Temperature dependences of channel mobility and threshold voltage in 4H- and 6H-SiC MOSFETs
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Temperature dependences of channel mobility and threshold voltage in 4H- and 6H-SiC MOSFETs S. Harada1, 3, R. Kosugi1, 3, J. Senzaki1, 3, S. Suzuki1, 2, W. J. Cho1, 3, K. Fukuda1, 3, and K. Arai1, 3 1 Ultra-low-loss power device technology research body, 2R&D association for future electron devices, 3Electrotechnical laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki, Japan ABSTRACT We have investigated the effect of polytype and oxidation condition on the temperature dependence of channel mobility and threshold voltage in 4H- and 6H-SiC MOSFETs. The behaviors of the channel mobility are apparently different for 4H- and 6H-SiC MOSFETs. In contrast to the polytype effect, dry and wet oxidation samples have almost similar channel mobilities. The variation of the threshold voltage with temperature is proportional to the number of the interface states near the conduction band extracted from n-type MOS capacitors. Therefore, we argue that the distribution of the interface states near the conduction band in p-type SiC MOS structure can be represented by that in n-type SiC MOS structure. Although the oxidation condition varies the distribution of the interface states in the energy range between 0.2 and 0.4 eV from the conduction band, it has little influence on the channel mobility.
INTRODUCTION 4H-SiC metal-oxide-semiconductor field effect transistor (MOSFET) is a promising candidate for high power electronic devices due to excellent physical properties of 4H-SiC. However, some problems still exist to achieve the high quality 4H-SiC MOSFETs. The most important problem is the low channel mobility of electrons in the surface inversion layer. N-channel 4H-SiC MOSFETs with a thermally grown gate oxide have a channel mobility less than 10 cm2/Vs, which is much lower than that in 6H-SiC (1, 2). It is proposed that higher density of interface trap (Dit) near the conduction band (Ec) causes the low channel mobility in 4H-SiC MOSFETs (2-6). Many researchers tried to reduce the interface states near Ec by modifying the oxidation conditions until now. However, the results obtained are not encouraging so far. In this study, we focus on the unique characteristics of the channel mobility and the threshold voltage varying drastically with temperature in 4H-SiC MOSFET. Their thermal behavior will provide the detailed information about the influence of the interface states. We investigated the effect of polytype and oxidation condition on the temperature dependence of channel mobility and threshold voltage. The interface states on the channel mobility is also discussed. EXPERIMENTAL Substrates used for MOSFETs fabrication were p-type 4H- and 6H-SiC (0001) wafers purchased from Cree Research Inc. Doping concentration (NA-ND) of the epitaxial layers was about 5x1015 cm-3. The designed geometries (length/width) of the channel region were 10/50 µm and 200/200 µm. Source and drain regions were formed by phosphorous ion implantation at 500 oC with a total dose of 7x1015 cm-2, followed by annealing at 1500 oC for 5 min in Ar ambient. Surface cleanin
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