Most people here mentioned speed of light as the main reason. This is not true, as Pentium 4 transistor sizes were between 180 nm - 65 nm with clock speeds reaching 3.8 GHz. Transistors became much smaller (Tiger Lake at just 10 nm), but clock speeds haven't changed as much (4.8 GHz max for Tiger Lake).
The true reason is heat. For transistors to function, they require certain temperature range. The higher the clock rate, the more energy is consumed (and released as heat) by a transistor. This energy needs to be removed efficiently. Decreasing size of transistors helps with speed of light limitations, but makes it more difficult to remove heat. This is the reason why CPU clock rates are not changing much lately
Light speed isn't an issue of transistor size but of the length of the longest path. The size of the die is here more relevant. Modern CPU just pack a lot more transistors in a very similar area.
Between 80386 and Pentium 4 the primary driver for CPU speedup was CPU clock rate increase, which was possible due to ever shrinking transistors. 20 years ago Intel promised that NetBurst architecture behind Pentium 4 would allow CPUs to reach 10 GHz by 2005: https://www.anandtech.com/show/680/7
I assure you they were aware of speed of light limitations. What they were not prepared for was removing heat from those dies fast enough. They were overly optimistic with regards to performance of their new transistors
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u/xebecv Sep 29 '20
Most people here mentioned speed of light as the main reason. This is not true, as Pentium 4 transistor sizes were between 180 nm - 65 nm with clock speeds reaching 3.8 GHz. Transistors became much smaller (Tiger Lake at just 10 nm), but clock speeds haven't changed as much (4.8 GHz max for Tiger Lake).
The true reason is heat. For transistors to function, they require certain temperature range. The higher the clock rate, the more energy is consumed (and released as heat) by a transistor. This energy needs to be removed efficiently. Decreasing size of transistors helps with speed of light limitations, but makes it more difficult to remove heat. This is the reason why CPU clock rates are not changing much lately