Simply put-every device that has the ability to control electron flow-electrically-is an active device. Controlling it electrically would require a control current/control voltage. All transistors and vacuum tubes fall into the category of active devices.
And making use of the controlled electron motion is what electronics is all about. All active components have Gain & Directionality, And since there’s no free lunch-they’re all powered by some external energy source.
On the other hand-stuff like R,C & L’s are passive-incapable of controlling electron flow electrically-no gain-IF you’re under the impression that electronics is all about active devices-Passive devices include the DIODE-which is also the most important semiconductor device.
Random Insight-Passive components have much better drift characteristics than active components-hence, it would be great if somehow the amplifier operation/gain depended on passive components rather than active components-this was the basic idea that built the Op-Amp which has its gain depending only on the passive components in the feedback network.
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A follow up from our facebook page-The diode is-in general-a passive device-but the tunnel diode has its own story-the tunnel diode amplifier uses the tunnel diode as an active device-but we then turn to the two views of passivity-thermodynamic & incremental-it may then, only be referred to as an incremental active device(the fact that it amplifies). It is still thermodynamically passive(passive in terms of heat/power dissipation & controlling electricity electrically).