International Technology Roadmap for Semiconductors 2003の要求清浄度について － シリコンウエハ表面と雰囲気環境に要求される清浄度, 分析方法の現状について －

A survey of Gallium Nitride HEMT for RF and high power applications

Recent advances on dielectrics technology for SiC and GaN power devices

This article reviews the development of SiC and GaN devices for power-switching applications in the context of four specifically identified application requirements: (1) high-blocking voltage, (2) high-power efficiency, (3) high-switching speed, and (4) normally OFF operation. Specific device and material characteristics, such as ON resistance, parasitic capacitances, and energy-gap values, are compared and discussed in relation to the identified application requirements. Following a review of the fundamental limitations of silicon as a material, this article describes the material advantages that motivated the development of commercially available Schottky diodes and transistors using SiC. The last section analyzes the potential of GaN to enable further technical progress beyond the theoretical limit of Si and to significantly reduce the cost of power-electronic switches.

Power-switching applications beyond silicon: Status and future prospects of SiC and GaN devices

In this brief, the InAlN/GaN high-electron mobility transistors (HEMTs) on silicon substrate with high Johnson’s figure-of-merit (J-FOM) are presented. A trilayer photoresist of polymethylmethacrylate (PMMA)/copolymer/PMMA associated with a T-shaped gate is used to reduce the parasitic resistance while maintaining high current gain cutoff frequency. The small dc-to-RF transconductance dispersion of only 1.1% suggests a good quality SiN x passivation layer, and the $f_{\rm MAX}$ of 101 GHz and $f_{T}$ of 60 GHz can be simultaneously obtained with a 0.11- $\mu \text{m}$ foot length and 1.5- $\mu \text{m}$ source-drain distance. In addition, the three-terminal OFF-state breakdown measurements reveal a source-drain breakdown voltage (BV $_{\rm DS})$ of 21 V ( $V_{\rm DG}=31$ V). The results lead to a high J-FOM of 1.3 THz $\,\cdot \,\text{V}$ , which has not been reported for the InAlN/GaN HEMTs on silicon substrate.

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101-GHz InAlN/GaN HEMTs on Silicon With High Johnson’s Figure-of-Merit

This paper reports on the processing and characterization of AlN/GaN metal-oxide-semiconductor high-electron mobility transistors (MOS-HEMTs). The devices employ thermally grown Al2O3 as a gate dielectric and surface protection and passivation, which is an approach that provides an opportunity to define the ohmic contact areas by wet etching of Al (and optimization of this processing step) prior to the formation of Al2O3 and ohmic metal deposition. The devices also employ a new process technique that significantly suppresses leakage currents on the mesa sidewalls. Fabricated devices exhibited good direct current and radio frequency performance. A high peak current, i.e., ~ 1.5 A/mm, at VGS = +3 V and a current-gain cutoff frequency fT and maximum oscillation frequency fMAX of 50 and 40 GHz, respectively, were obtained for a device with 0.2-μm gate length and 100- μm gate width. Additionally, a robust method for the extraction of the small-signal equivalent circuit suitable for process optimization is described. It relies on intimate process knowledge and device geometry to determine equivalent circuit elements of the fabricated AlN/GaN MOS-HEMTs.

AlN/GaN MOS-HEMTs With Thermally Grown $\hbox{Al}_{2} \hbox{O}_{3}$ Passivation

A simple method is reported for fabrication of AlN/GaN MOS-HEMTs. Ultra-thin Al 2 O 3 , which is formed using thermal oxidation of evaporated Al, was used for surface passivation and as a gate dielectric. Prior to formation of Al 2 O 3 , the Al protects the very sensitive AlN epilayer from exposure to processing liquid chemicals. Fabricated two-finger AlN/GaN MOS-HEMTs with 3 m gate length and 200 m gate width showed good gate control of drain currents up to a gate bias of 3 V and achieved a maximum drain current, I DSmax , of ~900~mA/mm. The peak extrinsic transconductance, G max , of the device is ~100~mS/mm at V DS = 8=V. Capacitance-voltage ( C - V ) characteristics of Al 2 O 3 /AlN/GaN circular test MOS structures were observed and measured. They exhibited no hysteresis, indicating the good quality of the thermally grown Al 2 O 3 for realising AlN/GaN MOS-HEMTs for high power and high frequency applications.

Surface passivation of AlN/GaN MOS-HEMTs using ultra-thin Al2O3 formed by thermal oxidation of evaporated aluminium

A new method for realising enhancement mode AlGaN/GaN devices using a localized gate-foot oxidation is described. Thermal oxidation of the AlGaN barrier layer converts the top surface/part of this layer into aluminium and gallium oxides, which serve as a good gate dielectric and improve the gate leakage current by several orders of magnitude compared to a Schottky gate. The oxidation process leaves a thinner AlGaN barrier which can result in normally off operation. Without special precaution, however, the oxidation of the AlGaN barrier is not uniform from the top but occurs at higher rates at the defect/dislocation sites. This makes it impossible to control the barrier thickness and so rendering the barrier useless. To avoid the problem of non-uniform oxidation, a thin layer of Aluminum is first deposited on the barrier layer and oxidized to form Al 2 O 3 on top. This additional oxide layer seems to ensure uniform oxidation of the AlGaN barrier layer underneath on subsequent further oxidation. Preliminary results of the fabricated 2μm × 100μm AlGaN/GaN MOS-HEMTs with a partially oxidized barrier layer showed a maximum drain current of more than 700 m A/mm at high gate bias of 5V with very less current compression.

