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Comment by fsckboy | Hacker News Reader

>the current in a CPU transistor is in the milliampere range

? you sure about that? in a single transistor? over what time period, more than nanoseconds? milliamps is huge, and there are millions of transistors on a single chip these days, and with voltage drops of ... 3V? .7V? you're talking major power. FETs should be operating on field more than flow, though there is some capacitive charge/discharge.

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Single transistors in modern processes switch currents orders of magnitude lower than milliamps. More like micro- to picoamps. There's leakage to account for too as features get smaller and smaller due to tunneling and other effects, but still in aggregate the current per transistor is tiny.

Also the transistors are working at 1V or lower, but as you say they are FETs and don't have the same Vbe drop as a BJT.

You are right, I mixed this up. If you take a CPU running at 100 W with 10 billion transistors (not quite realistically assumed to all be wired in parallel) at 1 V, you would get an average of 0.01 microamps. So the factor would reduce to roughly 10^5.

Wait a minute, a lot of those transistors are switching the same currents since they are in series. Also, FETs only draw most current while switching, so in between switches there's almost no flow of electrons. So in fact you cannot calculate things that way.

Yes, as I said the parallel assumption is not quite realistic, and the number is an average, covering all states a transistor may be in. So it amounts to a rough lower bound for when a transistor is switching.

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