Transponders do indeed have a thermonic valve, just like an old cathode ray tube. There's like a box of electrons in there that bounce around but don't start bouncing around until the source of electrons is hot, and just like old TV's, transponders take time to warm up before they respond to SSR Interrogations. Turn the transponder to standby when taxi-ing, then it's fully warmed up when you switch it to 'On/Alt' during your pre-takeoff immediate actions. Turning it 'On/Alt' straight from 'Off' won't damage it, it will just take 30secs to 1min to respond to SSR Interrogations (not good if you're on a radar assisted departure). Turning it on whilst on the ground will inevitably clutter radar screens. If you're not using your transponder whilst airborne, you'd better leave on 'STBY' rather than 'Off' in case you need to suddenly switch it on and dial in 7700.