– What is limiting computer processors to operate beyond the current range of clock frequencies (from 3 to up 5GHz)?
In: Engineering
The limitation on clock speed is caused by a concept known as the “Critical Path” through the CPU. Each of the 100s transistors used to make a calculation, (add, subtract, write to mem, read from mem, etc) need time to potentially change states. To go from a 1 to 0 or a 0 to a 1. The clock speed must be slower than the slowest possible calculation step so that in a worst case all operations can occur and fully complete within 1 cycle.
Modern chip use tons of techniques, one of which is called pipelining, to try to run operations in stages to circumvent this limitation. For example while a math operation is calculated, the values for the next calculation can be loaded into place ready for the next cycle. This creates interesting challenges when the result of that second calculation depends on the second, but that is the price you pay for speed in that case.
In addition as others have mentioned, beyond simplifying the structure for a shorter critical path (part of why Apples new M1 Chips are so much more efficient), you can make the switches flip faster. However this is a thermal issue. A stored 1 value or 0 value changing into the opposite requires current to flow in or out of the transistor, which generates heat which must be removed or the transistor will degrade or even melt. The more you have flipping faster, the more heat you get.
Lastly, you can shorten the critical path physically but making it shorter but designing the CPU die so that components that talk to each other are close by or making the transistors themselves smaller through this cant be done in all cases. We have been building CPUs with components so small that the actual speed of electric voltage moving though wires is starting to become relevant.
For context, an Intel i9 lists a 5.3 GHz clock speed. In one clock cycle, light – the fastest thing in the universe – travels only 5.66 centimeters and electric voltage (signals) moves much slower than that in metal, some where slightly slower than speed of light depending on other factors
Edit: speed of light
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