They saw two forms of uses during WW2, both low-tech uses and high-tech uses. Crystal detectors went out of favor soon after vacuum tubes came to prominence, a

They saw two forms of uses during WW2, both low-tech uses and high-tech uses. Crystal detectors went out of favor soon after vacuum tubes came to prominence, and eventually many researchers almost forgot them entirely due to their outdateness. Later work in microwave electronics made people rediscover their usefulness. Post-war advance in semiconductors made it possible to manufacture small-signal and microwave diodes consistently, finally making them no longer notoriously inconsistent devices. They remain in use for RF/microwave circuit today. Of course, there was also the work by the Indian physicist Jagadish Chandra Bose, an outlier who used crystal detectors to reach millimeter-wave frequency around 60 GHz already in the year 1900 before everyone else. But as far as I can see, his work had limited influence since it was far ahead of his time. In Crystal Fire: The Birth of the Information Age, a definite book on semiconductor history, it described the amazement of this rediscovery: > In the mid-1930s [George] Southworth was trying to detect ultrashort radio waves around a tenth of meter long—what he then dubbed “hyper-frequencies” and are today called microwaves—using specially designed vacuum tubes. But he was having little success. Inherent time lags in the flow of electrons through them were simply too great for the tubes to cope with these extremely short, rapidly oscillating waves. Copper oxide rectifiers didn’t work any better, either. > Frustrated, Southworth decided to try one of the old “cat’s whisker” crystal detectors he had used in radio sets during the Great War, when he served in the Army Signal Corps. These quirky devices had fallen gradually out of favor when vacuum tubes became popular during the 1920s. By the mid-1930s it had become almost impossible to buy one in an ordinary radio store. > So Southworth hopped a train bound for lower Manhattan, where he knew of a secondhand radio market on Cortlandt Alley, near Canal Street. Rummag ing around on the dusty back shelves of one tiny shop, he soon found a few old cat’s-whisker detectors. After bargaining with the shopkeeper, he carried them back to Holmdel, where he dusted them off and carefully inserted one into his receiving apparatus. He began searching around on its surface for a suitable hot spot. Finally, after hunting almost an hour, he found a good one. > And it worked! At last, he could detect his ultrashort-wave, hyper-frequency radiation.