Electronics Engineering · CHAPTER 01 · FOCUSED LESSON
Selenium photoconductivity reveals how light can become an electrical signal.
Willoughby Smith needed a stable high-resistance material for testing undersea cables. Selenium behaved inconsistently until light was recognized as the hidden variable.
Smith supervised the manufacture and laying of many submarine telegraph cables for the Gutta Percha Company. He wanted a semiconducting test element with high resistance that could remain connected while cable work continued.
Selenium rods appeared suitable in the laboratory, but their electrical behaviour changed during use. Instead of discarding the material as unreliable, Smith investigated the conditions around it.
In 1873 Smith connected the variation to exposure to light. Selenium is photoconductive: incident light changes the number of mobile charge carriers and therefore changes resistance and current. In the commonly used form, stronger illumination lowers resistance and increases conductivity.
This created a direct transducer from light intensity to an electrical quantity. Brightness could now modulate a current and become a signal.
Selenium cells became central to early proposals for transmitting pictures and to later photoelectric devices. Selenium also remains important in modern thin-film CIGS photovoltaics, where the acronym refers to copper, indium, gallium and selenium.
The model shows resistance falling and current rising as illumination increases.
THE IDEA TO REMEMBERThe inconsistent result was data. Once light was identified as the hidden variable, a cable-testing problem became a foundation of electronic imaging.
MATERIAL
SeleniumINPUT
Light intensityOUTPUT
Change in resistance and currentCOURSE SOURCE · “ELECTRONICS AND TECHNOLOGY” · ALI CHOURBA