T. Egawa

Surface passivation effects on AlGaN/GaN high-electron-mobility transistors with SiO2, Si3N4, and silicon oxynitride

Surface passivation effects were studied on AlGaN/GaN high-electron-mobility transistors (HEMTs) using SiO2, Si3N4, and silicon oxynitride (SiON) formed by plasma enhanced chemical vapor deposition. An increase of IDmax and gmmax has been observed on the passivated (SiO2, Si3N4 and SiON) HEMTs when compared with the unpassivated HEMTs. About an order of magnitude low IgLeak and three orders of magnitude high IgLeak was observed on Si3N4 and SiO2 passivated HEMTs, respectively, when compared with the unpassivated HEMTs. The increase of IgLeak is due to the occurrence of surface related traps, which was confirmed by the observation of kink and hysteresis effect on dc and ac IDS–VDS characteristics, respectively. Though the Si3N4 passivated HEMTs show better dc characteristics, the breakdown voltage (BVgd) characteristics are not comparable with SiO2, SiON passivated and unpassivated HEMTs. The SiON is also a very promising candidate as a surface passivant for AlGaN/GaN HEMTs because it shows better BVgd wit...

Enhancement of breakdown voltage by AlN buffer layer thickness in AlGaN∕GaN high-electron-mobility transistors on 4in. diameter silicon

Enhancement of breakdown voltage (BV) with the increase of AlN buffer layer thickness was observed in AlGaN∕GaN high-electron-mobility transistors (HEMTs) grown by metalorganic chemical vapor deposition on 4in. Si. The enhancement of device performance with AlN buffer thickness (200 and 300nm) is due to the reduction of electrically active defects from Si substrate. The reduction of defects from Si with the increase of AlN thickness was confirmed by x-ray rocking curve measurements. Not much change has been observed in ON-state BV (BV:ON) values except in devices with 500‐nm-thick buffer layer. About 46% enhancement in OFF-state BV (BV:OFF) was observed on 200μm wide HEMTs with 300nm thick AlN buffer layer when compared to HEMTs with 8nm thick AlN buffer layer. The location of junction breakdown in the device was identified as GaN∕AlN∕Si interface. The measured specific on-resistance (Ron) values for 200 and 400μm wide HEMTs with 300nm thick buffer layers were 0.28 and 0.33mΩcm2, respectively. About an or...

High-electron-mobility AlGaN∕AlN∕GaN heterostructures grown on 100-mm-diam epitaxial AlN/sapphire templates by metalorganic vapor phase epitaxy

Al0.26Ga0.74N∕AlN∕GaN heterostructures with 1-nm-thick AlN interfacial layers were grown on 100-mm-diam epitaxial AlN/sapphire templates and sapphire substrates by metalorganic vapor phase epitaxy. It was found that AlN/sapphire templates significantly enhanced the electron mobility of the two-dimensional electron gas (2DEG) confined at the GaN channel. This can be explained by the high-crystal-quality GaN channel realized by the use of epitaxial AlN/sapphire templates as substrates. The very high Hall mobilities of approximately 2100cm2∕Vs at room temperature and approximately 17000cm2∕Vs at 77K with a 2DEG density of approximately 1×1013∕cm2 were uniformly obtained for AlGaN∕AlN∕GaN heterostructures on 100-mm-diam epitaxial AlN/sapphire templates. The Hall mobility of AlGaN∕AlN∕GaN heterostructures on epitaxial AlN/sapphire templates reached a very high value of 25500cm2∕Vs at 15K.

Characterization of different-Al-content AlxGa1−xN/GaN heterostructures and high-electron-mobility transistors on sapphire

AlxGa1−xN/GaN (0.20⩽x⩽0.52) heterostructures (HSs) were grown on a sapphire substrate by atmospheric pressure metalorganic chemical vapor deposition with good uniformity and two-dimensional-electron-gas (2DEG) mobilities of 936, 1163, 1310, 1274, and 911 cm2/V s for different-Al-contents of 20%, 27%, 34%, 42%, and 52%, respectively. 2DEG mobility increase up to the Al content of 34% and then it slowly decreases for high Al-content AlGaN/GaN HSs. An increase of sheet carrier density with the increase of Al content has been observed. A small hump photoluminescence peak of e2DEG1–h has been observed in both 34% and 42% Al-content AlGaN/GaN heterostructures. High Al-content (52%) heterostructure has exhibited a distinguished e2DEG1–h peak. The increase of surface roughness and granular size of AlGaN/GaN heterostructures with the increase of Al content is due to the increase of lattice mismatch between GaN and AlGaN layers. High-electron-mobility transistors (HEMTs) have been fabricated and characterized using...

High-temperature effects of AlGaN/GaN high-electron-mobility transistors on sapphire and semi-insulating SiC substrates

The high-electron-mobility transistors (HEMTs) have been demonstrated on both sapphire and semi-insulating (SI) SiC substrates, and the dc characteristics of the fabricated devices were examined at temperatures ranging from 25 to 500 °C. The decrease in drain current and the transconductance with the increase of temperature have been observed. The decrease ratio of transconductance and drain current was similar for both the HEMTs on sapphire and SI–SiC substrates at and above 300 °C. The HEMTs on SiC substrates showed better dc characteristics after being subjected to thermal stress up to 500 °C. Although the SiC-based HEMTs showed better characteristics up to the temperature of 300 °C, compared with the sapphire-based HEMTs, similar dc characteristics were observed on both at and above 300 °C. For high-temperature applications (⩾300 °C), additional cooling arrangements are essential for both devices.

