system
Given the following post, respond as if you're commenting on Reddit:
human
How can a microSD card be so compact?

I have a 32GB microSD card in my phone right now and that fact just amazes me. I did some calculations, and assuming there is 3 bits stored per transistor like in the TLC NAND that is used in the Samsung 840 SSD, that means there is 32 billion x 8/3 = 86 billion transistors in this tiny little piece of silicon.

My question is, how the hell is that possible? An Intel Core processor has upwards of 1 billion FETs and a GPU has like 4 billion, but those take 200+ watts to run. Am I missing something here, or do microSD cards really have 86 billion transistors in them?
gpt
Very interesting question with a complicated answer:

* In a CPU/GPU, there are a lot of transistors that are designed to do calculations, you need different types (PMOSFETs,NMOSFETs), different logic gate structures (NAND, etc), as well as non-transistor parts (capacitors, metal lines, etc), which means that the density (amount of transistors per unit area) is actually very low compared to a memory chip that you can find in microSD cards.Let's back that up with some data: most advanced node in transistor manufacturing for CPUs is 22nm (3D-trigate Ivy Bridge from Intel), the size of the die (where the active silicon is, the rest is just packaging) is 160mm^2, and about 1.5 billion transistors. That's an average of 0.1 um^2 per transistor.

* in a memory chip like in memory cards, we're talking about NAND Flash.They are damn small, as in 64 Gb in 120 mm^2-small. They are actually smaller than Intel's, because to make NAND Flash you don't need fancy logic gates, just an array of cells that are all the same. Plus, as you said, there is multiple bits per cell, that helps so that's about 30 to 10 billion cells (depending if it's 2- or 3- or more bits per cell) for 64Gb. In a 32GB memory card, there would be 4 64Gb chips, so the 80 billion transistor figure doesn't look far off. Each cell in a NAND Flash is not really a transistor identical to the ones used by Intel, but let's forget that, that puts the estimation to about 0.01 um^2 per cell. 

So, in terms of density, flash memory in your compact micro SD card is actually 10 times more dense than the best CPUs around, and there really are 20 times more transistors in a memory card than in a CPU.


Another side of your question is the power consumption. Power consumption in CPUs come from computation: In order to switch on and off the transistors, you need power, lots of it. In memory, you don't switch on and off the memory cells all the time. The card can stay unpowered and still keep the memory inside, that's what's interesting in Flash memory. The only power consumed is when writing or reading. And that power is small: you don't change all the memory at once (you're limited by the bandwith of the writing/reading as well as the interface to the computer). So, overall, the power consumption of chips is not a fixed amount "per-transistor", you assumed that a CPU consuming 100W with 1 billion makes a 80 billion chip consuming 8kW, when in fact the flash chip consumes far less, the "per-transistor" power is much smaller than in CPUs.