1. ULSI Device Development Laboratories, NEC Corporation, 1120 Shimokuzawa, Sagamihara, Kanagawa 229, Japan
2. Microelectronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305, Japan
3. NEC Scientific Information System Development, Ltd., 34 Miyukigaoka, Tsukuba, Ibaraki 305, Japan