A diode is a two-terminal electronic device that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from the heated cathode to the plate. The most common type of diode is the semiconductor diode, which uses a crystal of silicon or germanium as the junction between two pieces of metal.