Diode
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A diode is a two-terminal electronic component that conducts electric current primarily in one direction. It has low resistance in one direction and high resistance in the other.
Close-up view of a silicon diode. The anode is on the right side; the cathode is on the left side (where it is marked with a black band). The square silicon crystal can be seen between the two leads.
A high power vacuum tube diode used in radio equipment as a rectifier
Various semiconductor diodes. Left: A four-diode bridge rectifier. Next to it is a 1N4148 signal diode. On the far right is a Zener diode. In most diodes, a white or black painted band identifies the cathode into which electrons will flow when the diode is conducting. Electron flow is the reverse of conventional current flow.
A vacuum tube containing two power diodes
Tunnel diode
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A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo E
1N3716 tunnel diode (with 0.1" jumper for scale)
10 mA germanium tunnel diode mounted in test fixture of Tektronix 571 curve tracer
8–12 GHz tunnel diode amplifier, circa 1970
I vs. V curve of 10 mA germanium tunnel diode, taken on a Tektronix model 571 curve tracer.