SDRAM elementary cell
Submitted by SDRAM Technology on Thu, 07/16/2009 - 22:42
As shown in Figure 1 a SDRAM elementary cell has two main parts:
- a transistor;
- and a capacitor;
The transistor plays the role of a switch and the capacitor is a place to store electrical charge. In order to store a physical “1” in a SDRAM cell the following processes take place:
- Transistor opens
- The charges/electrons are flowing and fill the capacitor
- The transistor closes
Due to the fact that the transistor (switch) and the capacitor (storage place) are not ideal, the charge which filled the capacitor is leaking away very quickly. Thus, the information written in the cell (“1”) will be lost after a very short time. Due to this fact the information written in the cell should be periodically refreshed. The periodicity of refreshing is 64 ms (milliseconds).
A physical “0” is obtained by opening intentionally the transistor and in this way the electrons will leave the transistor. Interestingly, a cell with a “0” inside should by refreshed as well. In this case there are no electrons in the capacitor, nothing can be lost. However, electrons not only can leave unintentionally the capacitor, they can also enter unintentionally (from our point of view) the capacitor and thus destroying the state “0”.
Figure 1: Schematic SDRAM elementary cell