# Selenium

| What is (and was) Se good for? An interesting story can be told here.
|  | Se was good for making rectifiers for 250 V DC, always needed for running the vacuum tubes in radios and early TVs. The rectifying junction consisted of Se - CdSe, and it worked without anybody knowing why.
|  | Many tablets as shown below were switched in series (with a central screw) as shown in the top picture or, in somewhat later developments, contained in a flat housing
Se - CdSe rectifier tablet | Selenium based rectifiers for 250 V. Bottom: Famous Siemens&Halske bridge rectifier © Jan Wüsten, used with permission.
| This is just one example of semiconductor technology that existed before semiconductor theory was "invented"
|  | Other examples are the Cu-Oxide rectifier ("Kupferoxydul") and, most important, the early "crystal detector" radio.
| Working with Se rectifiers had one side effect that is practically unknown with today's semiconductors. Whenever the contraption blew up (which happened every now and then) it emitted the most unpleasant smell imaginable because some Se compounds where produced (HSe is the stinkiest gas known to humankind, and all other Se gases are similar in that respect).
|  | That helped to diagnose the cause of malfunctioning radios and gave rise to a word play only possible in German: Selengleichrichter = Selen-gleich-riecht-er.

With frame

2.1.1 Basic Properties

© H. Föll (Semiconductor Technology - Script)