Characterization of Trap States in AlGaN/GaN MIS-High-Electron-Mobility Transistors under Semi-on-State Stress.

Devices under semi-on-state stress often suffer from more severe current collapse than when they are in the off-state, which causes an increase in dynamic on-resistance. Therefore, characterization of the trap states is necessary. In this study, temperature-dependent transient recovery current analysis determined a trap energy level of 0.08 eV under semi-on-state stress, implying that interface traps are responsible for current collapse. Multi-frequency capacitance-voltage (C-V) testing was performed on the MIS diode, calculating that interface trap density is in the range of 1.37×1013 to 6.07×1012cm-2eV-1 from EC-ET=0.29 eV to 0.45 eV.

Improvement of Single Event Transient Effects for a Novel AlGaN/GaN High Electron-Mobility Transistor with a P-GaN Buried Layer and a Locally Doped Barrier Layer.

In this paper, a novel AlGaN/GaN HEMT structure with a P-GaN buried layer in the buffer layer and a locally doped barrier layer under the gate (PN-HEMT) is proposed to enhance its resistance to single event transient (SET) effects while also overcoming the degradation of other characteristics. The device operation mechanism and characteristics are investigated by TCAD simulation. The results show that the peak electric field and impact ionization at the gate edges are reduced in the PN-HEMT due to the introduced P-GaN buried layer in the buffer layer. This leads to a decrease in the peak drain current (Ipeak) induced by the SET effect and an improvement in the breakdown voltage (BV). Additionally, the locally doped barrier layer provides extra electrons to the channel, resulting in higher saturated drain current (ID,sat) and maximum transconductance (gmax). The Ipeak of the PN-HEMT (1.37 A/mm) is 71.8% lower than that of the conventional AlGaN/GaN HEMT (C-HEMT) (4.85 A/mm) at 0.6 pC/µm. Simultaneously, ID,sat and BV are increased by 21.2% and 63.9%, respectively. Therefore, the PN-HEMT enhances the hardened SET effect of the device without sacrificing other key characteristics of the AlGaN/GaN HEMT.

Physics and chemistry of nitrogen dioxide (NO2) adsorption on gallium nitride (GaN) surface and its interaction with the yellow-luminescence-associated surface state.

Surface states have been a longstanding and sometimes underestimated problem in gallium nitride (GaN) based devices. The instability caused by surface-charge-trapping in GaN-based transistors is practically the same problem faced by the inventors of the silicon (Si) field effect transistors more than half a century ago. Although in Si this problem was eventually solved by oxygen and hydrogen-based passivation, in GaN, such breakthrough has yet to be made. Apparently, some of this surface charge originates in molecules adsorbed on its surface. Here, it is shown that the charge density associated with the GaN yellow band desorbs upon mild heat treatment in vacuum and re-adsorbs on exposure to the air. Selective exposure of GaN to nitrogen dioxide (NO2) reproduces this surface charge to its original distribution, as does exposure to air. Residual gas analysis of the gases desorbed during heat treatment shows a large concentration of nitric oxide (NO). These observations suggest that selective adsorption of NO2 is responsible for the surface charge that deleteriously affects the electrical properties of GaN. The physics and chemistry of this NO2 adsorption, reported here, may open a new path in the search for passivation to improve GaN device reliability.

Charge Trapping and Emission during Bias Temperature Stressing of Schottky Gate GaN-on-Silicon HEMT Structures Targeting RF/mm Wave Power Amplifiers.

For operation as power amplifiers in RF applications, high electron mobility transistor (HEMT) structures are subjected to a range of bias conditions, applied at both the gate and drain terminals, as the device is biased from the OFF- to ON-state conditions. The stability of the device threshold voltage (Vt) condition is imperative from a circuit-design perspective and is the focus of this study, where stresses in both the ON and OFF states are explored. We see rapid positive threshold voltage increases under negative bias stress and subsequent recovery (i.e., Vt reduces), whereas conversely, we see a negative Vt shift under positive stress and Vt increase during the subsequent relaxation phase. These effects are correlated with the thickness of the GaN layer and ultimately result from the deep carbon-acceptor levels in the C-GaN back barrier incorporated to screen the buffer between the silicon substrate and the epitaxially grown GaN layer. Methods to mitigate this effect are explored, and the consequences are discussed.

