High-speed alterable, nonvolatile MIS memory

A new type of Floating Si-Gate Tunnel Injection MIS (FTMIS) memory devide that switches within the nanosecond range was fabricated. The write/erase time of this device is found to be almost two orders of magnitude shorter than MAOS or MNOS devices. This device consists of a silicon substrate, oxide (20-40 Å), highly resistive polycrystalline silicon film (200-1000 Å) as a floating gate, gate oxide (800-1500Å), and gate electrode. Most conventional tunnel injection memory devices utilize traps distributed near the insulator-insulator interface as the site of charge storage. This limits the switching speed. In our structure, the floating gate overcomes this weak point and the transition time constant depends only on the thin tunnelable oxide thickness. The FTMIS device features high-speed write and erase (<20ns), rectangular hysteresis loop, large difference between on/off levels (Δ Vth
\lsim 50V
) and semipermanent retention (similar to MNOS). The gate oxide is formed by thermal oxidation of the upper part of the floating gate in an island structure. Thus leakage through pinholes has little influence on memory retention.