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Development of enhancement mode AlGaN/GaN MOS-HEMTs using localized gate-foot oxidation

BaTiO3 and Ba0.9925 Er0.0075 TiO3 ceramics were investigated regarding their dielectric and microstructure properties via conventional solid state reaction method. The phase pure samples were obtained when heated at 1400°C for overnight. The effect of Er3+ doped into BaTiO3 on dielectric properties and microstructural properties was investigated for composition of BaTiO3 and Ba0.9925 Er0.0075 TiO3 . The analysis was made by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Impedance Analyzer. The XRD patterns of BaTiO3 and Ba0.9925 Er0.0075 TiO3 are phase pure and identical with tetragonal perovskite structure with space group of P4mm. The lattice parameters and unit cell volume of BaTiO3 increased by doping with Erbium as the crystallite size decreased. Measurements of dielectric properties were carried out as a function of temperature up to 200°C at different frequencies. Ba0.9925 Er0.0075 TiO3 exhibit the high value of dielectric constant (e=6179) at Curie temperature (TC ) of 120°C. SEM analysis of BaTiO3 and Ba0.9925 Er0.0075 TiO3 ceramics showed that the grain sizes of BaTiO3 and Ba0.9925 Er0.0075 TiO3 were ranged from 3.3µm-7.8µm and 2.2µm-4.7µm respectively.

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Dielectric and microstructural properties of BaTiO3 and Ba0.9925Er0.0075TiO3 ceramics

The ever increasing demand for higher power devices at higher frequencies has prompted much research recently into the aluminium nitride/gallium nitride high electron mobility transistors (AlN/GaN HEMTs) in response to theoretical predictions of higher performance devices. Despite having superior material properties such as higher two-dimensional electron gas (2DEG) densities and larger breakdown field as compared to the conventional aluminium gallium nitride (AlGaN)/GaN HEMTs, the AlN/GaN devices suffer from surface sensitivity, high leakage currents and high Ohmic contact resistances. Having very thin AlN barrier layer of ∼ 3 nm makes the epilayers very sensitive to liquids coming in contact with the surface. Exposure to any chemical solutions during device processing degrades the surface properties, resulting in poor device performance. To overcome the problems, a protective layer is employed during fabrication of AlN/GaN-based devices. However, in the presence of the protective/passivation layers, formation of low Ohmic resistance source and drain contact becomes even more difficult.
In this work, thermally grown aluminium oxide (Al2O3) was used as a gate di- electric and surface passivation for AlN/GaN metal-oxide-semiconductor (MOS)-HEMTs. Most importantly, the Al2O3 acts as a protection layer during device processing. The developed technique allows for a simple and effective wet etching optimisation using 16H3PO4:HNO3:2H2O solution to remove Al from the Ohmic contact regions prior to the formation of Al2O3 and Ohmic metallisation. Low Ohmic contact resistance (0.76Ω.mm) as well as low sheet resistance (318Ω/square) were obtained after optimisation.
Significant reduction in the gate leakage currents was observed when employing an additional layer of thermally grown Al2O3 on the mesa sidewalls, particularly in the region where the gate metallisation overlaps with the exposed channel edge. A high peak current ∼1.5 A/mm at VGS=+3 V and a current-gain cutoff frequency, fT , and maximum oscillation frequency, fMAX , of 50 GHz and 40 GHz, respectively, were obtained for a device with 0.2 μm gate length and 100 μm gate width. The measured breakdown voltage, VBR, of a two-finger MOS-HEMT with 0.5μm gate length and 100 μm gate width was 58 V.
Additionally, an approach based on an accurate estimate of all the small-signal equivalent circuit elements followed by optimisation of these to get the actual element values was also developed for AlN/GaN MOS-HEMTs. The extracted element values provide feedback for further device process optimisation. The achieved results indicate the suitability of thermally grown Al2O3 for AlN/GaN-based MOS-HEMT technology for future high frequency power applications.

S. Taking

Papers

AlN/GaN MOS-HEMTs With Thermally Grown $\hbox{Al}_{2} \hbox{O}_{3}$ Passivation

Surface passivation of AlN/GaN MOS-HEMTs using ultra-thin Al2O3 formed by thermal oxidation of evaporated aluminium

Development of enhancement mode AlGaN/GaN MOS-HEMTs using localized gate-foot oxidation

Dielectric and microstructural properties of BaTiO3 and Ba0.9925Er0.0075TiO3 ceramics

AlN/GaN MOS-HEMTs technology