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electronic device in which electricity flows through solid semiconductor crystals (silicon, gallium arsenide, germanium) rather than through vacuum tubes. The first solid-state device was the "cat's whisker" (1906), in which a fine wire was moved across a solid crystal to detect a radio signal. Transistors, made of one or more semiconductors, are at the heart of modern solid-state devices; in the case of integrated circuits, millions of transistors can be involved. See also electronics.