A Study on the Dynamic Switching Characteristics of p-GaN HEMT Power Devices.

This study employs an innovative dynamic switching test system to investigate the dynamic switching characteristics of three p-GaN HEMT devices. The dynamic switching characteristics are different from the previous research on the dynamic resistance characteristics of GaN devices, and the stability of GaN devices can be analyzed from the perspective of switching characteristics. Based on the theory of dynamic changes in threshold opening voltage and capacitance caused by electrical stress, the mechanism of dynamic switching characteristics of GaN HEMT devices is studied and analyzed in detail. The test results have shown that electrical stress induces trap ionization within the device, resulting in fluctuations in electric potential and ultimately leading to alterations in two critical factors of the dynamic switching characteristics of GaN HEMT devices, the parasitic capacitance and the threshold voltage. The dynamic changes in capacitance before and after electrical stress vary among devices, resulting in different dynamic switching characteristics. The test system is capable of extracting the switching waveform for visual comparison and quantitatively calculating the changes in switching parameters before and after electrical stressing. This test provides a prediction for the drift of switch parameters, offering pre-guidance for the robustness of the optimized application scheme.

Electronic Properties of Group-III Nitride Semiconductors and Device Structures Probed by THz Optical Hall Effect.

Group-III nitrides have transformed solid-state lighting and are strategically positioned to revolutionize high-power and high-frequency electronics. To drive this development forward, a deep understanding of fundamental material properties, such as charge carrier behavior, is essential and can also unveil new and unforeseen applications. This underscores the necessity for novel characterization tools to study group-III nitride materials and devices. The optical Hall effect (OHE) emerges as a contactless method for exploring the transport and electronic properties of semiconductor materials, simultaneously offering insights into their dielectric function. This non-destructive technique employs spectroscopic ellipsometry at long wavelengths in the presence of a magnetic field and provides quantitative information on the charge carrier density, sign, mobility, and effective mass of individual layers in multilayer structures and bulk materials. In this paper, we explore the use of terahertz (THz) OHE to study the charge carrier properties in group-III nitride heterostructures and bulk material. Examples include graded AlGaN channel high-electron-mobility transistor (HEMT) structures for high-linearity devices, highlighting the different grading profiles and their impact on the two-dimensional electron gas (2DEG) properties. Next, we demonstrate the sensitivity of the THz OHE to distinguish the 2DEG anisotropic mobility parameters in N-polar GaN/AlGaN HEMTs and show that this anisotropy is induced by the step-like surface morphology. Finally, we present the temperature-dependent results on the charge carrier properties of 2DEG and bulk electrons in GaN with a focus on the effective mass parameter and review the effective mass parameters reported in the literature. These studies showcase the capabilities of the THz OHE for advancing the understanding and development of group-III materials and devices.

Effect of High-Temperature Storage on Electrical Characteristics of Hydrogen-Treated AlGaN/GaN High-Electron-Mobility Transistors.

In this paper, high-temperature storage of hydrogen-treated AlGaN/GaN HEMTs is conducted for the first time to study the effect of high temperature on the electrical characteristics of the devices after hydrogen treatment, and it is found that high-temperature storage can effectively reduce the impact of hydrogen on the devices. After hydrogen treatment, the output current and the maximum transconductance of the device increase, and the threshold voltage drifts negatively. However, after high-temperature treatment at 200 °C for 24 h, the output current, threshold voltage, and the maximum transconductance of the device all approach their initial values before hydrogen treatment. By using low-frequency noise analysis technology, the trap density of the hydrogen-treated AlGaN/GaN HEMT is determined to be 8.9 × 1023 cm-3·eV-1, while it changes to 4.46 × 1022 cm-3·eV-1 after high-temperature storage. We believe that the change in the electrical characteristics of the device in hydrogen is due to the passivation of hydrogen on the inherent trap of the device, and the variation in the electrical properties of the device in the process of high-temperature storage involves the influence of two effects, namely the dehydrogenation effect and the improvement of the metal-semiconductor interface caused by high temperatures.

Using U-Net convolutional neural network to model pixel-based electrostatic potential distributions in GaN power MIS-HEMTs.

