(i) Thermionic emission : It can be caused by supplying the minimum required energy by heating the metal surface to a suitable temperature.
(ii) Field emission : It can be caused by applying an electric field to the metal surface of the order of `10^(8)Vm^(-1).`
(iii) Photelectric emission : It can be caused by applying the minimum required energy by illuminating the metal surface with light of suitable frequency.