There's something I don't get - I am always under the impression that the more transistors we pack in, the more energy it consumes and the hotter it gets (assuming we haven't shrunk the die). However, the Apple A8 has around 3 billion transistors while the Intel Haswell has half the transistor count, yet the A8 is able to run cool and takes much less energy to compute.

Can anyone explain the difference?

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    \$\begingroup\$ It's not the number of transistors, but how many of them are switching. \$\endgroup\$ – Olin Lathrop Jan 19 '15 at 23:46
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    \$\begingroup\$ 22nm vs 20nm as well. Smaller gates = less power. If their core voltages are different, that would matter as well. \$\endgroup\$ – Aaron D. Marasco Jan 20 '15 at 1:21

It is the clock frequency, i.e. frequency of the switching, and the number of transistors actually being switched at the same time that consume the power, and consequently, generate the heat, and not the number of transistors.

For example there could be a large amount of transistors in a area of the die that aren't being used, and so, if they are in the off (closed) state, then they will not draw any current, so less energy and less heat.

However, if some active area is switching at a higher clock frequency, then (on average) they are open for more of the time, and so draw more current, thus energy and heat.

Hope that helps.

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    \$\begingroup\$ It's not "open" or "closed" that really matters, but rather charging/discharging inevitable capacitance. Changing state matters far more than what the state is. \$\endgroup\$ – Chris Stratton Jan 20 '15 at 1:30
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    \$\begingroup\$ Well, leakage currents can be important nowadays, depends on the technology. Parts of chips are shut down to reduce power. \$\endgroup\$ – TEMLIB Jan 20 '15 at 1:42

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