SRAM vs. SDRAM

  • Thread starter Thread starter flekso
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flekso

If SRAM consists of 4-6 times transistors as SDRAM, how come it costs
gazillion times as much ?
Shouldn't it be viable to produce a 128MB stick for the same amount of money
it takes to produce 512MB DRAM ?
 
If SRAM consists of 4-6 times transistors as SDRAM, how come it costs
gazillion times as much ?
Shouldn't it be viable to produce a 128MB stick for the same amount of
money it takes to produce 512MB DRAM ?
Click to expand...

Economics - ie supply and demand, economies of scale and all that stuff.

Cheers
Anton
 
flekso said:
If SRAM consists of 4-6 times transistors as SDRAM, how come it costs
gazillion times as much ?
Shouldn't it be viable to produce a 128MB stick for the same amount of money
it takes to produce 512MB DRAM ?
Click to expand...

The size of a chip and it's cost is not a linear relationship. Let's
say a wafer has 20 defects. If you can fit 100 chips on the wafer,
you'll yield 80 good chip. Now, if your smaller chips are 1/6 the
size, you could fit maybe 700 of them on the wafer (think about the
edges), yielding 680 good chips, 8.5 (not 6) times as many. Pardon my
gross guesstimates, but the point is valid.
 
The size of a chip and it's cost is not a linear relationship. Let's
say a wafer has 20 defects. If you can fit 100 chips on the wafer,
you'll yield 80 good chip. Now, if your smaller chips are 1/6 the
size, you could fit maybe 700 of them on the wafer (think about the
edges), yielding 680 good chips, 8.5 (not 6) times as many. Pardon my
gross guesstimates, but the point is valid.
Click to expand...

Surely SRAM has some redundancy to such defects? eg extra transistors.
 
Dave said:
Surely SRAM has some redundancy to such defects? eg extra transistors.
Click to expand...

Not generally. An individual chip can't generally route around bad
transistors. With something as big and as expensive as CPUs, sometimes you
can route around a a whole region of a chip (ie, as is rumored to be the way
they pick some chips to be celerons -- if there's a defect in part of the
cache region.)... but I doubt that would be economical for a cheap bulk part
like RAM.
 
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