Cathode

A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery.

A battery or galvanic cell in use has a cathode that is the positive terminal since that is where conventional current flows out of the device.

It is continued externally by electrons moving into the battery which constitutes positive current flowing outwards.

In a diode, the cathode is the negative terminal at the pointed end of the arrow symbol, where current flows out of the device.

The word was coined in 1834 from the Greek κάθοδος (kathodos), 'descent' or 'way down', by William Whewell, who had been consulted[2] by Michael Faraday over some new names needed to complete a paper on the recently discovered process of electrolysis.

Previously, as related in the first reference cited above, Faraday had used the more straightforward term "exode" (the doorway where the current exits).

Therefore, "exode" would have become inappropriate, whereas "cathode" meaning 'West electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by the magnetic reference.

In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the cathode's function any more, but more importantly because, as we now know, the Earth's magnetic field direction on which the "cathode" term is based is subject to reversals whereas the current direction convention on which the "exode" term was based has no reason to change in the future.

[5][9] The filament is a thin wire of a refractory metal like tungsten heated red-hot by an electric current passing through it.

These are often called photocathodes and are used in phototubes used in scientific instruments and image intensifier tubes used in night vision goggles.

[citation needed] When P and N-doped layers are created adjacent to each other, diffusion ensures that electrons flow from high to low density areas: That is, from the N to the P side.

Similarly, holes diffusing into the N-doped layer become minority carriers and tend to recombine with electrons.

In equilibrium, with no applied bias, thermally assisted diffusion of electrons and holes in opposite directions across the depletion layer ensure a zero net current with electrons flowing from cathode to anode and recombining, and holes flowing from anode to cathode across the junction or depletion layer and recombining.

[citation needed] Like a typical diode, there is a fixed anode and cathode in a Zener diode, but it will conduct current in the reverse direction (electrons flow from anode to cathode) if its breakdown voltage or "Zener voltage" is exceeded.