This study demonstrates a novel use of the U-Net convolutional neural network (CNN) for modeling pixel-based electrostatic potential distributions in GaN metal-insulator-semiconductor high-electron mobility transistors (MIS-HEMTs) with various gate and source field plate designs and drain voltages. The pixel-based images of the potential distribution are successfully modeled from the developed U-Net CNN with an error of less than 1% error relative to a TCAD simulated reference of a 500-V electrostatic potential distribution in the AlGaN/GaN interface. Furthermore, the modeling time of potential distributions by U-Net takes about 80 ms. Therefore, the U-Net CNN is a promising approach to efficiently model the pixel-based distributions characteristics in GaN power devices.

Improvement Performance of p-GaN Gate High-Electron-Mobility Transistors with GaN/AlN/AlGaN Barrier Structure.

This study demonstrates a particular composited barrier structure of high-electron-mobility transistors (HEMTs) with an enhancement mode composed of p-GaN/GaN/AlN/AlGaN/GaN. The purpose of the composite barrier structure device is to increase the maximum drain current, reduce gate leakage, and achieve lower on-resistance (Ron) performance. A comparison was made between the conventional device without the composited barrier and the device with the composited barrier structure. The maximum drain current is significantly increased by 37%, and Ron is significantly reduced by 23%, highlighting the synergistic impact of the composite barrier structure on device performance improvement. This reason can be attributed to the undoped GaN (u-GaN) barrier layer beneath p-GaN, which was introduced to mitigate Mg diffusion in the capping layer, thus addressing its negative effects. Furthermore, the AlN barrier layer exhibits enhanced electrical properties, which can be attributed to the critical role of high-energy-gap properties that increase the 2DEG carrier density and block leakage pathways. These traps impact the device behavior mechanism, and the simulation for a more in-depth analysis of how the composited barrier structure brings improvement is introduced using Synopsys Sentaurus TCAD.

Improved Vth Stability and Gate Reliability of GaN-Based MIS-HEMTs by Employing Alternating O2 Plasma Treatment.

The Vth stability and gate reliability of AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) with alternating O2 plasma treatment were systematically investigated in this article. It was found that the conduction band offset at the Al2O3/AlGaN interface was elevated to 2.4 eV, which contributed to the suppressed gate leakage current. The time-dependent dielectric breakdown (TDDB) test results showed that the ALD-Al2O3 with the alternating O2 plasma treatment had better quality and reliability. The AlGaN/GaN MIS-HEMT with the alternating O2 plasma treatment demonstrated remarkable advantages in higher Vth stability under high-temperature and long-term gate bias stress.

Research on the Reliability of Threshold Voltage Based on GaN High-Electron-Mobility Transistors.

With the development of high-voltage and high-frequency switching circuits, GaN high-electron-mobility transistor (HEMT) devices with high bandwidth, high electron mobility, and high breakdown voltage have become an important research topic in this field. It has been found that GaN HEMT devices have a drift in threshold voltage under the conditions of temperature and gate stress changes. Under high-temperature conditions, the difference in gate contact also causes the threshold voltage to shift. The variation in the threshold voltage affects the stability of the device as well as the overall circuit performance. Therefore, in this paper, a review of previous work is presented. Temperature variation, gate stress variation, and gate contact variation are investigated to analyze the physical mechanisms that generate the threshold voltage (VTH) drift phenomenon in GaN HEMT devices. Finally, improvement methods suitable for GaN HEMT devices under high-temperature and high-voltage conditions are summarized.

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.

Design of Inner Matching Three-Stage High-Power Doherty Power Amplifier Based on GaN HEMT Model.

