Tsutomu Uesugi

GaN power device and reliability for automotive applications

Many power switching devices are used in a hybrid vehicle (HV) and an electric vehicle (EV) systems. For future development of the HV/EV, higher performances than Si power device, for example, low on-resistance, high speed, high operation temperature, are strongly required. GaN power devices are promising candidate for the requirements. Present status of the GaN power device development is presented. Reliability of the GaN power device was also discussed.

Advanced SiC and GaN power electronics for automotive systems

A power switching device is one of the key elements to determine the performance of hybrid electric vehicles (HEVs) and pure electric vehicles (EVs). Recently, the power devices using wide-bandgap semiconductors, such as SiC and GaN, have been intensively developed for the future HEVs and EVs. In this paper, we review a role of the power devices in these automotive systems, the required device characteristics, and the recent status of SiC and GaN power devices.

A concept of SOI RESURF lateral devices with striped trench electrodes

This paper presents a concept of silicon-on insulator lateral devices based on a reduced surface field (RESURF) principle by striped trench electrodes formed along the current flow direction. These trench electrodes reduce the electric field at the pn junctions sandwiched between the electrodes. We experimentally applied this RESURF technology to a conventional pn/sup -/ lateral diode. As a result, the breakdown voltage was increased from 56 to 104 V without varying the impurity concentration and the length of the n/sup -/ region. This means that the RESURF effect was achieved with the striped trench electrodes. The LDMOS with this RESURF technology was evaluated by simulations. This would be available for 80-V class lateral MOSFETs, used in the forthcoming 42-V automotive systems.

As-grown deep-level defects in n-GaN grown by metal–organic chemical vapor deposition on freestanding GaN

Traps of energy levels Ec −0.26 and Ec −0.61 eV have been identified as as-grown traps in n-GaN grown by metal–organic chemical vapor deposition by using deep level transient spectroscopy of the Schottky contacts fabricated by resistive evaporation. The additional traps of Ec −0.13 and Ec −0.65 eV have been observed in samples whose contacts are deposited by electron-beam evaporation. An increase in concentration of the Ec −0.13 and Ec −0.65 eV traps when approaching the interface between the contact and the GaN film supports our argument that these traps are induced by electron-beam irradiation. Conversely, the depth profiles of as-grown traps show different profiles between several samples with increased or uniform distribution in the near surface below 50 nm. Similar profiles are observed in GaN grown on a sapphire substrate. We conclude that the growth process causes these large concentrations of as-grown traps in the near-surface region. It is speculated that the finishing step in the growth process ...

New 3-D lateral power MOSFETs with ultra low on-resistance

This paper presents a new 3D lateral power MOSFET which has a double gate and a trench gate/drain structure. The double gate structure decreased its channel resistance, and the trench gate/drain structure decreased its n drift resistance. We realized this structure using solid phase epitaxy and a conventional trench technology. From experimental results, a breakdown voltage of 49.5 V and a specific on-resistance of 42 m/spl Omega//spl middot/mm/sup 2/ were obtained.

Current collapse imaging of Schottky gate AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation measurement

Two-dimensional current collapse imaging of a Schottky gate AlGaN/GaN high electron mobility transistor device was achieved by optical electric field-induced second-harmonic generation (EFISHG) measurements. EFISHG measurements can detect the electric field produced by carriers trapped in the on-state of the device, which leads to current collapse. Immediately after (e.g., 1, 100, or 800 μs) the completion of drain-stress voltage (200 V) in the off-state, the second-harmonic (SH) signals appeared within 2 μm from the gate edge on the drain electrode. The SH signal intensity became weak with time, which suggests that the trapped carriers are emitted from the trap sites. The SH signal location supports the well-known virtual gate model for current collapse.

Vertical device operation of AlGaN/GaN HEMTs on free-standing n-GaN substrates

We report on the demonstration of normally off and normally on vertical AlGaN/GaN high electron mobility transistors (HEMTs). The normally off device shows the threshold voltage of 1.6 V. The normally on device shows the normalized on resistance of 1.48 mOmega-cm2 and the maximum drain current density of 3.9 kA/cm2. Before then, the dependence of threshold voltage on the thickness of n-GaN in the structure of AlGaN/n-GaN/p-GaN was studied experimentally.

Interface Properties of Al2O3/n-GaN Structures with Inductively Coupled Plasma Etching of GaN Surfaces

The effects of the Cl2-based inductively coupled plasma (ICP) etching of GaN on the interface properties of Al2O3/GaN structures prepared by atomic layer deposition (ALD) were investigated. We used n-GaN layers grown on freestanding n+-GaN substrates with low dislocation density. The ICP etching caused slight disorder of the chemical bonds at the GaN surface and monolayer-level interface roughness at the Al2O3/GaN interface, resulting in poor capacitance–voltage (C–V) characteristics due to high-density interface states including nitrogen-vacancy (VN) related levels. The postannealing process in N2 at 400 °C drastically improved the C–V characteristics, probably owing to the partial recovery of the VN-related defects and the increased ordering of chemical bonds in the GaN surface region.

Growth and characteristics of low dislocation density GaN grown on Si(111) from a single process

From one uninterrupted growth process, GaN films were deposited on maskless stripe-patterned Si(111) substrates using the facet-initiated growth technique. The epilayer with a flat surface has a thickness of ∼1.3μm. The influence of stress on the behavior of dislocations in the crystal during growth was observed by the transmission electron microscopy (TEM). Concentrated lines of dislocations were found along the coalescence boundaries by atomic force microscopy (AFM). Few dislocations were detected in the other area. The average threading dislocation density of the GaN layer was decreased to ∼1.7×108cm−2. These dislocations have pure or partial screw dislocation characteristics.

Deposition of aluminum oxide layer on GaN using diethyl aluminum ethoxide as a precursor

We performed a feasibility study on the deposition of aluminum oxides (AlOx) on GaN using diethyl aluminum ethoxide [(C2H5)2AlOC2H5] as a precursor by electron-cyclotron-resonance-assisted chemical vapor deposition. We determined the refractive index and permittivity of the deposited AlOx layer to be 1.59 and 8.3, respectively. An x-ray photoelectron spectroscopy (XPS) study showed that the energy positions of the Al- and O-core levels were very close to those of the reference crystalline Al2O3 and that there was no significant peak related to carbon in the AlOx film. XPS also indicated a bandgap of 7.0 eV and oxygen composition of 1.48 for the AlOx layer deposited on GaN. We found good capacitance-voltage (C-V) behavior for the Ni/AlOx/n-GaN diode including accumulation and depletion behavior at room temperature. Even at high temperatures, the C-V slope remained unchanged indicating relatively low interface state densities near the midgap.