O.T.: Basic Transistor Engineering Question


I am a chemist and really not at all any kind of engineer.

In regard to flash memory MOSFET configurations, why is the NOR (parallel or random access) setup designed to use hot electron (holes, too?) injection to program while the NAND version (serial, block access) exclusively uses Fowler-Nordheim injection?

The latter appears more efficient, seems to use less power, and spares the transistor of excess aging.

I would really like to know why the hot carrier injection design is still used.


It is a complex battle of both technology trade-offs and vested interests by semiconductor manufacturers.
See this article for appliations in mobile hansets:

Quoting from the end of the article: "Right now, NAND still lags NOR in read speed, while NOR is considerably behind NAND in density. Until one technology can bridge the advantage gap offered by the other, there will continue to be cases, such as in the NEC FOMA 900iL handset, where NAND and NOR can, and will, co-exist."

INTEL and ST are continuing to push their NOR technologies and now are competing better on density by using 4 levels in a cell to store 2 bits per cell.




But why even use the hot electron injection design in NOR cells?


Some NOR flash parts do use Fowler-Nordheim tunneling for both program and erase (vs. the majority of NOR flash that uses hot electron injection for program and Fowler-Nordheim tunneling for erase). I'm not sufficiently conversant with the issues to know why most NOR flash parts still use hot electron injection.

Edited: 5 Nov 2007, 2:58 p.m.

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