Thermionic Emission: What Causes Electrons to be Emitted?

[Cheman]

Thermionic Emission...

Why exactly does thermionic emission happen? ie - Why does heating a metal cause electrons to be emitted?
I know that it must be that the heat is giving the electrons enough energy to escape from their atoms but in what form is this? ie - I know that electrons can be emitted when hit by a photon with the required energy, but what is this "heat" all about"?

Thanks.

 

[FZ+]

Heat is a type of kinetic energy.

 

[Integral]

When the temperature is high enough there will be some electrons on the surface which have kinetic energies high enough to escape the surface of the material. The higher the temperature the more electrons that will escape.