[Dwolb]

It has to do with small amounts of leakage current on the device level. If you just use 1 type of transistor (p or n -type) then when you change from on/off or off/on you will lose more current than if you use both types in your circuit. Sum this across billions of transistors and the equation starts to make sense.

[williadc]

Noise rejection is another important advantage of CMOS. I don't know enough about these materials to know whether it applies here or not.

[RollAHardSix]

Can you explain what the importance of this leakage is? Electricity costs 'power-wise' it makes sense but are there other reasons to want to avoid transistor leakage?

Disclaimer: Not a hardware guy but am utterly fascinated, ooh's and ahh's may spontaneously result.

[Dwolb]

I'll speak to this a bit...

Transistor logical state, typically logical 1 or logical 0, is represented physically by positive and negative charges, respectively. Therefore, we can change the device logical output of 1 or 0 by changing the charge output as positive or negative.

However, to change the charge of a piece of metal from positive to negative (or neutral to negative... it doesn't matter because we're worried about relative change), you need to physically move electrons (charge) on or off the metal. This movement of charge is real 'work' if you're unable to fully recover the charge when the device changes state. This results in spent energy.

Leakage current is important to consider mainly for power and heat transfer. As we require devices to increase computing performance and decrease power consumption, it is important to use techniques that economically reduce power. As power consumption of a chip increases, the spent energy of a toggling transistor becomes heat that needs to be removed in order to ensure proper functionality (in the worst case, the chip burns itself up). Therefore, proper thermal simulation and validation must be made at the system level (depending on what type of designer you are).