p-GaN) high electron mobility transistor (HEMT) is proposed and investigated for high power electronics applications. In the proposed device, an AlInN/AlN/GaN double heterostructure has been used. In addition, InAlGaN back-barrier has also been employed to improve the DC performance of the device. The double AlInN/GaN heterostructures offer low resistance, resulting to a high two-dimensional electron gas (2DEG) density. As a result the current drive efficiency of the device is significantly improved. On the other hand, an inserted InAlGaN back-barrier confines the electrons in the channel which further enhances the performance of the device. To validate the simulation results, calibration of the device models has been performed with experimental results. Results show that the proposed device exhibits low on-resistance (Ron), high drain current (Ids) and high transconductance (gm) as compared to the state-of-the-art devices. Also, the processing steps have been proposed for the possible fabrication of the proposed device. Finally, the optimization of the device design parameter is performed to achieve the optimal device performance with respect to Ron.

On the surface oxidation and band alignment of ferroelectric Sc0.18Al0.82N/GaN heterostructures

Ferroelectric scandium aluminum nitride (ScAlN), owing to its unique properties and seamless integration with the mainstream GaN technology, has attracted considerable attention as one of the most promising materials for next-generation multifunctional electronics. To date, however, a detailed understanding of the fundamental surface properties and the band alignment of ScAlN/III-nitride heterostructure has remained elusive. In this work, with the use of high-resolution X-ray photoelectron spectroscopy measurements, we demonstrate the existence of a considerable oxidation layer on ScAlN when it is exposed to air, which shows a significant impact on the material characterization and electronic structure evaluation. By excluding the possible impact from the surface oxide layer, the band alignment of Sc0.18Al0.82N/GaN can be correctly determined. Simulation results further showed that the Sc0.18Al0.82N barrier layer could offer excellent charge carrier confinement and a high density of two-dimensional electron gas (2DEG) at the heterostructure interface, which is needed for GaN-based high electron mobility transistors (HEMTs) with enhanced performance. This work not only identifies the band alignment between Sc0.18Al0.82N and GaN for future novel ScAlN/III-nitride device applications but also highlights the importance of considering the prevalent surface oxidation issue in the emerging ferroelectric nitride family.

Strain relaxation and self-heating effects of fin AlGaN/GaN HEMTs

In this paper, we present a methodology of 3D electro-thermo-mechanical simulation to analyze the strain relaxation and self-heating effects of fin AlGaN/GaN high electron mobility transistors (HEMTs). The free boundaries of narrow fins cause strain relaxation of the AlGaN barrier and a non-uniform strain distribution near the AlGaN/GaN interface. The strain relaxation not only reduces the surface piezoelectric polarization charges (PPCs), but also introduces space PPCs in AlGaN/GaN, leading to a reduction of two-dimensional electron gas density and a positive shift of threshold voltage (V th). The simulated V th shift with fin width agrees well with experimental results from literature. In addition, the inter-fin trenches facilitate more efficient lateral heat spreading and suppress the self-heating effect compared with the planar HEMTs with the same effective gate width.

Influence of AlGaN n-type doping and AlN thickness on the two-dimensional electron gas density (ns) and resistance (R2DEG)

In this paper, electrical characterizations by C(VG) and ID(VG) and comparison with 1D Schrödinger-Poisson simulations are carried out to investigate the effects of AlN layer thickness and n-type AlGaN barrier doping on the two-dimensional electron gas resistance (R2DEG). Specifically, these effects are related to either the electron sheet density (ns) or the mobility (µ). We show that varying the AlN thickness from 0.7 nm (ns = 7.8 × 1012 cm−2) to 1.5 nm (ns = 9.0 × 1012 cm−2) leads to a linear increase of ns. However, simultaneous degradation of the transport properties probably due to intensifying roughness mechanism at high ns tends to limit the improvement of the device properties. Furthermore, we show that the heavily doped AlGaN barrier slightly increases ns and reduces the 2DEG mobility to ∼ 1770 cm2.V−1.s−1 leading to a degraded 2DEG resistance. However, the overall resistance is improved (R = 323 Ω/□) compared to the undoped case (R = 380 Ω/□) due to the simultaneous contribution of two conducting channels at VG = 0 V. To go further, the polarization charges at the AlGaN/AlN and AlN/GaN interfaces are computed in the 1D Schrödinger-Poisson simulations to account for the presence of the AlN layer. A reduced polarization for very thin AlN layers is considered to account for the experimental results. Finally, a very simple empirical model is proposed that predicts the enhancement of polarization charges (σ) as a function of AlN thickness.

