Impact of the charge transport in the conduction band on the retention of Si-nitride based memories

An improved model for charge injection through ONO gate stacks, that comprises carrier transport in the conduction band of the silicon nitride (Si₃N₄), is used to investigate the program/retention sequence of Si₃N₄ based (SONOS/TANOS) non volatile memories without making assumptions on the initial distribution of the trapped charge at the beginning of retention. We show that carrier transport in the Si₃N₄ layer impacts the spatial charge distribution and consequently several other aspects of the retention transient. The interpretation of the Arrehnius plots of the high temperature retention data, typically used to infer the trap depth from the retention activation energy is discussed. The model provides a simple explanation of the small threshold voltage increase observed during retention experiments of thick tunnel oxide ONO stacks.