Diodes are widely used electronic devices that act as one-way switches for current. A well-known example is the LED (light-emitting diode), but there is a special class of diodes designed to make use of the phenomenon known as "quantum tunneling". Called resonant-tunneling diodes (RTDs), they are among the fastest semiconductor devices and are used in countless practical applications, such as high-frequency oscillators in the terahertz band, wave emitters, wave detectors, and logic gates, to take only a few examples. RTDs are also sensitive to light and can be used as photodetectors or optically active elements in optoelectronic circuits.