From metal surface emission of electron can be obtained by following methods.

$(1)$Thermionic emission : When metals are heated (current is passed through filament of metal) electrons are given sufficient heat (thermal) energy, electrons escape from the metal such emission is called thermionic emission.

Example : From diode, triode or TV tube electrons are emitted by using thermionic emission.

$(2) $Field emission : When metal surface are applied electric field of $10^{8} \frac{\mathrm{V}}{\mathrm{m}}$, electron are emitted from it. Such emission is called field emission.

$(3) $Photoelectric emission : When on a clean metal surface incident radiation of proper frequency, electron are emitted from it. Such emission is called photoelectric emission.

Electrons emitted are called photoelectron.