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Just a simple question,

In https://en.wikipedia.org/wiki/Random-access_memory says that each bit of data in RAM/DRAM stored in a single pair of transistor and capacitor in memory cell.

So let's says the RAM has 4GB memory that mean it can store about 4billionx8bit = 32billion bit of data.

So that i want to ask is, is that true that there are at least 32 billion capacitor in a single 4GB DRAM ? and is that possible to put 32 billion capacitor that can fit on a single board of RAM size? or am I just wrong?

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    \$\begingroup\$ What makes you think you can't fit 32 billion capacitors yet it is ok to have 64 billion transistors? \$\endgroup\$
    – Trevor_G
    Oct 31 '17 at 16:31
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    \$\begingroup\$ Yeah, just a little curious thing cross in my mind, iam a web developer, i dont really know that things before... :) \$\endgroup\$ Oct 31 '17 at 16:45
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    \$\begingroup\$ A capacitor is just two wires/plates separated by an insulator. It's much easier to fabricate a small capacitor compared to a small transistor. \$\endgroup\$
    – horta
    Oct 31 '17 at 17:27
  • \$\begingroup\$ @horta yeah i know it. so it mean there are 'at least' 32 billion transistor in a 4GB RAM too, am i right? i know it more complicated to produce transistor because it has logic operator inside it. Also I believe the amount of transistor are much more than the capacitor in a single RAM, but who's bigger or take more space? 1 capacitor or 1 transistor? \$\endgroup\$ Oct 31 '17 at 17:45
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    \$\begingroup\$ And to blow your mind, your 4GB memory in addition to 32 billion capacitors has 32 billion transistors. \$\endgroup\$ Oct 31 '17 at 18:31
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Yes, they really have that many capacitors in that small of an area.

There are two dominant technologies to do this: stacked capacitor DRAMs and trench capacitor DRAMs.

Stacked capacitors basically use a number of layers of metal and insulator to build a capacitor of reasonable capacity in a small surface area.

Trench capacitor DRAMs basically etch a "trench" (a deep, V-shaped one) in the silicon, the deposit a layer of metal, another of insulator, and another of metal.

Either way, you end up with a relatively large capacitance for the surface area. The capacitance is still quite small by most normal standards though. For example as of 2017, a Samsung DRAM has a capacitance around 7.4 fF per cell.

To get meaningful results from such a small capacitance, most DRAMs actually have some extra capacitors in addition to those used for the storage.

To read a cell, you charge one of these spare cells (one that's physically close to the cell you want to read) with approximately half the charge you'd use to store a 1 in a normal memory cell. One easy way to do that is to use two capacitor cells together, so feeding the same voltage and duration of charge pulse into them results in half the charge in the capacitor.

Then you read back the values from the spare cell and the memory cell and feed them both into a differential amplifier (the "sense amp"). This helps cancel most common mode noise on the bit lines, so the signal coming out of the sense amp is a fairly clean low or high value, with substantially better noise immunity (and from it, improved reliability) compared to just reading the voltage from the capacitor by itself.

In addition, a typical DRAM will have some extra banks of memory. When the chip is being tested at the factory, they may find that one of the normal banks of memory has a defect. If so, the chip will typically include some fuses (or anti-fuses) that can be blown to substitute a spare bank for the defective one, so a chip can still usually meet spec, despite a defect or two.

Thus, a DRAM chip will typically have even more capacitors than you get from computing its size based on what it's rated to hold (though, in all honesty, the increase is fairly small, at least as a percentage--though with something like a 32 GB memory, even a small percentage works out to an absolute number that's fairly large).

As a final note: a DRAM chip has to have a fair amount of circuitry (decoders, sense amps, etc.) in addition to the DRAM cells themselves. As a really simple rule of thumb, figure that the actual cells occupy about half the chip area, and the associated circuitry the other half.

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  • \$\begingroup\$ Nice answer, but it would be really nice to mention the ballpark value of these capacitors, in nano-pico-femto Farades. \$\endgroup\$ Oct 31 '17 at 17:53
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    \$\begingroup\$ According to the article provided by Tek, each memory capacitor is about 10-12 fF (0.010 - 0.012 pF) in 21-nm technology. \$\endgroup\$ Oct 31 '17 at 19:29
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Yes, that's right.

The capacitors are VERY small!

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It's worth a mention that one of the key variables that dictate how much capacitance a capacitor has is the inverse of the distance between the "plates".

enter image description here

That is, if you half the distance between the plates, you double the capacitance.

When you scale things down to the chip level that distance gets incredibly small compared to your typical discrete capacitor.

As such it is possible to create workable capacitors in the nM sizes that fit in their billions on a silicon chip.

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