Why must cathode materials have a low electronic work function?

Cathode materials in electron tubes must have a low electronic work function because this value represents the amount of energy required to release electrons from the material's surface.12

In thermionic emission, which is the primary mechanism for electron release in vacuum tubes, electrons must acquire enough kinetic energy through heat to overcome the forces holding them within the metal. A lower work function means that electrons can be emitted more easily and efficiently at lower operating temperatures.13

If a material has a high work function, such as tungsten, it requires significantly higher temperatures to achieve a useful emission current, which is less efficient and can impact the longevity of the device. By using materials like barium, strontium, or thorium, engineers can ensure a sufficient flow of electrons while maintaining practical operating conditions.31