What does it says the transconductance efficiency (gm/Id) in an amplifier?

I understand the concept of the transconductance but it's not clear what does the transconductance efficiency (gm/Id) mean. Since gm is the variation of Id to the Vgs or vin voltage... What is the variation of Id to Vgs by Id???

Answer

I'm a little unclear what you're actually asking. If you're asking why we care about transconductance efficiency, that I think I can answer.

If you're asked to design an amplifier, and you need a specific transconductance gm, the transconductance efficiency tells you the drain current you need to achieve that transconductance. That determines the current consumption and power dissipation of the amplifier.

Normally, the lower current consumption and less power dissipation, the happier your customer will be...