Showing papers by "Jun Suda published in 2015"

Ultrahigh-Voltage SiC p-i-n Diodes With Improved Forward Characteristics

Abstract: Silicon carbide (SiC) p-i-n diodes having five different n−-layer ( $i$ -layer) thicknesses from 48 to $268~\mu $ m are fabricated. The forward characteristics of SiC p-i-n diodes are significantly improved by carrier-lifetime enhancement. After this improvement, the differential on-resistance is inversely proportional to the square root of current density for all the diodes with different thicknesses of n−-layer. As a result, the forward current density–voltage characteristics can be approximately expressed by a parabolic function, as in the case of Si p-i-n diodes. Using a 268- $\mu $ m-thick n−-layer, the lifetime enhancement, and an improved space-modulated junction termination extension structure, a very high blocking voltage over 26.9 kV and low differential on-resistance of 9.7 m $\Omega \cdot $ cm $^{2}$ are achieved.

Impact Ionization Coefficients in 4H-SiC Toward Ultrahigh-Voltage Power Devices

Abstract: A temperature dependence of impact ionization coefficients in 4H-SiC was studied in a wide range of electric field toward the accurate designing of ultrahigh-voltage devices. The photomultiplication measurement was conducted for various photodiodes with different multiplication layer structures to obtain multiplication factors and ionization coefficients in a wide range of electric field strength. Especially, using multiplication layer structure with low doping concentration, the hole impact ionization coefficient was extracted at low electric field of 1 MV/cm. In high-temperature measurement, the hole ionization coefficient decreased with the increase of temperature, as observed in other semiconductor materials. For the electron ionization coefficient, however, its temperature dependence was very small and values obtained at room temperature could be used, at least up to 150 °C.

Interface Properties of 4H-SiC ( $11\bar {2}0$ ) and ( $1\bar {1}00$ ) MOS Structures Annealed in NO

Abstract: Interface properties of 4H-SiC (11 $\bar {2}$ 0) and (1 $\bar {1}$ 00) metal–oxide–semiconductor (MOS) structures annealed in nitric oxide are characterized by conductance, high–low, and C – $\psi _{\mathbf {\textit {s}}}$ methods. Compared with 4H-SiC (0001) MOS structures, generation of very fast interface states by nitridation is much smaller in 4H-SiC ( $11\bar {2}0$ ) and ( $1\bar {1}00$ ). The effective mobility of planar MOSFETs fabricated on Al+-implanted p -body doped to $1\times 10^{\mathrm {\mathbf {17}}}$ cm $^{\mathrm {\mathbf {-3}}}$ is 103 cm $^{\mathrm {\mathbf {2}}}$ /Vs on (1 $\bar {1}$ 00), 92 cm $^{\mathrm {\mathbf {2}}}$ /Vs on (11 $\bar {2}$ 0), and 20 cm $^{\mathrm {\mathbf {2}}}$ /Vs on (0001). The mobility-limiting factors are discussed on the basis of experimental results. The high channel mobilities for ( $11\bar {2}0$ ) and ( $1\bar {1}00$ ) MOSFETs can be correlated with the lower density of fast interface states generated by nitridation.

Oxidation-induced majority and minority carrier traps in n- and p-type 4H-SiC

Abstract: We investigated majority and minority carrier traps in lightly doped n- and p-type 4H-SiC epitaxial layers before and after thermal oxidation using deep level transient spectroscopy and minority carrier transient spectroscopy. We detected oxidation-induced new minority carrier traps, HO1 (EV + 0.36 eV) and HO2 (EV + 0.54 eV) for n-type 4H-SiC, and EO1 (EC − 0.59 eV) and EO2 (EC − 0.84 eV) for p-type 4H-SiC after thermal oxidation at 1300 °C. After subsequent Ar annealing at 1550 °C, the HO1, HO2, and EO1 centers disappeared, whereas the concentration of the EO2 center increased. The properties of these carrier traps are discussed.

Temperature dependence of current gain in 4H-SiC bipolar junction transistors

Abstract: The temperature dependence of current gain from 140 to 460 K in 4H-SiC bipolar junction transistors (SiC BJTs) was investigated. The current gain increased from 110 to 1200 with decreasing temperature from 460 to 200 K. The high current gain at the low temperature can be ascribed to the enhanced incomplete ionization of aluminum acceptors in the base layer, resulting in the increase in injection efficiency. However, when the temperature was further reduced from 200 to 140 K, the current gain decreased from 1200 to 515, which is caused by high injection condition in the base layer, because of a very low hole concentration below 200 K.

