A new memory structure using threshold shifting from charge stored in nanocrystals of silicon (≊5nm in size) is described. The devices utilize direct tunneling and storage of electrons in the nanocrystals. The limited size and capacitance of the nanocrystals limit the numbers of stored electrons. Coulomb blockade effects may be important in these structures but are not necessary for their operation. The threshold shifts of 0.2–0.4 V with read and write times less than 100’s of a nanosecond at operating voltages below 2.5 V have been obtained experimentally. The retention times are measured in days and weeks, and the structures have been operated in an excess of 109 cycles without degradation in performance. This nanomemory exhibits characteristics necessary for high density and low power.

Results of a Monte Carlo study of carrier multiplication in silicon bipolar and field-effect transistors and of electron injection into silicon dioxide are presented. Qualitative and, in most cases, quantitative agreement is obtained only by accounting for the correct band structure, all relevant scattering processes (phonons, Coulomb, impact ionization), and the highly nonlocal properties of electron transport in small silicon devices. In addition, it is shown that quantization effects in inversion layers cause a shift of the threshold energy for impact ionization which is very significant for the calculation of the substrate current in field-effect transistors. Conservation of parallel momentum, image-force corrections, dynamic screening of the interparticle Coulomb interaction, and improvements to the WKB approximation are necessary to treat correctly the injection of electrons from silicon into silicon dioxide. The validity of models-analytic or Monte Carlo-which treat hot-electron transport with oversimplified physical approximations is argued against. In a few words, there is no shortcut. 0 I995 Ametican Institute of Physics.

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