Challenges in material processing and reliability issues in AlGaN/GaN HEMTs on silicon wafers for future RF power electronics & switching applications: A critical review

This paper demonstrates highly efficient GaN high-electron-mobility transistors (HEMTs) on GaN substrates with reduced interface contamination. By applying a hydrofluoric acid-based pre-growth treatment to a GaN substrate, the Si impurity concentration at the interface between the GaN substrate and the epitaxial layer can successfully be reduced. RF performance was enhanced by pre-growth treatment owing to the suppression of Si-induced parasitic loss. As a result, GaN HEMTs on GaN substrates exhibited an excellent power-added efficiency of 82.8% at a 2.45 GHz. To the best of our knowledge, this exceeds that of the previously reported discrete GaN HEMTs at around this frequency range.

Over 80% power-added-efficiency GaN high-electron-mobility transistors on free-standing GaN substrates

Due to the excellent material characteristics, AlGaN/GaN HEMTs are widely used in high power and high frequency applications. However, the existence of damages, defects and dislocations still degrade the device features because of trapping effects. In this paper, investigations of trapping effects are summarized and discussed to understand the inner mechanism, including the gate-lag and drain-lag transient responses, the current collapse of pulsed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{I}_{\mathrm {DS}}$ </tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\mathrm {DS}}$ </tex-math></inline-formula> , the dependence of frequency dispersion, the transient current reduction with pulsed-RF excitation and the kink effect. In addition, recent methods of suppressing trapping effects are reviewed, including surface passivation, GaN cap layer, gate/source field plate, buffer engineering and structure modification. The profits and shortages of each method are compared and discussed. 

An Overview on Analyses and Suppression Methods of Trapping Effects in AlGaN/GaN HEMTs

Recent observation of high density polarization-induced two-dimensional electron gases in ultra-thin N-polar GaN layers grown on single-crystal AlN has enabled the development of N-polar high electron mobility transistors (HEMTs) on AlN. Such devices will take advantage of thermal and power handling capabilities of AlN, while simultaneously benefitting from the merits of N-polar structures, such as a strong back barrier. We report the experimental demonstration of N-polar GaN/AlGaN/AlN HEMTs on single-crystal AlN substrates, showing an on-current of 2.6 A/mm with a peak transconductance of 0.31 S/mm. Small-signal RF measurements revealed speeds exceeding ft/ fmax = 68/100 GHz. These results pave the way for developing RF electronics with excellent thermal management based on N-polar single-crystal AlN.

N-polar GaN/AlGaN/AlN high electron mobility transistors on single-crystal bulk AlN substrates

Conventional silicon (Si)-based power devices face physical limitations—such as switching speed and energy efficiency—which can make it difficult to meet the increasing demand for high-power, low-loss, and fast-switching-frequency power devices in power electronic converter systems. Gallium nitride (GaN) is an excellent candidate for next-generation power devices, capable of improving the conversion efficiency of power systems owing to its wide band gap, high mobility, and high electric breakdown field. Apart from their cost effectiveness, GaN-based power high-electron-mobility transistors (HEMTs) on Si substrates exhibit excellent properties—such as low ON-resistance and fast switching—and are used primarily in power electronic applications in the fields of consumer electronics, new energy vehicles, and rail transit, amongst others. During the past decade, GaN-on-Si power HEMTs have made major breakthroughs in the development of GaN-based materials and device fabrication. However, the fabrication of GaN-based HEMTs on Si substrates faces various problems—for example, large lattice and thermal mismatches, as well as ‘melt-back etching’ at high temperatures between GaN and Si, and buffer/surface trapping induced leakage current and current collapse. These problems can lead to difficulties in both material growth and device fabrication. In this review, we focused on the current status and progress of GaN-on-Si power HEMTs in terms of both materials and devices. For the materials, we discuss the epitaxial growth of both a complete multilayer HEMT structure, and each functional layer of a HEMT structure on a Si substrate. For the devices, breakthroughs in critical fabrication technology and the related performances of GaN-based power HEMTs are discussed, and the latest development in GaN-based HEMTs are summarised. Based on recent progress, we speculate on the prospects for further development of GaN-based power HEMTs on Si. This review provides a comprehensive understanding of GaN-based HEMTs on Si, aiming to highlight its development in the fields of microelectronics and integrated circuit technology.

