Thermionic emission and vacuum tube devices

Inside any metal, the conduction electrons are free to wander through the lattice but not free to leave it — escaping into the surrounding vacuum takes at least as much energy as the material's work function, exactly the same barrier that governs the photoelectric effect. A photon can supply that energy in one hit; heat supplies it statistically, by raising the electrons' thermal energy distribution until a useful fraction of them have enough to clear the barrier on their own. That only becomes significant at very high temperatures — cathodes in real vacuum tubes typically run above 1000°C, not merely "warm" — and it's electrons alone that leave; the atoms of the lattice stay exactly where they are.

Once loose in the vacuum, those emitted electrons are exactly the raw material a vacuum tube needs. The simplest device, a diode, adds a second electrode (the plate) held at a positive voltage relative to the heated cathode: electrons are pulled across the gap when the plate is positive, but nothing flows the other way if it's negative, since a cold plate emits nothing on its own. That one-way behaviour is what let early diodes rectify AC into DC.

A triode adds a third electrode — a wire grid — between cathode and plate. Because the grid sits much closer to the electron stream than the plate does, even a small voltage on it changes the electric field the electrons feel right where they're leaving the cathode, throttling or opening the flow toward the plate. The grid voltage doesn't add current of its own; it controls how much of the cathode's emission gets through. A small signal on the grid can therefore produce a much larger swing in the plate current — amplification — which is exactly what made the triode the foundational active component of electronics before the transistor replaced it.