Resonant tunnel diodes as submillimetre-wave sources

Resonant-tunnelling diodes (RTDs) are one of the only quantum-transport devices that operate effectively at room temperature, and are well known for their high-speed coherent generation and switching characteristics associated with the negative differential resistance (NDR) region. This paper addresses their capability as high-frequency coherent sources. Two types of sources will be addressed here: (i) quasi-sinusoidal oscillators connected to a resonant circuit, and (ii) relaxation oscillators connected to a high-frequency energy-storage element, such as a length of transmission line. To date, the highest-frequency sinusoidal oscillator was a waveguide-mounted InAs-AlSb RTD that performed up to 712 GHz. Transmission-line relaxation oscillators have been demonstrated only at microwave frequencies but show promise as submillimetre-wave sources. Their output waveform consists of a sequence of pulses having a repetition rate determined by the electrical delay of the transmission line and a pulse width determined by the switching time of the diode. Because they do not require De-bias stability in the NDR region, they are quite amenable to power-combining techniques, such as parallel arrays, and could behave in an analogous manner to the atomic species in a mode-locked laser.