The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industry's quest for devices and/or architectures based on new materials. Here, we provide a review of electronic devices based on two-dimensional materials, outlining their potential as a technological option beyond scaled complementary metal-oxide-semiconductor switches. We focus on the performance limits and advantages of these materials and associated technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. We also discuss the use of two-dimensional materials as an enabling factor for flexible electronics and provide our perspectives on future developments.

Electronics based on two-dimensional materials

GaN and AlGaN have shown great potential in next-generation high-power electronic devices; however, they are plagued by a high density of interface states that affect device reliability and performance, resulting in large leakage current and current collapse. In this review, the authors summarize the current understanding of the gate leakage current and current collapse mechanisms, where awareness of the surface defects is the key to controlling and improving device performance. With this in mind, they present the current research on surface states on GaN and AlGaN and interface states on GaN and AlGaN-based heterostructures. Since GaN and AlGaN are polar materials, both are characterized by a large bound polarization charge on the order of 1013 charges/cm2 that requires compensation. The key is therefore to control the compensation charge such that the electronic states do not serve as electron traps or affect device performance and reliability. Band alignment modeling and measurement can help to determi...

Electronic surface and dielectric interface states on GaN and AlGaN

We have investigated the relationship between improved electrical properties of Al2O3/AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) and electronic state densities at the Al2O3/AlGaN interface evaluated from the same structures as the MOS-HEMTs. To evaluate Al2O3/AlGaN interface state densities of the MOS-HEMTs, two types of capacitance-voltage (C-V) measurement techniques were employed: the photo-assisted C-V measurement for the near-midgap states and the frequency dependent C-V characteristics for the states near the conduction-band edge. To reduce the interface states, an N2O-radical treatment was applied to the AlGaN surface just prior to the deposition of the Al2O3 insulator. As compared to the sample without the treatment, the N2O-radical treated Al2O3/AlGaN/GaN structure showed smaller frequency dispersion of the C-V curves in the positive gate bias range. The state densities at the Al2O3/AlGaN interface were estimated to be 1 × 1012 cm−2 eV−1 or less around the midgap and 8 × 1012 cm−2 eV−1 near the conduction-band edge. In addition, we observed higher maximum drain current at the positive gate bias and suppressed threshold voltage instability under the negative gate bias stress even at 150 °C. Results presented in this paper indicated that the N2O-radical treatment is effective both in reducing the interface states and improving the electrical properties of the Al2O3/AlGaN/GaN MOS-HEMTs.

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Characterization of interface states in Al2O3/AlGaN/GaN structures for improved performance of high-electron-mobility transistors

https://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/68840/1/1-s2.0-S1369800117317304-main.pdf

State of the art on gate insulation and surface passivation for GaN-based power HEMTs

To investigate current linearity and operation stability of metal–oxide–semiconductor (MOS) AlGaN/GaN high electron mobility transistors (HEMTs), we have fabricated and characterized the Al2O3-gate MOS-HEMTs without and with a bias annealing in air at 300 °C. Compared with the as-fabricated (unannealed) MOS HEMTs, the bias-annealed devices showed improved linearity of I D–V G curves even in the forward bias regime, resulting in increased maximum drain current. Lower subthreshold slope was also observed after bias annealing. From the precise capacitance–voltage analysis on a MOS diode fabricated on the AlGaN/GaN heterostructure, it was found that the bias annealing effectively reduced the state density at the Al2O3/AlGaN interface. This led to efficient modulation of the AlGaN surface potential close to the conduction band edge, resulting in good gate control of two-dimensional electron gas density even at forward bias. In addition, the bias-annealed MOS HEMT showed small threshold voltage shift after applying forward bias stress and stable operation even at high temperatures.

/pdf/current-linearity-and-operation-stability-in-al2o3-gate-455f6isdsw.pdf

Current linearity and operation stability in Al2O3-gate AlGaN/GaN MOS high electron mobility transistors

AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOSHFETs) with Al2O3 gate oxide which was deposited by atomic layer deposition (ALD) were fabricated and their performance was then compared with that of AlGaN/GaN MOSHFETs with HfO2 gate oxide. The capacitance (C)–voltage (V) curve of the Al2O3/GaN MOS diodes showed a lower hysteresis and lower interface state density than the C–V curve of the HfO2/GaN diodes, indicating better quality of the Al2O3/GaN interface. The saturation of drain current in the ID–VGS relation of the Al2O3 AlGaN/GaN MOSHFETs was not as pronounced as that of the HfO2 AlGaN/GaN MOSHFETs. The gate leakage current of the Al2O3 MOSHFET was five to eight orders of magnitude smaller than that of the HfO2 MOSHFETs.

