Switching and storage characteristics of MIS memory transistors

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Transistors with memory hawe been constructed in the form of MIS field-effect transistors in which the gate insulator consists of a double layer. Closest to the silicon is a silicon dioxide layer, no more than 15A thick. The importance of this layer will be discussed. It is covered by another layer, which may be silicon nitride, 200-800A thick. Aluminum oxide and silicon dioxide have also been tried as the second layer. At the interface between these two insulator layers, and at an energy level inside the silicon forbidden band, are traps with a density as high as 2×1014cm-2in the form of disorder states. These traps are donor type and may each give off an electron when the silicon is biased positively for a short time with respect to the insulator, turning the transistor ON. When the polarity is reversed the electrons are recaptured by the traps, neutralizing them and turning the transistor OFF. The charge transport is by tunneling.