Electrically-programmable semiconductor memories with buried injector

Abstract

Each memory cell of an electrically-programmable semiconductor memory has a field-effect transistor with a charge-storage region. Efficient and fast injection of hot carriers into the charge-storage region is achieved by vertical punch-through of a depletion layer to a buried injector region, by application of programming voltages to a control gate and to the surface of the punch-through region. Non-injected carriers are removed via at least the transistor drain during the programming. A well-defined punch-through region can be obtained with a higher-doped boundary region at at least one side of the punch-through region to restrict the lateral spread of the depletion layer(s) and prevent parasitic connections. This permits closer spacing of the injector region to other regions of the memory cell, e.g. source and drain regions, and the injector region may adjoin an inset insulating field pattern. A compact cell array layout can be formed with a common connection region for the injector regions of two adjacent cells and for either a source or drain region of four other adjacent cells. The control gate and an erase gate may both be coupled in the same manner to the charge-storage region, and the cell can be operated with complementary voltage levels for writing and erasing. A feed-back mechanism with the start of injection from the punch-through and injector regions can provide a well-defined charge level limit for the erasure.