Electron gate currents and threshold stability in the n-channel stacked gate MOS tetrode

A new structure is given for the n-channel stacked gate MOS tetrode which consists of a polycrystalline silicon buried control gate and thermally grown oxide for the offset gate insulator. As a result of the large band-bending in the offset gate depletion region of an operating tetrode, some drain current electrons surmount the Si-SiO2energy barrier and are injected into the oxide. Since the electron trapping is relatively small in the thermal-oxide offset gate insulator, it was possible to measure gate currents of up to2 times 10^{-4}A/cm². The gate current was measured as a function of the drain current, the drain voltage and the offset gate voltage. The resulting behavior confirms previous models of the tetrode device. Since electron trapping is much less in thermally grown oxide than in deposited pyrolytic oxide which was used formerly, the offset gate threshold voltage shifts less. As a result of this effect the new structure is used to advantage in fabricating the n-channel stacked gate tetrode in that the drain current is comparatively insensitive to changes in the offset gate voltage.