Showing papers by "Takahiro Makino published in 2017"

Impacts of gate bias and its variation on gamma-ray irradiation resistance of SiC MOSFETs

Abstract: Gamma-ray irradiation into vertical type n-channel hexagonal (4H)-silicon carbide (SiC) metal-oxide-semiconductor field effect transistors (MOSFETs) was performed under various gate biases. The threshold voltage for the MOSFETs irradiated with a constant positive gate bias showed a large negative shift, and the shift slightly recovered above 100 kGy. For MOSFETs with non- and a negative constant biases, no significant change in threshold voltage, Vth, was observed up to 400 kGy. By changing the gate bias from positive bias to either negative or non-bias, the Vth significantly recovered from the large negative voltage shift induced by 50 kGy irradiation with positive gate bias after only 10 kGy irradiation with either negative or zero bias. It indicates that the positive charges generated in the gate oxide near the oxide–SiC interface due to irradiation were removed or recombined instantly by the irradiation under zero or negative biases.

High Gamma Ray Tolerance for 4H-SiC Bipolar Circuits

Abstract: A high gamma radiation hardness of 4H-SiC circuits is performed. The OR NOR circuits are based on emitter coupled logic (ECL), using integrated bipolar NPN transistors. Gain degradation in individual bipolar junction transistors (BJT) is minimal up to a dose of 38 Mrad (SiO2), but for the dose of 332 Mrad (SiO2) a degradation of 52% is observed. The SiC BJTs show higher radiation hardness than existing Si-technology and high stability under temperature stress. It is proposed that the oxide charge-dominated recombination is the key base current recombination mechanism contributing to gain degradation. An improvement in the gain is seen after annealing at 400 °C for 1800 s due to the possible annealing of some of the oxide defects contributing to the oxide charge.

Optimum structures for gamma‐ray radiation resistant SiC‐MOSFETs

Abstract: 【はじめに】東京電力福島第一原発の廃炉作業のためには、高放射線環境下でも長寿命なロボットが必要となっ ている。その実現には高い耐放射線性を有するパワーデバイスが不可欠であり、我々は次世代パワーエレトロニ クス用材料である炭化ケイ素( SiC )に着目し、その金属-酸化膜-半導体 電界効果トランジスタ( MOSFET )の耐 放射線性強化技術を開発している。これまで、放射線照射による SiC MOSFET の劣化の主原因は酸化膜中に発生 する正の固定電荷であることを見出しているが[1,2]、Si MOS デバイスにおいては、正の固定電荷発生量は酸化膜 厚に依存し、ゲート酸化膜が薄いほど高い耐放射線性を有することが報告されている[3]。そこで本研究では、よ り耐放射線性の高い SiC MOSFET の構造条件に関する知見を得るために、ゲート酸化膜厚の異なる SiC MOSFET にガンマ線照射を行い特性の変化を調べた。 【実験方法】実験には耐圧 1.2 kV、定格電流 20 A、オン抵抗 100 mΩ( Vg=20 V )のサンケン電気製の 4H-SiC DMOSFET を用いた。ゲート酸化膜厚がガンマ線照射効果へ及ぼす影響を調べるため、酸化膜厚 35 nm と 60 nm の試料を用意した。これらに窒素雰囲気中、室温にて Co ガンマ線(線量率 1~10 kGy/h)を 6845 kGy(SiO2)まで照射 し、Vd=10 V のときの Id-Vg特性からしきい値電圧 ( Vth )を算出した。 【結果】図 1 に、膜厚の異なる二つのサンプルの Vthの吸収線量依存を示す。両者とも 0.5 kGy で若干の減少を見 せた後、横ばいとなり、400 kGy 以上で大きくマイナス電 圧方向へのシフトを示した。しかし、酸化膜厚 60 nm の MOSFET の方が高線量での低下が大きく、より少ない線量 でノーマリーオンへと至った。これより、SiC MOSFET に おいても酸化膜厚は放射線による特性劣化に影響を及ぼし、 酸化膜が薄い方が高い耐性を持つことが判明した。本発表 では、より耐放射線性に優れた構造・作製プロセスの提案 を行うためゲート酸化膜の窒化処理とガンマ線耐性の関係 についても報告を行う。 【参考文献】 [1] T. Yokoseki et al., Mater.Sci. Forum 821-823, 705 (2015). [2] T. Ohshima et al., Jpn. J. Appl. Phys. 55, 01AD01 (2015). [3] James R. Schwank, IEEE Nuclear and Space Radiation Effects Conference (2002), SectionIII. 【謝辞】本研究は原子力基礎基盤イニシアチブによる助成を受けて行われた。 図 1 酸化膜厚の異なるサンプルの Vthの線量依存 ●及び▲は、それぞれ、ゲート酸化膜厚 35 nm 及び 60 nm の MOSFET の結果 -6 -4 -2 0 2 4

