propagation delay (idea)

This is the time that an incoming signal takes to propagate from the input to the output of a logic gate. It´s a main factor of the speed of devices, such as the CPU of your computer. The reason for this delay usually lies, like almost always in electrical engineering, in the time it takes to charge/discharge a capacitor. Consider the following CMOS inverter:

         VDD
          |
          |
        |-
     +--|
     |  |-
     |    |
A ---+    +---- B
     |    |
     |  |-
     +--|
        |-
          |
          |
         VSS

To propagate a change in the input A to the output B your have to open one transistor and to close the other one. This means that the gate capacitors have to be charged/discharged. There are also other, so-called parasitic capacitances, like from interconnects, that have to be considered. Due to the minuscule size of today´s logic structures this process won´t take long, but cannot be neglected.

Thís charge/discharge mechanism is also the main reason why a logic circuit needs power to operate.

What design decisions influence the propagation delay ?

• Device complexity The more stages a device has, the longer takes the signal to propagate through them. Important: This does not necessarily have an impact on throughput. When you´ve got some sort of pipeline structure, you can have more than one data set processed at the time.
• Gate capacity The smaller, less time is taken. That means that in the future, with structures becoming smaller and smaller, devices will become faster. This also helps keeping power consumption low, as there is less charge needed. On the opposite side, parasitic capacitances become more important.
• Structure size The smaller structures become, the higher their resistance, limiting the charge/discharge current.
• Transistor design Today´s transistors have a much lower on resistance, this allows higher currents and lower propagation delays.
• Heat radiation Charging means current, current means heat. The faster you load, the higher the current. In addition, faster circuits can have more charge/discharge cycles per given time. This means that faster circuits radiate more heat. You remember the times of those 486/25 machines that didn´t even need a fan? Now think of today´s quazillion GHz computers... (Yeah, they´re even back to water cooling like in the good old Cray times)