Yasunori Tateno

High temperature operation of AlGaN/GaN HEMT

We investigated high temperature operation of AlGaN/GaN HEMTs. At channel temperature of 269 degC, a linear gain of 12.3 dB and a power added efficiency of 53.6% were achieved at 2.14 GHz, less than 50 V operations. These are sufficient performance to practical application. At channel temperature of 368 degC, the linear gain was 10.4 dB and a power added efficiency of 43.9% was achieved. We also investigated the temperature dependence of equivalent circuit values, and found that the temperature dependence of saturated output power and the linear gain is originated from the temperature dependence of electron velocity in the channel.

Epitaxial graphene formation on 3C-SiC/Si thin films

By forming a thin 3C-SiC film on Si substrates and by annealing it at ~1500 K in vacuo, few-layer graphene is formed epitaxially on Si substrates. In this graphene-on-silicon (GOS) technology, graphene grows at least on three major low-index Si surfaces: (1 1 1), (1 0 0) and (1 1 0), which allows tuning of structural and electronic properties of epitaxial graphene by simply controlling the crystallographic orientation of the surface. A typical example can be found in the two types of graphene formed on 3C-SiC(1 1 1) surfaces; the one on 3C-SiC(1 1 1)/Si(1 1 1) shows a Bernal stacking with an interfacial buffer layer, while the one on 3C-SiC(1 1 1)/Si(1 1 0) shows a non-Bernal stacking without an interfacial buffer layer. Inserting an AlN interlayer between Si and 3C-SiC significantly contributes to improvement of the GOS quality. Moreover, thanks to the sealing effect of the AlN layer against Si out-diffusion, we can apply chemomechanical polishing of SiC surface to reduce the surface roughness, which can further accentuate the effect of H2 annealing of the surface. As a result, a D to G band intensity ratio as low as 0.4 is obtained.

High-efficiency and wide-band single-ended 200W GaN HEMT power amplifier for 2.1 GHz W-CDMA base station application

In this paper, we present high drain efficiency of 34% and wide-band characteristics of single-ended 200W GaN HEMT power amplifier for 2.1 GHz W-CDMA application. We also confirmed that GaN HEMT power amplifier has low memory effect and adjacent channel leakage power ratio (A.C.L.R.) is sufficiently reduced for the application by utilizing digital predistortion (D.P.D.) system.

High Quality Graphene Formation on 3C-SiC/4H-AlN/Si Heterostructure

The growth of graphene on 3C-SiC/Si heterostructure is a promising approach, which provides low production cost, high scalability and easiness of nanoelectromechanical system fabrication. However, the quality of graphene is still insufficient for device applications due to mediocre morphological and structural quality of the 3C-SiC epilayers compared to bulk SiC crystals and to excessive Si out-diffusion from the Si substrate. Here, we propose a solution of inserting a 4H-AlN layer between 3C-SiC and Si, which allows us to polish the 3C-SiC film without worrying about enhancement of the Si out-diffusion despite the thinning after the polishing. With this insertion, a considerable quality improvement is achieved in our graphene on silicon.

Novel Instability-Probing Simulation for Power Amplifiers

A novel instability-probing simulation for power amplifiers is presented. Instabilities of power amplifiers with use of very-high performance pHEMT should be observed from a single-gate pHEMT among the multi-gate configuration of the pHEMT, where the power amplifiers are not yet symmetric. A conventional orthogonal (even- and odd-) mode analysis, based on the symmetric power combining, is not complete to predict various instabilities in the power amplifiers. This paper demonstrates a novel instability-probing simulation method that incorporates an ideal transformer. The proposed method successfully predicted microwave and millimeter-wave spurious oscillations observed for a Ku-band MMIC power amplifier

A 1.8-2.3GHz wideband and compact power amplifier module using AlGaN/GaN HEMTs

We have developed a wideband and compact power amplifier module (PA-module) for a driver stage amplifier of a base station transmitter system using AlGaN/GaN high electron mobility transistors (HEMTs), a nonhermetic package, clip leads, and a printed circuit board (PCB). Linear gain of 30dB, gain flatness of less than 0.5dBp-p, and saturation output power of more than 40dBm were achieved from 1.8GHz to 2.3GHz, operating at drain bias voltage (Vds) of 50V and gate bias voltage (Vgs) of -3V. We demonstrated an adjacent channel leakage power ratio (ACLR) of -50dBc at average output power of 19dBm in practical 2-carrier W-CDMA signals, dissipating 13W. This PA-module covered wide frequency band and its package volume was only 2cm/sup 3/. It could be compactly incorporated into various base station transmitter systems without difficulty.

Operation Mechanism of GaN-based Transistors Elucidated by Element-Specific X-ray Nanospectroscopy

With the rapid depletion of communication-frequency resources, mainly due to the explosive spread of information communication devices for the internet of things, GaN-based high-frequency high-power transistors (GaN-HEMTs) have attracted considerable interest as one of the key devices that can operate in the high-frequency millimeter-wave band. However, GaN-HEMT operation is destabilized by current collapse phenomena arising from surface electron trapping (SET), which has not been fully understood thus far. Here, we conduct quantitative mechanistic studies on SET in GaN-HEMTs by applying element- and site-specific photoelectron nanospectroscopy to a GaN-HEMT device under operation. Our study reveals that SET is induced by a large local electric field. Furthermore, surface passivation using a SiN thin film is demonstrated to play a dual role: electric-field weakening and giving rise to chemical interactions that suppress SET. Our findings can contribute to the realization of high-capacity wireless communication systems based on GaN-HEMTs.

Time-Resolved Micro-Beam X-Ray Absorption Fine Structure (XAFS) Measurement to Investigate the Cause of a Current Collapse of GaN-HEMTs

The purpose of our work was to carry out a direct measurement of a GaNs surface condition under a voltage stress, and to identify a cause of a current collapse. Using pulsed S-parameters measurements immediately after the voltage stress was applied for the GaN HEMT, equivalent circuit parameters were extracted, and we estimated the virtual gate length which induced the current collapse. And we carried out time resolved micro-beam X-ray Absorption Fine Structure (XAFS) measurement after the voltage stress was applied. As a result, by the applied voltage stress, an X-ray Absorption Near Edge Structure (XANES) spectrum of gallium, Ga, was found to be changed. This means the surface condition of GaN was changed in the virtual gate region by the voltage stress.

Analyses of Chemical States at SiN x /GaN Interface by HAXPES

We have analyzed the chemical state at the SiNx/GaN interface, which affects collapse phenomenon in GaN-HEMT. In the specimen fabrication, we employed O2and N2-plasma for GaN surface cleaning prior to the SiNx deposition, since the O2-plasma had been confirmed to reduce the collapse compared to the N2-plasma. HAXPES analyses demonstrated that the O2-plasma grew more metallic Ga at the interface than N2-plasma. The metallic Ga is thought to be a good leakage path, through which the trapped electrons can escape from the interface to the electrodes.

Recent Progress on GaN HEMTs

This paper is overviewed the recent progress on GaN HEMTs. High power, high frequency and high efficiency performance are reported. In addition, the results about long-term reliabilities and good manufacturability demonstrate that GaN HEMTs are ready for mass production.