Yasunori Tokuda

Remarkable breakdown voltage enhancement in AlGaN channel high electron mobility transistors

The channel layer substitution of a wider bandgap AlGaN for a conventional GaN in high electron mobility transistors (HEMTs) is an effective method of enhancing the breakdown voltage. We demonstrated a remarkable breakdown voltage enhancement in these AlGaN channel HEMTs. The obtained maximum breakdown voltages were 463 and 1650V in the Al0.53Ga0.47N∕Al0.38Ga0.62N HEMT with the gate-drain distances of 3 and 10μm, respectively. This result is very promising for the further higher power operation of high-frequency HEMTs.

Classification and origins of GaAs oval defects grown by molecular beam epitaxy

Abstract By extensive surface and cross sectional microscopic observations, oval defects on single GaAs and multiple GaAs/AlGaAs epilayers grown by molecular beam epitaxy (MBE) were investigated. It is found that there are at least seven different representative kinds of oval defects which possess distinct morphological characteristics. Among six different types of oval defects without macroscopic core particulates, four types are ascribed to surface microscopic contaminations, while other two types to the Ga source material. However, the majority of oval defects with our MBE system were usually the type with macroscopic core particulates and were found to originate at the epilayer/substrate interface. They are, therefore, attributed to surface macroscopic contaminations.

Fabrication of Terahertz Planar Metamaterials Using a Super-Fine Ink-Jet Printer

Super-fine ink-jet (SIJ) printing technology is applied to the fabrication of terahertz metamaterials. A silver film is fabricated using an SIJ printer with silver paste ink, and it is confirmed that the film behaves as a good conductor in the terahertz frequency region. Then, basic terahertz metamaterials such as metal wire-grid structures and split-ring resonators are printed on high-resistivity silicon substrates. The terahertz responses of the printed samples agree with those expected from their structures. SIJ printing is one of the ideal methods for fabricating terahertz metamaterials owing to its rapidity, simplicity, flexibility, and sufficient accuracy.

AlGaN Channel HEMT With Extremely High Breakdown Voltage

Enhanced performance of RF power modules is required in a next-generation information society. To satisfy these requirements, we designed a novel high-electron mobility transistor (HEMT) structure employing wider bandgap AlGaN for a channel layer, which we called AlGaN channel HEMT, and investigated it. The wider bandgap is more effective for higher voltage operation of HEMTs and contributes to the increase of output power in RF power modules. As a result, fabricated AlGaN channel HEMTs had much higher breakdown voltages than those of conventional GaN channel HEMTs with good pinchoff operation and sufficiently high drain current density without noticeable current collapse. Furthermore, specific on-state resistances of fabricated AlGaN channel HEMTs were competitive with the best values of reported GaN- and SiC-based devices with similar breakdown voltages. These results indicate that the proposed AlGaN channel HEMTs are very promising not only for an information-communication society but also in the power electronics field.

Spectroscopic Characterization of Low-Temperature Grown GaAs Epitaxial Films

Femtosecond time-resolved reflectance and Raman scattering studies have been made on GaAs epitaxial layers grown at temperatures between 200 and 300° C and subsequently annealed. A subpicosecond carrier lifetime (∼0.25 ps) has been measured for a sample grown at 250° C and annealed at 600° C. Raman measurements using a back scattering geometry show a strong TO phonon band for samples grown at 200° C and 250° C, while it is absent for samples grown at 275 and 300° C. The band width of the LO band increases with decreasing growth temperature. A phonon band corresponding to As precipitates is also observed at 200 cm-1 for samples grown at 200 and 250° C. A strong correlation is found between the measured carrier lifetime and Raman profile.

First Operation of AlGaN Channel High Electron Mobility Transistors

A channel layer substitution of a wider bandgap AlGaN for conventional GaN in high electron mobility transistors (HEMTs) is one possible method of enhancing the breakdown voltage for higher power operation. Wider bandgap AlGaN, however, should also increase the ohmic contact resistance. We utilized a Si ion implantation doping technique to achieve sufficiently low resistive source/drain contacts, and realized the first HEMT operation with an AlGaN channel layer. This result is very promising for the further higher power operation of high-frequency HEMTs.

Isolation edge effect depending on gate length of MOSFETs with various isolation structures

The gate length (L) dependence of the isolation edge effect is investigated for MOSFETs with various isolation structures. We extract the isolation edge effect for a single L by comparing with an H-shaped gate MOSFET which did not have any influence from the isolation edges. For shallow trench isolation (STI), the isolation edge effect is enhanced for L around the onset of the short channel effect (SCE) and is more prominent for a trench edge with a deeper dip. On the other hand, for the local oxidation of silicon (LOCOS) isolation with an elevated field oxide edge (i.e., the birds beak), the isolation edge effect operates in the opposite direction against the cases of STI, though it is enhanced around the SCE appearance point. The L dependence is successfully explained using the charge sharing model where the charge shared by the mixing effect between the SCE and the (inverse) narrow width effect [(I)NWE] is introduced at the channel corners. The enhancement of the isolation edge effect results from that the fraction of the charge shared by the mixing effect depends on L. In addition, the difference between STI and LOCOS occurs because the mixing effect for STI is opposite to that for LOCOS.

Emission spectra of single quantum well lasers with inhomogeneous current injection

Emission spectra of a tandem‐type GaAs single quantum well laser diode were investigated under pulsed operating conditions. By controlling the two injection current levels, we could force the device to operate not only at the lowest (n=1) quantized state transition but also at the second (n=2) transition. Anomalous pulse responses, moreover, were found for the two lasing modes which were simultaneously observed in time‐integrated spectra. The dynamic emission behavior was utilized to achieve a variety of intensity combinations of the two radiation modes at the widely different wavelengths.

Widely separate wavelength switching of single quantum well laser diode by injection-current control

It is demonstrated for the first time that by injection‐current control, the lasing wavelength was able to be switched from the lowest (n=1) quantized state transition to the second (n=2) transition for a single quantum well laser diode. The separation in energy was as wide as >50 meV. This novel function of a laser was realized as a result of appropriately increased threshold gain. This diode, moreover, acted as a multiple wavelength‐emitting laser.

Improved recombination lifetime of photoexcited carriers in GaAs single quantum well heterostructures confined by GaAs/AlAs short‐period superlattices

Photoluminescence (PL) decay time measurements at 77 and 300 K are reported from 6.1 nm GaAs single quantum well heterostructures (SQWH’s) confined by GaAs/AlAs short‐period superlattices (SPS’s) or ternary AlGaAs alloys with similar Al content, prepared by molecular beam epitaxy. The SQW PL intensity exhibits a single exponential decay with a time constant of 1.6 ns for SQWH’s confined by SPS’s and 0.3 ns for SQWH’s confined by AlGaAs alloys at 77 K. From comparison of the decay rates in both types of the sample, it is found that the radiative recombination efficiency is improved by a factor of about 6 in SPS confined SQWH’s. This higher efficiency is attributed to the improved heterointerfaces in addition to the enhanced radiative recombination rate due to the increased overlap of electron and hole wave functions in the narrow SQW.