Unsupervised Learning by Spike-Timing-Dependent Plasticity in a Mainstream NOR Flash Memory Array—Part I: Cell Operation

This article and its part II demonstrate the possibility to operate a mainstream NOR Flash memory array as an artificial synaptic array learning without external supervision according to the spike-timing-dependent plasticity (STDP) rule. As a first mandatory step to this aim, suitable array working conditions allowing not only selective cell programming but also selective cell erase are here identified, overcoming the block erase operation typical of any Flash memory technology. The proposed array working conditions exploit channel hot-electron injection and hot-hole injection at the drain side to achieve selective, bidirectional, and virtually analog tuning of cell threshold voltage, with no need for changes in the design of a common-ground double-polysilicon NOR array. In part II, the new working scheme will be shown to allow for a straightforward implementation of STDP and unsupervised learning in the array.