High performance of InGaN LEDs on (111) silicon substrates grown by MOCVD

We report on the high-performance of InGaN multiple-quantum well light-emitting diodes (LEDs) on Si (111) substrates using metal-organic chemical vapor deposition. A high-temperature thin AlN layer and AlN-GaN multilayers have been used for the growth of high-quality GaN-based LED structure on Si substrate. It is found that the operating voltage of the LED at 20 mA is reduced to as low as 3.8-4.1 V due to the formation of tunnel junction between the n-AlGaN layer and the n-Si substrate when the high-temperature AlN layer is reduced to 3 nm. Because Si has a better thermal conductivity than sapphire, the optical output power of the LED on Si saturates at a higher injected current density. When the injected current density is higher than 120 A/cm/sup 2/, the output power of the LED on Si is higher than that of LED on sapphire. The LED also exhibited the good reliability and the uniform emission from a large size wafer. Cross-sectional transmission electron microscopy observation indicated that the active layer of these LEDs consists of the dislocation-free pyramid-shaped (quantum-dot-like) structure.

Effects of annealing on Ti, Pd, and Ni/n-Al/sub 0.11/Ga/sub 0.89/N Schottky diodes

Schottky diodes of Ti, Pd, and Ni/n-Al/sub 0.11/Ga/sub 0.89/N have been fabricated and the barrier heights were measured to be 0.60, 0.95 and 0.97 eV using current-voltage (I-V) measurements and to be 0.67, 1.15 and 1.22 eV using capacitance-voltage (C-V) measurements. Annealed Schottky diodes are show higher I-V and C-V barrier heights when compared with as-deposited Ti Schottky diodes except high temperature annealed (450/spl deg/C/30 min-500/spl deg/C/1 hr) Ti Schottky diodes. The I-V barrier height of Ti/n-Al/sub 0.11/Ga/sub 0.89/N increases up to the annealing temperature 350/spl deg/C/5 min and it decreased for higher annealing temperatures. The C-V barrier height increases up to the annealing temperature 150/spl deg/C/5 min for Ti (1.63 eV), 250/spl deg/C/5 min for both Pd (1.68 eV) and Ni (1.53 eV) Schottky diodes respectively. The increase of barrier heights for low temperature annealing is due to intimate contact between metal and semiconductor. Rectifying behavior has been observed up to the annealing temperature 450/spl deg/C/1 hr for Ni/n-Al/sub 0.11/Ga/sub 0.89/N and 500/spl deg/C/1 hr for both Ti and Pd/n-Al/sub 0.11/Ga/sub 0.89/N Schottky diodes. An increase of surface average roughness has been observed for the annealed Pd and Ni Schottky diodes except Ti Schottky diodes. Al/sup 0.11/Ga/sub 0.89/N surface behaves more like ceramic with both Pd and Ni than semiconductor.

Temperature dependence of gate–leakage current in AlGaN/GaN high-electron-mobility transistors

We report on the studies of the temperature dependence of gate–leakage current in AlGaN/GaN high-electron-mobility transistors (HEMTs) for the temperature range 20–400 °C. The results show that the temperature dependence of gate–leakage current for AlGaN/GaN HEMTs at subthreshold regime (VGS=−6.5 V) have both negative and positive trends. It has been observed that the leakage current decreases with the temperature up to 80 °C. Above 80 °C, the leakage current increases with the temperature. The negative temperature dependence of leakage current with the activation energy +0.61 eV is due to the impact ionization. The positive temperature dependence of leakage current with the activation energy −0.20 eV is due to the surface related traps, and the activation energy −0.99 eV is due to the temperature assisted tunneling mechanism. The drain voltage at a fixed drain–leakage current reveals the occurrence of both positive (+0.28 V/K) and negative (−0.53 V/K) temperature coefficients.

Comparative study of drain-current collapse in AlGaN/GaN high-electron-mobility transistors on sapphire and semi-insulating SiC

The drain-current collapse at high drain voltage has been studied in AlGaN/GaN high-electron-mobility transistors (HEMTs) on both semi-insulating (SI)–SiC and sapphire substrates using small frequency (120 Hz) sinusoidal wave superimposed dc IDS–VDS characteristics. Low drain-current collapses were observed in AlGaN/GaN HEMTs on SI–SiC substrate when compared with the HEMTs on sapphire substrates. Two and three thermally activated deep traps were observed on SiC-based and sapphire-based HEMTs, respectively. The existence of an additional deep trap (ΔE=0.61 eV) could be associated with the material defects/ dislocations responsible for the severe drain current collapse in sapphire-based HEMTs. The white-light illuminated IDS–VDS characteristics support the existence of more number of deep traps in the sapphire-based HEMTs.

The infrared optical functions of AlxGa1−xN determined by reflectance spectroscopy

Infrared reflectivity measurements have been carried out on samples with structures of GaN/sapphire and AlxGa1−xN/GaN/sapphire as well as sapphire substrates. Analyses of the reflectance data of sapphire using the Kramers–Kronig technique and fitting of the reflectance spectra of GaN and AlxGa1−xN samples using analytical expressions have been made. The high-frequency dielectric constant e∞ and the transverse phonon frequency ωTO, are found to vary from 5.15 to 4.2 and from 559.7 to 586.4 cm−1, respectively, when the composition x is varied from 0 to 0.35 at room temperature. The E2 mode, which arises from the disordered state of the alloys, has been observed in the reflectivity spectrum of AlxGa1−xN, and the intensity of the peak is enhanced by increasing the Al content.