Comparative study of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with Al2O3 and HfO2 gate oxide

This paper discusses the capacitance‐voltage (C‐V) characteristics of Al2O3/InSb/Si (1 1 1) MOS diodes grown using MBE via InSb bi-layer with special care to the surface reconstruction. This growth technique is based on our finding that the InSb layer grown on a Si (1 1 1) substrate is rotated by 30 ◦ with respect to the substrate under certain initial conditions. This rotation drastically reduces the lattice mismatch from 19.3% to 3.3%, and improves the crystal quality of an InSb layer. To investigate the possibilities of InSb MOSFETs on Si substrates, we fabricated MOS diodes having an Al2O3 insulator film deposited by atomic layer deposition. C‐V characteristics were measured both at RT and 77 K. It was found that the InSb grown on Si shows a degraded C‐V curve compared to the InSb substrate, even though the mobility of the grown layer is quite high. We also investigated the effects of InSb thickness on the C‐V characteristics of the MOSFETs. It was found that the quality of MOS diodes first degrades when decreasing the InSb thickness from 1 μm to 50 nm; further reduction of the InSb thickness improves it again. It was demonstrated that the MOS diode having a 10 nm InSb layer shows a good C‐V curve, which is comparable to that of the InSb substrate. Finally, we discussed the possibility of the InSb/Si pseudomorphic MOSFETs.

https://iopscience.iop.org/article/10.1088/0268-1242/27/4/045007/pdf

Characterization of Al 2 O 3 /InSb/Si MOS diodes having various InSb thicknesses grown on Si(1 1 1) substrates

Al2O3/InSb/Si quantum well MOSFETs were fabricated with a thin InSb channel layer grown directly on Si(111) substrates The InSb thickness ranged from 6 to 25 nm These thicknesses are close to the critical thickness of InSb on Si, when the InSb layer is grown using a special technique called surface reconstruction controlled epitaxy, which reduces the lattice mismatch from 193 to 33% by rotating the in-plane InSb axis by 30° with respect to the Si(111) substrate Good FET characteristics were observed for 10 nm InSb channel devices The dependence of the device properties on InSb channel thickness was investigated The enhancement of effective mobility for thin InSb channel devices was demonstrated, which indicates the crystal quality improvement when approaching the critical thickness

https://iopscience.iop.org/article/10.7567/JJAP.52.04CF01/pdf

Effective Mobility Enhancement in Al2O3/InSb/Si Quantum Well Metal Oxide Semiconductor Field Effect Transistors for Thin InSb Channel Layers

We have studied “gate-first process” in the fabrication of the Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) to improve the electrical properties of the MOSHFETs, where Al2O3 gate oxide is deposited before ohmic contact alloying. The interface state density of the Al2O3/GaN MOS diodes fabricated by the gate-first process was more than one order of magnitude smaller than that of the diodes fabricated by the conventional “gate-last process” where Al2O3 gate oxide was deposited after ohmic contact alloying. In addition, the saturation of the drain current at large gate voltages in the ID-VGS characteristics of the Al2O3/AlGaN/GaN MOSHFETs was relaxed and the maximum drain current was increased by employing the gate-first process. The hysteresis width of the ID-VGS curve of the MOSHFETs fabricated by the gate-first process was smaller than that of the devices fabricated by the gate-last process. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Eiji Miyazaki

Papers

Comparative study of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with Al2O3 and HfO2 gate oxide

Characterization of Al 2 O 3 /InSb/Si MOS diodes having various InSb thicknesses grown on Si(1 1 1) substrates

Effective Mobility Enhancement in Al2O3/InSb/Si Quantum Well Metal Oxide Semiconductor Field Effect Transistors for Thin InSb Channel Layers

Improvement of the electrical properties of Al2O3/AlGaN/GaN MOSHFETs by gate‐first process

Improvement of the Interface Quality of the Al 2 O 3 / III -Nitride Interface by (NH 4 ) 2 S Surface Treatment for AlGaN/GaN MOSHFETs