Temperature dependence of forward characteristics for ultrahigh-voltage SiC p–i–n diodes with a long carrier lifetime

Abstract: Forward characteristics of ultrahigh-voltage 4H-SiC p–i–n diodes having four different n−-layer (i-layer) thicknesses from 48 to 198 µm were investigated in the temperature range from room temperature to 573 K. After enhancement of carrier lifetimes in i-layers, nearly ideal forward characteristics (differential on-resistance = 1.1–5.5 mΩ cm2 at 100 A/cm2) were obtained at room temperature. The forward voltage drop decreased with temperature, which is consistent with the temperature dependence of junction voltage. The differential on-resistance exhibited a slight increase at elevated temperatures, which can mainly be ascribed to the increase in substrate resistance.

Impacts of surface roughness scattering on hole mobility in germanium nanowires

Abstract: The hole mobility in rectangular cross-sectional germanium nanowires was calculated taking into account phonon and surface roughness scattering (SRS). SRS was modeled based on atomistic description of electronic states, and the impacts of SRS on hole mobility were analyzed.

Influence of Conduction-Type on Thermal Oxidation Rate in SiC(0001) with Various Doping Densities

Abstract: It was discovered that the oxidation rate for SiC depended on the conduction type. The oxidation was performed for SiC(0001) with nitrogen doping (n-type) in the range from 2×1016 cm-3 to 1×1019 cm-3, and aluminum doping (p-type) in the range from 2×1015 cm-3 to 1×1019 cm-3, exhibiting a clear dependence. For n-type SiC the oxide thickness increases for higher doping density, and for p-type the thickness decreases. Note that in the case of Si oxidation, there exists very little difference of oxidation rate between the conduction types in such low doping density, and the dependence is peculiar to SiC.

MOS Structures Annealed in NO

Abstract: Interface properties of 4H-SiC (11¯ 20) and (1¯ metal-oxide-semiconductor (MOS) structures annealed in nitric oxide are characterized by conductance, high-low, and C-ψs methods. Compared with 4H-SiC (0001) MOS structures, generation of very fast interface states by nitridation is much smaller in 4H-SiC (11¯ 20) and (1¯ 100). The effective mobility of planar MOSFETs fabricated on Al + -implanted p-body doped to 1×10 17 cm �3 is 103 cm 2 /Vs on (1¯ 100), 92 cm 2 /Vs on (11¯ 20), and 20 cm 2 /Vs on (0001). The mobility-limiting factors are discussed on the basis of experimental results. The high channel mobilities for (11¯ and (1¯ 100) MOSFETs can be correlated with the lower density of fast interface states generated by nitridation.

Impacts of orientation and cross-sectional shape on hole mobility of Si nanowire MOSFETs

Abstract: We fabricated 〈100〉, 〈110〉, 〈111〉, and 〈112〉 p-channel gate-all-around Si nanowire (SiNW) MOSFETs, cross sections of which are rectangles with various widths, and investigated the hole mobility of the SiNW MOSFETs using the double L m method. Measured hole mobilities of SiNW MOSFETs were about 80–140 cm2/Vs at surface carrier density of 1 × 1013 cm−2. The dependences of the hole mobility on orientations and cross-sectional shapes of nanowires were analyzed. The orientation and geometry dependences can be explained by the band structures calculated by tight-binding approximation.

High-Temperature Operation of Electrostatically-Excited Single-Crystalline 4H-SiC Microcantilever Resonators

Abstract: We fabricated electrostatically-excited single-crystalline 4H-SiC microcantilever resonators with various thicknesses and lengths. Their resonant characteristics were investigated from room temperature (RT) up to 600°C. The resonant frequency of the cantilevers decreased with increasing temperature. From the results, the temperature dependence of Young’s modulus of single-crystalline 4H-SiC was obtained, i.e., 3% decrement with increasing temperature from RT to 600°C. The cantilevers with different thicknesses showed different temperature dependences of the quality factor. A 2-μm-thick cantilever exhibited a high quality factor (Q) (250,000) at RT and the Q decreased to 6,000 at 600°C, which can be explained by thermoelastic damping. On the other hand, a Q of a 0.45-μm-thick cantilever was still high (50,000) even at 600°C.