The amount of power that a chip dissipates per unit area is called its power density, and there are two types of power density that concern processor architects: dynamic power density and static power density.

Dynamic Power Density

Each transistor on a chip dissipates a small amount of power when it is switched, and transistors that are switched rapidly dissipate more power than transistors that are switched slowly. The total amount of power dissipated per unit area due to switching of a chip’s transistors is called dynamic power density. There are two factors that work together to cause an increase in dynamic power density: clockspeed and transistor density.

Increasing a processor’s clockspeed involves switching its transistors more rapidly, and as I just mentioned, transistors that are switched more rapidly dissipate more power. Therefore, as a processor’s clockspeed rises, so does its dynamic power density, because each of those rapidly switching transistors contributes more to the device’s total power dissipation. You can also increase a chip’s dynamic power density by cramming more transistors into the same amount of surface area.
In addition to clockspeed-related increases in dynamic power density, chip designers must also contend with the fact that even transistors that aren’t switching will still leak current during idle periods, much like how a faucet that is shut off can still leak water if the water pressure behind it is high enough. This leakage current causes an idle transistor to constantly dissipate a trace amount of power. The amount of power dissipated per unit area due to leakage current is called static power density.