the triode grid-leak detector. Crystal radios were kept as emergency backup radios on ships. During World War II in Nazi-occupied Europe the radio saw use as an easily constructed, easily concealed clandestine radio by Resistance groups. After World War II, the development of modern semiconductor diodes finally made the galena cat whisker detector obsolete. Development of the theory of semiconductor rectification Semiconductor devices like the crystal detector work by quantum mechanical principles; their operation cannot be explained by classical physics. The birth of quantum mechanics in the 1920s was the necessary foundation for the development of semiconductor physics in the 1930s, during which physicists arrived at an understanding of how the crystal detector worked. The German word halbleiter, translated into English as "semiconductor", was first used in 1911 to describe substances whose conductivity fell between conductors and insulators, such as the crystals in crystal detectors. Felix Bloch and Rudolf Peierls around 1930 applied quantum mechanics to create a theory of how electrons move through a crystal. In 1931, Alan Wilson created quantum band theory which explains the electrical conductivity of solids. Werner Heisenberg conceived the idea of a hole, a vacancy in a crystal lattice where an electron should be, which can move about the lattice like a positive particle; both electrons and holes conduct current in semiconductors. A breakthrough came when it was realized that the rectifying action of crystalline semiconductors was not due to the crystal alone but to the presence of impurity atoms in the crystal lattice. In 1930 Bernhard Gudden and Wilson established that electrical conduction in semiconductors was due to trace impurities in the crystal, a "pure" semiconductor did not act as a semiconductor, but as an insulator (at low temperatures). The maddeningly variable activity of different pieces of crystal when used in a detector, and the presence of "active sites" on the surface, was due to natural variations in the concentration of these impurities throughout the crystal. Nobel Laureate Walter Brattain, coinventor of the transistor, noted: The "metallurgical purity" chemicals used by scientists to make synthetic experimental detector crystals had about 1% impurities which were responsible for such inconsistent results. During the 1930s progressively better refining methods were developed, allowing scientists to create ultrapure semiconductor crystals into which they introduced precisely controlled amounts of trace elements (called doping). This for the first time created semiconductor junctions with reliable, repeatable characteristics, allowing scientists to test