Meet the Cell

Before looking at the Flash Cell (or Transistor), let’s take a look at a MOSFET (Metal Oxide Field Effect Transistor). A MOSFET has three terminals: Source, Gate and Drain. When a positive charge is applied to the gate, the electrons from the p-type substrate get pulled towards it and form a thin channel below it. Think of this channel as a bridge that allows the electrons from the source to flow to the drain (MOSFET is on and stores a 1). When the positive voltage applied to the gate is taken away, the flow of electrons from source to drain stops (MOSFET is off and stores a 0). As computers store information in 0s and 1s, a MOSFET can only store 1s as long as external power is applied to its gate. Therefore, all the 1s will be turned into 0 as soon as external power is taken away, thereby causing loss of data.

Due to this reason, MOSFETs need to be modified before they can be used for storing data without the use of external power. Enter Flash Cell, which has an extra gate called the Floating Gate.

To program the cell, a high voltage is applied to the control gate. This voltage attracts the electrons and causes them to tunnel through the oxide layer and move into the Floating Gate. This changes the value of the cell to 0. If you remove the voltage applied to the gate, the electrons will stay in the floating gate and the cell will retain its value of 0. To erase the cell, apply the voltage across the channel with reversed polarity as before. The electrons are removed from the floating gate thereby changing the value of the cell to 1. Thus this cell can store two states, 1 or 0.

SLC vs MLC

There are two kinds of flash technologies in use today: Single-Level Cell (SLC) flash and Multi-Level Cell (MLC) flash. The key difference between the two is the amount of data one cell can store. SLC flash is capable of storing 1 bit per cell, unlike MLC flash which can store 2 bits per cell. Below is an image of a SLC chip and a MLC chip. If you look at them, they are indistinguishable and look the same from the outside. So, for effectively the same die area, MLC technology provides you with double the storage capacity.

Image Courtesy of SuperTalent MLC vs SLC whitepaper

SLC transistor has two states, 0 and 1. MLC flash transistor can have four states 00, 01, 10, 11. A 0 or 1 is determined by the threshold voltage V_t. This voltage is calculated by the amount of charge on the floating gate of the flash cell. For example when the threshold voltage is high enough, around 4.0 V, the cell will be read as programmed or 1. No charge or threshold voltage < 4.0 V will cause the cell to be read as erased. Now in MLC flash, you can have different threshold voltage levels and have them be read as different states, thereby being able to store more data. An example of different states in SLC and MLC flash is shown below

To better understand the difference between SLC and MLC flash, think of a cell as a glass of water. The SLC glass can have two levels, empty (erased or 1) and full (programmed or 0). MLC glass can have four levels: empty (Fully erased or 11), 1/3 full (Partially erased or 10), 2/3 full (Partially programmed or 01), and full (Fully programmed or 00).

As MLC has four levels, the read time of MLC flash is higher when compared to SLC. This is because the reading application has to take some extra time to determine the exact reading of the cell. Another advantage that SLC flash has over the MLC flash is in the endurance department. Usually the endurance of SLC flash is 10x more than MLC flash. Therefore SLC flash is used in solid state disks made for servers where reliability is of prime importance.

Now that we know how flash memory works at the low level, let’s look at how data is read, written and erased.