Because tunneling is inherently a very fast phenomenon that is not transit-time limited, the resonant-tunneling diode is considered among the fastest devices ever made. Furthermore, it does not suffer from minority charge storage. It has been demonstrated that as a mixer it can detect radiation up to 2.5 THz, and as an oscillator it can generate 700 GHz signals. Maximum operational oscillation frequency has been projected to be over I THz. Tunneling, on the other hand, is more difficult to supply high current and the output power of an oscillator is limited. The resonant-tunneling diode has also been used in fast pulse-forming circuits and trigger circuits. Other applications that have been mentioned include the frequency multiplier, harmonic generator and parity generator. The unique feature of multiple current peaks can result in efficient functional devices that can perform more complex functions with a single device where conventional design would take many more components. Examples are multi-value logic and memory. The resonant-tunneling diode also serves as the building block for other three-terminal devices such as the resonant-tunneling bipolar transistor and the resonant-tunneling hot-electron transistor. It has been incorporated in structures to study hot-electron spectroscopy.