I have once written about SRAM and DRAM, and today I would like to more about them.
SRAM consists of a simplest flip-flop, e.g. serial connection of two inverters (NOT gates), and two transistors for read and write.
Below is a schematic picture of a single cell of SRAM with 1-bit information (0 or 1).
Since each inverter uses two transistors, SRAM is composed of 6 transistors.
Since flip-flop realizes its constant output of 1 or 0 while a current is running in loop, SRAM can keep an information as long as the power for the circuit is on. It is said there would be no significant leakage of current for so long as 10 years.
For SRAM one only have to arrange 6 transistors.
And since it is composed of only transistors, it has an affinity to other CMOS’ logic process (all technologies for making transistors).
In consequence, SRAM processing is friendly for beginners.
DRAM uses a capacitor to keep an information. The output is 1 when it is charged and 0 when not. By the way, capacitors is in Japan more likely called condensers. Of course, I will write here as capacitors.
Below is a schematic picture of a single cell of DRAM with 1-bit information.
The structure is much simpler and the size per a cell much smaller than SRAM, so DRAM can provide a great amount of capacity at lower cost. Indeed, DRAM is only one tenth in size of SRAM.
On the other hand, the early development of capacitors in DRAM is much elaborate. Once the method for it was confirmed, a mass production with lower cost is expected.
The major problem of using DRAM is leakage of charge from the capacitor. Currently the excellent capacitor would lose its charge in 1 second. So a process of “refreshment” is required while using. Usually a refresh controller is equipped by DRAM and recharge its capacitors, for example, once in 1/20 second.
Main memory modules of PC are definitely DRAM. While DRAM costs less than 10,000 yen for 2GB capacity, SRAM for it would cost 100,000 -1,000,000 yen. It is thanks to DRAM, that PCs of today are cheap.
Although prices of SRAM and DRAM are both dramatically being reduced, SRAM would cost 10 times as DRAM forever, so long as the structural difference of SRAM-6T(transistors) and DRAM-1T+1C(capacitor) remains.