page_title: Charge trap flash
section_title: MirrorBit operation – getting 2 bits onto the cell
hierarchical_section_title: Charge trap flash / MirrorBit Flash memory / MirrorBit operation – getting 2 bits onto the cell
caption_attribution_description: English: Cross section of a charge trapping memory cell being erased using hot hole injection
context_page_description: Charge trap flash is a semiconductor memory technology used in creating non-volatile NOR and NAND flash memory. It is a type of floating-gate MOSFET memory technology, but differs from the conventional floating-gate technology in that it uses a silicon nitride film to store electrons rather than the doped polycrystalline silicon typical of a floating-gate structure. This approach allows memory manufacturers to reduce manufacturing costs five ways:
  Fewer process steps are required to form a charge storage node
  Smaller process geometries can be used
  Multiple bits can be stored on a single flash memory cell.
  Improved reliability
  Higher yield since the charge trap is less susceptible to point defects in the tunnel oxide layer
  While the charge trapping concept was known earlier, it wasn't until 2002 that AMD and Fujitsu produced high-volume charge-trapping flash memory. They began the commercial production of charge-trapping flash memory with the introduction of the GL NOR flash memory family. The same business, now operating under the Spansion name, has produced charge trapping devices in high volume since that time.
context_section_description: During CHE programming (Figure 2) the hot electrons are injected from the channel into the charge trapping layer toward the biased drain end of the channel, but not from the floating source end of the channel.  By allowing the transistor's source and drain to switch from one end of the channel to the other, charges can be injected and stored into the charge trapping layer over either end of the channel.
  In a similar way, one end of the charge trapping cell can be erased by placing the erasing field at one end or the other of the channel, allowing the other end to float as shown in Figure 3.  Band-to-band Hot Hole Erase creates holes that are trapped locally some of which recombine with electrons to remove the charge from that end of the charge trap.
caption: Figure 3. Erasing removes electrons from the charge trap through hot hole injection
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