NAND cells are the fundamental component of flash memory. The term «NAND» refers to the logical structure underlying data storage in these cells, specifically NAND logic (NOT-AND). NAND cells operate using a transistor channel (source-drain) and two gates: a control gate and a floating gate. The floating gate is isolated from the control gate and transistor channel by an oxide layer. When electrons are pushed through this oxide layer into the floating gate using storage voltage (via the tunneling effect), they remain trapped there permanently – even without sustained voltage.
NAND Flash Read Operation
To read the memory cell, a read voltage is applied to the transistor, and the current flowing between the source and drain is measured. If the floating gate is charged, meaning there are many electrons in the floating gate, the state is read as zero. This is because no current flows between the source and drain
Electron Release via Erase Voltage
Electrons are released again using the erase voltage. If only a few electrons remain in the floating gate, the state "one" is read because current flows between the source and drain.
All NAND cells wear out over time, and the oxide layer degrades. The thicker the oxide layer, the slower this process progresses. Accordingly, NAND memory with larger shrinks lasts longer than those with smaller shrinks. The period during which stored information remains error-free is called retention. NAND cells with a thick oxide layer achieve higher retention.
SLC (Single-Level Cell) memory recognizes only two charge states: almost no electrons or a large number of electrons in the floating gate. In contrast, MLC (Multi-Level Cell) memory stores four different charge states per cell using different voltage levels, corresponding to two bits per cell.
Worn-out SLC NAND can be read significantly longer than worn-out MLC NAND. With only two charge states, assignment remains easy, even if the charge level is no longer as distinct as in a new cell. In contrast, with four charge states (MLC NAND), assignment is much more difficult. Even minor wear is enough to make charge states unrecognizable. As a result, SLC memory allows a much higher number of write and read cycles per flash cell compared to MLC memory. Additionally, SLC memory is much faster than MLC memory due to the clear voltage differences between charge states.
There is an option to use MLC NAND while storing only one bit per cell. This approach is known as Fast Page Mode, with some manufacturers referring to it as MLC+ or Turbo Mode. The advantage of Fast Page Mode, as the name suggests, lies primarily in speed. Read and write speeds are increased at the expense of storage capacity. Any MLC NAND memory can be operated in Fast Page Mode without requiring firmware adjustments.
However, the downside is that the endurance (lifespan) of individual flash cells (NAND) is only slightly higher than that of conventionally used MLC memory. While Fast Page Mode only stores two charge states, the voltage level difference remains just as small as with four charge states. Consequently, the disadvantages of MLC NAND come into play: susceptibility to errors due to difficult voltage level assignment, limited write and read cycles, and thus limited lifespan.
In pSLC (Pseudo Single-Level Cell) technology, sometimes referred to as "SLC Light," MLC NAND is also written with only one bit per cell. However, at the same time, the voltage difference between the two charge states is increased. Due to these clear voltage differences, the benefits of SLC memory become apparent. Charge states are easier to assign compared to Fast Page Mode or conventionally used MLC NAND, allowing more write and read cycles. At the same time, the significant voltage differences reduce susceptibility to data errors. As a result, pSLC memory provides significantly better data security and longevity than conventional MLC memory or MLC memory in Fast Page Mode.
To increase the voltage differences between charge states, the memory manufacturer must adjust the firmware. Unlike Fast Page Mode, pSLC technology requires specialized MLC NAND that supports pSLC operation.
With pSLC technology, the lifespan of MLC NAND is significantly increased. However, only half of the physical storage capacity remains available in pSLC mode. A 32GB pSLC memory module is physically a 64GB MLC module. Still, due to the increased voltage differences in charge states, the lifespan is not just doubled but increased by a factor of six. For many applications, pSLC memory is a worthwhile investment.
However, in terms of endurance, pSLC cannot match traditional SLC memory. pSLC is based on MLC NAND technology, which inherently limits its durability. As a result, true SLC NAND supports five times as many write and read cycles as pSLC NAND. SLC technology remains the undisputed leader in terms of endurance and is best suited for industrial applications.
When high storage capacities are required or when write and erase cycles are limited, industrial companies opt for pSLC or MLC memory. However, for small to medium storage capacities, investing in genuine SLC industrial memory pays off. If flash storage is exposed to extreme thermal conditions, investing in SLC memory is beneficial, as specialized SLC NAND exists for an extended temperature range of –40 to +90 degrees Celsius. While there are also MLC flash products for extended temperature ranges, they are screened only in the final production step before shipment and are therefore less reliable under extreme temperatures.
Regardless of whether SLC, pSLC, or MLC memory is used, choosing a manufacturer with industrial experience is worthwhile. Significant differences in reliability and longevity exist among manufacturers of SLC, pSLC, and MLC memory. One of the few manufacturers specializing exclusively in industrial NAND storage is Cactus Technologies. The Taiwanese company exclusively uses high-quality A-grade NAND from Toshiba and has a sophisticated, extensively tested firmware. As a result, Cactus Technologies' memory solutions are among the most reliable on the market, offering product lines based on SLC, pSLC, and MLC technology.
Cactus Technologies Production of NAND Flash Storage