Interface-state suppression of AlGaN/GaN Schottky barrier diodes with post-anode-annealing treatment

Owing to the high density and high mobility of two-dimensional electron gas (2DEG) induced by strong spontaneous polarization and piezoelectric polarization effect, AlGaN/GaN Schottky barrier diodes (SBDs) with high output current density and low on-resistance have proved to be a promising candidate. Anode of GaN SBD is the core structure, which affects the device performance such as turn-on voltage, reverse current, on-resistance, and breakdown voltage. Therefore, idealized Schottky junction with low interface state density is very important in achieving high-performance GaN SBD. In this work, AlGaN/GaN SBD with low work-function metal W as anode is fabricated, and the post-anode-annealing (PAA) treatment is found to be effective in promoting the bonding reaction between anode metal and GaN in the anode region. Comparing with GaN SBDs without PAA treatment, the interface state density decreases from 9.48×10<sup>15</sup> eV<sup>–1</sup>·cm<sup>–2</sup> to 1.77×10<sup>13</sup> eV<sup>–1</sup>·cm<sup>–2</sup> after PAA treatment. The reverse leakage current is reduced by two orders, which ascribes to the idealized anode interface with low interface state density. Meanwhile, the influence of interface state on carriers in the forward conduction process is also suppressed, and the differential on-resistance of the fabricated GaN SBDs decreases from 17.05 Ω·mm to 12.57 Ω·mm. It is obvious that the PAA process proves to be an effective method to suppress the interface states density at M/S interface, thus significantly improving the performance of GaN SBD, which is the key technology in fabricating the high-performance GaN device.

Low leakage current and high breakdown voltage of AlGaN/GaN Schottky barrier diodes with wet-etching groove anode

AlGaN/GaN heterojunction epitaxies with wide bandgap, high critical electric field as well as high density and high mobility two-dimensional electron gas have shown great potential applications in the next-generation high-power and high-frequency devices. Especially, with the development of Si-based GaN epitaxial technique with big size, GaN devices with low cost also show great advantage in consumer electronics. In order to improve the rectification efficiency of AlGaN/GaN Schottky barrier diode (SBD), low leakage current and low turn-on voltage are important. The GaN Schottky barrier diode with low work-function metal as anode is found to be very effective to reduce turn-on voltage. However, the low Schottky barrier height makes the Schottky interface sensitive to damage to groove surface, which leads to a high leakage current. In this work, a novel wet-etching technique with thermal oxygen oxidation and KOH corrosion is used to prepare the anode groove, and the surface roughness of groove decreases from 0.57 to 0.23 nm, compared with that of the dry-etching surface of groove. Meanwhile, the leakage current is suppressed from 1.5 × 10<sup>–6</sup> to 2.6 × 10<sup>–7</sup> A/mm. Benefiting from the great corrosion selectively of hot KOH solution to AlGaN barrier layer and GaN channel layer after thermal oxygen oxidation, the spikes of the edge of groove region caused by the nonuniform distribution of plasma in the cavity is improved, and the breakdown voltage of the fabricated AlGaN/GaN SBDs is raised from –1.28 to –1.73 kV.

Simulation Study on the Structure Design of p-GaN/AlGaN/GaN HEMT-Based Ultraviolet Phototransistors.

This work investigates the impacts of structural parameters on the performances of p-GaN/AlGaN/GaN HEMT-based ultraviolet (UV) phototransistors (PTs) using Silvaco Atlas. The simulation results show that a larger Al content or greater thickness for the AlGaN barrier layer can induce a higher two-dimensional electron gas (2DEG) density and produce a larger photocurrent. However, they may also lead to a larger dark current due to the incomplete depletion of the GaN channel layer. The depletion conditions with various Al contents and thicknesses of the AlGaN layer are investigated in detail, and a borderline between full depletion and incomplete depletion was drawn. An optimized structure with an Al content of 0.23 and a thickness of 14 nm is achieved for UV-PT, which exhibits a high photocurrent density of 92.11 mA/mm, a low dark current density of 7.68 × 10-10 mA/mm, and a large photo-to-dark-current ratio of over 1011 at a drain voltage of 5 V. In addition, the effects of other structural parameters, such as the thickness and hole concentration of the p-GaN layer as well as the thickness of the GaN channel layer, on the performances of the UV-PTs are also studied in this work.

Influence of the ZrO2 gate dielectric layer on polarization coulomb field scattering in InAlN/GaN metal–insulator–semiconductor high-electron -mobility transistors

An InAlN/GaN MIS-HEMT with ZrO2 gate dielectric layer and a Schottky-gate InAlN/GaN HEMT were fabricated. Subsequently, the influence of ZrO2 gate dielectric layer on polarization Coulomb field (PCF) scattering in InAlN/GaN MIS-HEMT was studied. Compared with InAlN/GaN HEMT, the ZrO2 gate dielectric layer in InAlN/GaN MIS-HEMT results in fewer additional polarization charge and higher two-dimensional electron gas density at the same gate-source voltage (VGS) conditions. Also, we established that both of these factors weaken PCF scattering intensity. However, the InAlN/GaN MIS-HEMT has a larger gate swing. Therefore, there is a smaller VGS was applied during regular operation of device. When the VGS is small, PCF scattering is the strongest scattering and play non-negligible impact in the size of total electron mobility. This study provides a new theoretical foundation for further enhancing the performance of InAlN/GaN MIS-HEMTs.