Kingston SSD Now V Series 128 GB Review

SSD Tech Refresher Course

A flash cell is the most basic entity involved in making the flash memory for the SSD. But 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.

Few of these flash cells can are grouped together to form a page which is 4 KB in size in most SSDs these days. A page is the smallest entity that is readable/writable in a SSD. Next, group 128 pages together to get a block which is the smallest entity that can be erased. Moving on with the grouping, 1024 of these blocks form a plane. Then these planes are grouped together to form a flash die.

The image below will help explain the concept of grouping further.

For a more detailed analysis of the SSD tech please read here, here and here.