Semiconductor devices play an indispensable role in contemporary electronics. In the early days of radio and television, transmitting and receiving equipment relied on vacuum tubes, but these have been almost completely replaced in the last three decades by solid-state devices, including transistors, diodes, integrated circuits, and other semiconductor devices. The only surviving vacuum tubes in radio and TV equipment are the picture tube in most TV receivers, imaging devices in studio TV cameras, and high power transmitting equipment. A thin slab of semiconductor can serve as a photocell. When the material is irradiated with an electromagnetic wave whose photons have at least as much energy as the band gap between the valence and conduction bands, an electron in the valence band can absorb a photon and jump to the conduction band, where it (and the hole it left behind) contribute to the conductivity. The conductivity therefore increases with wave intensity. Detectors for charged particles operate on the same principle. An external circuit applies a voltage across a semiconductor. An energetic charged article passing through the semiconductor collides inelastically with valence electrons, exciting them from the valence to the conduction band and creating pairs of holes and conduction electrons. The conductivity increases momentarily, causing a pulse of current in the external circuit. Solid-state detectors are widely used in nuclear and high-energy physics research; without them, some of the most recent discoveries could not have been made.