Memory density

Also called: density, bit density, storage density, bits per chip

How many bits a memory technology fits on a chip or in a given area. Smaller cells mean higher density and lower cost per bit.

Density measures how tightly a technology packs bits. It comes down to cell size: the smaller each memory cell, the more fit on a chip, and the cheaper each bit becomes.

Comparing the two kinds of RAM:

SRAM: 6 transistors per bit. Low density, high cost per bit.
DRAM: 1 transistor and 1 capacitor per bit. High density, low cost per bit.

That single fact explains a lot of computer design. Main memory, measured in gigabytes, has to be cheap, so it's DRAM. Caches, measured in kilobytes or megabytes, have to be fast, so they're SRAM. Density and speed pull in opposite directions, and systems use both.

Among non-volatile memories, flash won over EEPROM for the same reason: block erasing allows smaller cells, so flash is much denser.

Worked example

Example

Same area, different capacity

Suppose a DRAM cell takes one quarter of the area of an SRAM cell, and a chip has room for 64 Mi SRAM cells.

1.
In the same area, DRAM fits 4 × 64 Mi = 256 Mi cells.
2.
So the DRAM version stores 4 times as many bits.
3.
That's why gigabytes of main memory are DRAM, while the fast cache is SRAM.

Common mistakes

Thinking SRAM is denser because it's faster. Its 6-transistor cell is larger, so it's less dense.
Equating density with speed. Denser memory is usually slower, not faster.