An electronic device with two electrodes. In the obsolescent thermionic diode a heated cathode emits electrons, which flow across the intervening vacuum to the anode when a positive potential is applied to it. The device permits flow of current in one direction only as a negative potential applied to the anode repels the electrons. This property of diodes was made use of in the first thermionic radios, in which the diode was used to demodulate the transmitted signal (see modulation). In the semiconductor diode, a p−n junction performs a similar function. The forward current increases with increasing potential difference whereas the reverse current is very small indeed. See semiconductor; transistor.