Q.State one basic limitation of a vacuum diode as a rectifier as compared with a semiconductor p-n junction diode.

A vacuum diode depends entirely on thermionic emission, so its cathode must be continuously heated to operate -- this wastes power, takes time to warm up before it conducts properly, and limits the tube's operating life since a heated filament eventually fails. It is also physically bulky and fragile (a sealed glass envelope). A semiconductor p-n junction diode needs no heater at all, conducts (almost) instantly, and is far smaller, more robust and more power-efficient -- which is why it has replaced the vacuum diode in nearly every application.

Contrast the need for continuous heating (and its consequences) with a semiconductor diode's instant, heater-free operation.

• Naming only 'it's older technology' without giving the actual physical reason (the heater requirement).
• Forgetting the vacuum diode's rectifying ACTION itself is similar; the limitation is practical/efficiency, not that it fails to rectify.