GaN-based power high-electron-mobility transistors on Si substrates: from materials to devices

In this letter, we successfully achieved high-power radio frequency (RF) operation of AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on free-standing AlN substrate at X-band. The developed HEMT on AlN substrate comprised a 200 nm thick GaN channel and AlGaN buffer with an Al composition of 30%. Thanks to high breakdown voltage of the HEMT on AlN substrate, we successfully demonstrated 15.2 W mm−1 output power density at operating voltages of 70 V even without device technologies such as source-field plate and optimization of device dimension. Our results show that the potential of GaN HEMTs on AlN substrate as next-generation high-power RF devices.

https://iopscience.iop.org/article/10.35848/1882-0786/abec90/pdf

First demonstration of X-band AlGaN/GaN high electron mobility transistors using free-standing AlN substrate over 15 W mm -1 output power density

The gate leakage characteristics of n-GaN- and i-GaN-capped AlGaN/GaN high-electron-mobility transistor (HEMT) heterostructures grown on various substrates of Si, SiC, and GaN were investigated. HEMT heterostructures were grown by metal-organic vapor phase epitaxy, and the effect of n-GaN cap layer doping on the gate leakage characteristics was investigated depending on the dislocation densities. For i-GaN capped HEMT heterostructures grown on GaN substrates, the current–voltage characteristics were well explained by the theoretical calculation based on thermionic emission, thermionic-field emission, and field emission. Alternatively, for the AlGaN/GaN HEMT heterostructures grown on Si substrates that contain a high threading dislocation density of 8.2 × 109 cm−2, a drastic increase in the reverse leakage current of approximately five orders of magnitude was observed compared to the reverse leakage current observed for the i-GaN-capped HEMT heterostructures. Conductive atomic force microscope analysis revealed that the dislocation-induced surface pits acted as leakage paths only for the n-GaN-capped HEMT heterostructures. Furthermore, the temperature dependence of the leakage current through the surface pit was investigated using a conductive AFM system, and the temperature-dependent behavior associated with one-dimensional variable-range-hopping (1D-VRH) was confirmed. These results indicate that the surface pits have a large impact on the leakage characteristics of the n-GaN capped structures with high dislocation density, and the electron transport through dislocations based on 1D-VRH plays an important role. Furthermore, we demonstrated a significant reduction in the reverse leakage current for n-GaN-capped AlGaN/GaN HEMT heterostructures using low-dislocation-density GaN substrates, i.e., by reducing dislocation-induced leakage paths. Therefore, we believe that the HEMT heterostructures grown on GaN substrates have an advantage in suppressing frequency dispersion and current collapse with minimized impact on the gate leakage behavior of the Schottky-gated HEMTs with an n-GaN cap layer.

Impact of n-GaN cap layer doping on the gate leakage behavior in AlGaN/GaN HEMTs grown on Si and GaN substrates

https://iopscience.iop.org/article/10.35848/1347-4065/ac06d8/pdf

Growth of microcrystalline diamond films after fabrication of GaN high-electron-mobility transistors for effective heat dissipation

To improve the electron mobility of quantum well (QW) gallium nitride (GaN) high electron mobility transistors (HEMT), we investigated QW and conventional AlGaN/GaN heterostructures grown by metal organic vapor phase epitaxy. Using calculation and experimental results, we revealed that the primary reason for the reduced electron mobility of the QW GaN-HEMT is the increase in intrasubband scattering events because of excessive electron confinement, which is caused by a strong polarization electric field. A strained Al0.30Ga0.70N/Al0.86Ga0.14N buffer structure was applied to alleviate the electric field in the GaN channel while maintaining a strong electron confinement. It enables to increase the piezo polarization of Al0.30Ga0.70N and thus, reduces that of the GaN channel. Consequently, the electron mobility improved to 1420 cm2/Vs for the QW GaN-HEMT structure with the Al0.30Ga0.70N/Al0.86Ga0.14N buffer from 1100 cm2/Vs for the common QW GaN-HEMT structure. To the best of our knowledge, 1420 cm2/Vs at a two-dimensional electron gas density of $1.5\times 10^{{13}}$ cm−2 is the highest electron mobility in the QW double heterostructure.

Improved Channel Electron Mobility Through Electric Field Reduction in GaN Quantum-Well Double-Heterostructures

Thermally stable and low trap density SiNx/AlON bi-layer structure for AlGaN/GaN MIS-HEMTs

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