This paper introduces the structure and characteristics of an internal-matching high-power Doherty power amplifier based on GaN HEMT devices with 0.25 µm process platforms from the Nanjing Electronic Devices Institute. Through parameter extraction and load-pull testing of the model transistor, an EE_HEMT model for the 1.2 mm gate-width GaN HEMT device was established. This model serves as the foundation for designing a high-power three-stage Doherty power amplifier. The amplifier achieved a saturated power gain exceeding 9 dB in continuous wave mode, with a saturated power output of 49.7 dBm. The drain efficiency was greater than 65% at 2.6 GHz. At 9 dB power back-off point, corresponding to an output power of 40.5 dBm, the drain efficiency remained above 55%. The performance of the amplifier remains consistent within the 2.55-2.62 GHz frequency range. The measured power, efficiency, and gain of the designed Doherty power amplifier align closely with the simulation results based on the EE_HEMT model, validating the accuracy of the established model. Furthermore, the in-band matching design reduces the size and weight of the amplifier. The amplifier maintains normal operation even after high and low-temperature testing, demonstrating its reliability. In conjunction with DPD (digital pre-distortion) for the modulated signal test, the amplifier exhibits extremely high linearity (ACLR < -50.93 dBc). This Doherty power amplifier holds potential applications in modern wireless communication scenarios.

Study on the Hydrogen Effect and Interface/Border Traps of a Depletion-Mode AlGaN/GaN High-Electron-Mobility Transistor with a SiNx Gate Dielectric at Different Temperatures.

In this study, the electrical characteristics of depletion-mode AlGaN/GaN high-electron-mobility transistors (HEMTs) with a SiNx gate dielectric were tested under hydrogen exposure conditions. The experimental results are as follows: (1) After hydrogen treatment at room temperature, the threshold voltage VTH of the original device was positively shifted from -16.98 V to -11.53 V, and the positive bias of threshold was 5.45 V. When the VDS was swept from 0 to 1 V with VGS of 0 V, the IDS was reduced by 25% from 9.45 A to 7.08 A. (2) Another group of original devices with identical electrical performance, after the same duration of hydrogen treatment at 100 °C, exhibited a reverse shift in threshold voltage with a negative threshold shift of -0.91 V. The output characteristics were enhanced, and the saturation leakage current was increased. (3) The C-V method and the low-frequency noise method were used to investigate the effect of hydrogen effect on the device interface trap and border trap, respectively. It was found that high-temperature hydrogen conditions can passivate the interface/border traps of SiNx/AlGaN, reducing the density of interface/border traps and mitigating the trap capture effect. However, in the room-temperature hydrogen experiment, the concentration of interface/border traps increased. The research findings in this paper provide valuable references for the design and application of depletion-mode AlGaN/GaN HEMT devices.

A 28 GHz GaN 6-Bit Phase Shifter MMIC with Continuous Tuning Calibration Technique.

A 28 GHz digitally controlled 6-bit phase shifter with a precision calibration technique in GaN high-electron mobility transistor (HEMT) technology is presented for Ka-band phased-array systems and applications. It comprises six stages, in which stages 1 and 2 for 5.625° and 11.25° are designed in the form of a switched-line circuit, and stages 3, 4, and 5 for 22.5°, 45°, and 90° are designed in the form of a switched-filter circuit. The final stage 6 for 180° is designed in a single-to-differential balun followed by a single-pole double-throw (SPDT) switch for achieving an efficient phase inversion. A novel continuous tuning calibration technique is proposed to improve the phase accuracy. It controls the gate bias voltage of off-state HEMTs at the stage 6 SPDT switch for fine calibration of the output phase. Fabricated in a 0.15 µm GaN HEMT process using a die size of 1.75 mm2, the circuit produces 64 phase states at 28 GHz with a 5.625° step. The experimental results show that the Root-Mean-Square (RMS) phase error is significantly improved from 8.56° before calibration to 1.08° after calibration. It is also found that the calibration does not induce significant changes for other performances such as the insertion loss, RMS amplitude error, and input-referred P1dB. This work successfully demonstrates that the GaN technology can be applied to millimeter-wave high-power phased-array transceiver systems.

A Comparative Study on the Degradation Behaviors of Ferroelectric Gate GaN HEMT with PZT and PZT/Al2O3 Gate Stacks.

