This page is in response to a question about calculating the power density of a microwave FET (or HEMT). We'll "integrate" this page into the encyclopedia eventually. This is a great question, a lot of junior and senior people get confused on.
Question:
The unit of power density is W/mm, which length is the power divided by to get power density? Is it power/(channel length) or power/(transistor width)?
Answer:
Use the total transistor "periphery" in the calculation. Example: Four finger device, 80 micron unit gate fingers, 0.5 um gate length, puts out 1 watt.
Power density is 1/(4x80um*0.001mm/um)=3.125W/mm
Note that you could doe the calculation in mW/um and come up with the same answer in the same units:
Power density is 1000/(4x80um)=3.125W/mm
The 0.5um gate length does not enter the equation!