SLC vs MLC

SLC NAND Flash – Performance First

Single-level cell (SLC) NAND Flash memory is a NAND Flash at its simplest and best.

SLC NAND stores one bit of data per memory cell. SLC NAND offers relatively fast read and write capabilities, good endurance, and relatively simple error correction algorithms. SLC NAND can be more expensive per bit when compared to other NAND technologies since each bit cell stores only one bit of data.

Thus, users have to make a choice between cost and performance. If an application needs speed—like a high-performance media card, some hybrid disk drives, solid state devices (SSDs), or some embedded applications— SLC NAND may be the only choice.

MLC NAND Flash – Density and Value

In contrast to SLC NAND, multilevel cell (MLC) NAND stores two or more bits per memory cell. To determine the state of each bit, a voltage is applied and the resulting current is detected. In an SLC device, only one voltage level is required. If current is detected, then the bit stored is 1; if no current is detected, then the bit is 0.

For an MLC device, three different voltage levels are used to determine the state of both bits.

Below Figure illustrates the reference point voltages that are applied and the resulting bit values for both SLC and MLC devices.

Generally, MLC NAND offers twice the capacity as SLC NAND in the same size device and comes at a significantly lower cost-per-bit. Designers will have to make some trade-offs in terms of performance and reliability (since SLC NAND is about three times as fast as MLC NAND and offers over 10 times the endurance); but for many applications, MLC NAND offers the right combination of price and performance. In fact, MLC NAND represents nearly 80% of all NAND Flash shipments. And MLC NAND is the dominant Flash memory of choice for consumer class SSDs because its performance is superior to magnetic hard disk drives.