Fatigue phenomena of FTMIS memory transistors

Two operation modes of long endurance and their fatigue properties are described for a nonvolatile charge storage memory device which employs a floating silicon gate tunnel injection MIS (FTMIS) structure. The device is composed of an n-channel metal gate field effect transistor with a floating gate over tunnelable (20-35 Å) SiO2. The floating gate consists of highly resistive polycrystalline Si grains. Gate oxidation isolates each poly-Si grain, resulting in a structure of islands. This improves retention characteristics. The primary feature of these devices is that no fatigue phenomena are observed for 2 × 10¹²cycles continuous write-erase operation in the conventional operation mode. In addition, it is possible both to write and erase in the other operation mode with only positive pulses to the gate electrode. Furthermore, stored data is retained more than one year without any external power supply. Therefore the device is an excellent candidate for nonvolatile RAM applications as a semiconductor memory device.