Abnormal increase in soft-error sensitivity of back-biased thin-BOX SOI SRAMs

Abstract: Silicon-on-insulator (SOI) SRAMs supported by a thin buried-oxide (BOX) film have been exposed to wide-range high-energy heavy ions for simulating terrestrial and galactic radiation impacts Experimental results have demonstrated that a back-bias approach leads to a 100-times increase in their soft-error sensitivity compared to the counterpart zero-bias situation This is attributed to that back biasing may enhance radiation-induced potential fluctuation under BOX, which may spread and cause multi-cell errors in the top SOI circuits via the capacitance coupling principle

Radiation hardness of Ga 2 O 3 MOSFETs against gamma-ray irradiation

Abstract: Gallium oxide (Ga 2 O 3 ) is attractive for power devices owing to its wide bandgap of 4.5 eV and the availability of economical device-quality native substrates. Research on Ga 2 O 3 Schottky barrier diodes and field-effect transistors (FETs) has seen rapid recent progress [1]. An unexplored area of immense interest is the radiation tolerance of these devices, whose high-voltage and high-temperature capabilities are expected to find applications in extreme radiation environments such as space and nuclear facilities that impose stringent reliability requirements to ensure stable operations. This paper reports the first investigation into the effects of ionizing radiation on Ga 2 O 3 metal-oxide-semiconductor FETs (MOSFETs). A gamma-ray (y-ray) tolerance as high as 230 kGy(SiO 2 ) was demonstrated for the bulk Ga2O3 channel by virtue of the MOSFETs' stable on-current, on-resistance (R on ), and threshold voltage (V t ) against irradiation. Hysteresis in the transfer characteristics remained negligible after exposure to the highest dose. Radiation-induced degradations in the gate insulation and surface passivation, which could be attributed to dielectric damage and interface trap generation, were found to limit the overall radiation resistance of these devices.

Self-aligned 4H-SiC nMOSFETs for harsh environment electronics

Abstract: Low-parasitic-capacitance 4H-SiC nMOSFETs using a novel self-aligned process were suggested and demonstrated. In these nMOSFETs, device characteristics including parasitic capacitances (gate-source, gate-drain, drain-source capacitance) were investigated and low parasitic capacitance was achieved by the self-aligned structure.

Effects of SiH Groups on ELDRS Quantified by XPS with Combined Use of Gamma-ray and Electron-beam Irradiation

Abstract: There have been no studies to date estimating the quantitative relation between the increase of interface traps and SiH groups in silicon dioxide films for the effects of total ionizing dose including enhanced low-dose-rate sensitivity (ELDRS). In this study, SiH groups densities in silicon dioxide films are measured and compared with interface trap densities developed at different dose rates to evaluate the impacts of direct proton release mechanism on ELDRS by X-ray photoelectron spectroscopy (XPS) analysis with the combined use of gamma-ray and electron-beam irradiation. The measurement results in this study clearly show that ELDRS depends linearly on SiH groups densities. The linear relationship demonstrates ELDRS is caused by the direct proton release mechanism. The order of magnitude of ELDRS we observed is almost the same as the experimental results reported previously. We emphasize that our results provide the first experimental evidence that the direct proton release mechanism is a critical mechanism affecting ELDRS in addition to the space charge model and H 2 cracking.