In this paper, the degradation behaviors of the ferroelectric gate Gallium nitride (GaN) high electron mobility transistor (HEMT) under positive gate bias stress are discussed. Devices with a gate dielectric that consists of pure Pb(Zr,Ti)O3 (PZT) and a composite PZT/Al2O3 bilayer are studied. Two different mechanisms, charge trapping and generation of traps, both contribute to the degradation. We have observed positive threshold voltage shift in both kinds of devices under positive gate bias stress. In the devices with a PZT gate oxide, we have found the degradation is owing to electron trapping in pre-existing oxide traps. However, the degradation is caused by electron trapping in pre-existing oxide traps and the generation of traps for the devices with a composite PZT/Al2O3 gate oxide. Owing to the large difference in dielectric constants between PZT and Al2O3, the strong electric field in the Al2O3 interlayer makes PZT/Al2O3 GaN HEMT easier to degrade. In addition, the ferroelectricity in PZT enhances the electric field in Al2O3 interlayer and leads to more severe degradation. According to this study, it is worth noting that the reliability problem of the ferroelectric gate GaN HEMT may be more severe than the conventional metal-insulator-semiconductor HEMT (MIS-HEMT).

Construction of the Rhodobacter sphaeroides strain overproducing 5-aminolevulinic acid by insertion of endogenous promoter.

5-Aminolevulinic acid (ALA) is a precursor of heme and a natural amino acid synthesized in the cells of most living organisms. Currently, ALA is used as an ingredient in pharmaceuticals, supplements, cosmetics, feed, fertilizers, and other products. ALA is mainly produced by industrial fermentation by the photosynthetic bacterium Rhodobacter sphaeroides. In this study, we tried to improve the ALA productivity by R. sphaeroides using a genetic strategy to highly express ALA synthase (ALAS) genes. We inserted a constitutive promoter (PrrnB or Prsp_7571) upstream of genes encoding ALAS (hemA and/or hemT) to construct strains that constitutively express ALAS. The highest transcript levels of hemA were observed in the strain where PrrnB was inserted into the hemA promoter region and were 3.5-fold higher than those in the wild-type. The highest transcript levels of hemT were observed in the strain where PrrnB was inserted into the hemT promoter region and were 46-fold higher than those in the wild-type. The maximum ALAS activity was observed in crude cell extracts of the strain where PrrnB was inserted into the hemT promoter region under optimized growth conditions that was 2.7-fold higher than that in the wild type. This strain showed 12-fold accumulation of ALA compared to the wild-type. Thus, we improved ALA productivity without using exogenous DNA sequences. In the future, further improvement in ALA productivity may be expected by applying this approach to current industrial ALA-producing bacteria.

Trials and tribulations of highly effective modulator therapies in cystic fibrosis.

Highly effective modulator therapies (HEMTs) have revolutionised the management approach of most patients living with cystic fibrosis (CF) who have access to these therapies. Clinical trials have reported significant improvements across multiorgan systems, with patients surviving longer. However, there are accumulating case reports and observational data describing various adverse events following initiation of HEMTs including drug-to-drug interactions, drug induced liver injury, Stevens-Johnson syndrome, and neurocognitive symptoms including psychosis and depression, which have required discontinuation of therapy. Current clinical trials are assessing efficacy in younger patients with CF, yet long-term studies are also required to better understand the safety profile in the real-world setting across all ages and the impact of HEMT dose alteration or discontinuation.

Exploring the Potential of GaN-Based Power HEMTs with Coherent Channel.

The GaN industry always demands further improvement in the power transport capability of GaN-based high-energy mobility transistors (HEMT). This paper presents a novel enhancement-type GaN HEMT with high power transmission capability, which utilizes a coherent channel that can form a three-dimensional electron sea. The proposed device is investigated using the Silvaco simulation tool, which has been calibrated against experimental data. Numerical simulations prove that the proposed device has a very high on-state current above 3 A/mm, while the breakdown voltage (above 800 V) is not significantly affected. The calculated Johnson's and Baliga's figure-of-merits highlight the promise of using such a coherent channel for enhancing the performance of GaN HEMTs in power electronics applications.

Trap Characterization Techniques for GaN-Based HEMTs: A Critical Review.

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have been considered promising candidates for power devices due to their superior advantages of high current density, high breakdown voltage, high power density, and high-frequency operations. However, the development of GaN HEMTs has been constrained by stability and reliability issues related to traps. In this article, the locations and energy levels of traps in GaN HEMTs are summarized. Moreover, the characterization techniques for bulk traps and interface traps, whose characteristics and scopes are included as well, are reviewed and highlighted. Finally, the challenges in trap characterization techniques for GaN-based HEMTs are discussed to provide insights into the reliability assessment of GaN-based HEMTs.