Vacuum Diode

a diode that is a two-electrode vacuum tube. Used primarily as kenotrons, vacuum diodes are characterized by the absence of a reverse current and can withstand higher inverse voltages than can gas-filled and semiconductor diodes.

Vacuum diodes may be classified as low-voltage low-power diodes, whose inverse voltages may be no greater than 2 kilovolts (kV) and whose rectified currents are as high as 0.4 ampere (A); high-voltage low-power diodes, whose inverse voltages may be no higher than 30 kV and whose rectified currents may reach 0.002 A; high-voltage pulse-forming diodes, with inverse voltages that do not exceed 60 kV and rectified currents of up to 100 A; or high-voltage X-ray diodes, with inverse voltages no greater than 220 kV and rectified currents of up to 2 A.

With the growth of semiconductor electronics, vacuum diodes are being supplanted by semiconductor diodes, which are more efficient.