Page "Semiconductor" Paragraph 4

In 1833, Michael Faraday reported that the resistance of specimens of silver sulfide decreases when they are heated.

This is contrary to the behavior of metallic substances such as copper.

In 1874 Karl Ferdinand Braun observed conduction and rectification in metallic sulphides, and Arthur Schuster found that a copper oxide layer on wires has rectification properties that ceases when the wires are cleaned.

In 1839, A. E. Becquerel reported observation of a voltage between a solid and a liquid electrolyte when struck by light, the photovoltaic effect.

In 1873 Willoughby Smith observed that selenium resistors exhibit decreasing resistance when light falls on them.

Adams and Day observed the photovoltaic effect in selenium in 1876.