Hello there,
I assume we are talking about a regular diode, and not a special diode type like a tunnel diode.
I was taught that in order to qualify as an active device, the device has to be capable of delivering an infinite amount of energy.
That means voltage and current sources pretty much, and probably anything that can be made from voltage and current
sources.
A diode on the other hand, is a consumer of energy and therefore can not be an active device. Many people share this view also.
Another interesting thing about active devices is that they can have a large range of DIFFERENT current levels for the EXACT
same terminal voltage. For example, 5.000v and 100 amps, or 5.000v and 1 amp, with the very same device (by whatever
means it takes). Passive elements can only approximate this behavior.
My personal view is that anything (or it's model) that can supply energy is active, and anything that consumes energy is passive.
This isnt 100 percent accurate but it seems more like what most people i've talked to over the years think.
In other words, a power supply is active because it can deliver energy.
Another thing to think about is: Is a light dependent resistor (LDR) an active element? See if you can answer this one too.