Apacer SLC-Lite - more cost efficient than SLC while more reliable than MLC

Flash memory principle

Base cell of flash memory is floating gate MOS transistor (FGMOS).

Erased cell hasn’t charge (electrons) stored in gate and threshold voltage of transistor is Vt1. Charge is transfered into the gate during programming and increases threshold voltage to Vt0. During read operation Vref voltage is connected to gate and current between drain and source of transistor is evaluated. Cell is programmed if current is lower than predefined threshold. This status corresponds to logic level 0 (L). Oposit case is evaluated as logic level 1 (H). Erase operation removes charge from gate and threshold voltage returns in ideal case to value Vt1.

In the practice during programming isn’t stored always the same charge into gate, after erase operation some charge can remains stored in gate also read operation can disturb charge stored in adjacent cels. Threshold voltages difference decreases after each programming/erasing (P/E) cycle until finally reach value when it is not possible to distinguish between programmed and erased state. Because of this, every flash cell has limited number of P/E cycles. As dimmensions of cell decreases number of P/E cycles also decreases. SLC (single-level cell) flash SLC flash cell stores 1 bit. Cell is erased if Vt belongs to area 1, if it is located inside area 0 then cell is programmed.

MLC (multi-level cell) flash MLC flash stores 2 bits. If Vt belongs to area 11 – cell is fully erased, 01 – partially erased, 00 – partially programmed and 10 – fully programmed. Programming circuitry has to store four different charges into the gate. To achieve this precise distribution of charge a more sophisticated and time consuming programming algorithm is required. As a result, the programming time for MLC flash is up to 4X slower than that for SLC flash. Reading is also slower. Threshold voltages difference is also lower than that for SLC flash which drastically decreases number of P/E cycles.

SLC-Lite
SLC-Lite technology is based on MLC flash but firmware is changed in a such way that it only uses states 11 and 10 (maximal threshold voltage difference)

SLC, SLC-lite and MLC flash comparison (based on Toshiba flash chips)

The first samples of Apacer memory cards with capacity 4GB and 8GB we have in our stock. In case of interest please contact us at sales@soselectronic.com.