Vacuum tubes temporarily put an end to crystal detector research. Bose first patented a crystal detector in 1901. Around 1898, he invented a crystal detector . It usually employs a sodium iodide crystal detector. The crystal detector was the earliest type of semiconductor diode. In early receivers, a type of crystal detector often used was a "cat whisker detector". Some sets used a crystal detector (semiconductor diode) instead. The crystal detector was widely used prior to vacuum tubes becoming available. The detector can be a crystal detector or a Schottky barrier diode. In the 1930s, researchers miniaturized and improved the crystal detector for use at microwave frequencies. The crystal detector was the most successful of many detector devices invented during this era. A crystal detector (Type DC-1) and telephone circuit were connected to the secondary circuit.