Microwave and millimeter-wave QWITT diode oscillators

The authors present DC, microwave, and millimeter-wave characteristics of different quantum-well-injection transit-time (QWITT) devices. Small-signal and large-signal device models are used to provide physical design parameters to maximize the output power density at any desired frequency of operation. A peak output power density of 3.5-5 kW/cm² in the frequency range 5-8 GHz has been obtained from a planar QWITT oscillator. This appears to be the highest output power density obtained from any quantum-well oscillator at any frequency. This result also represents the first planar circuit implementation of a quantum-well oscillator. Good qualitative agreement between DC and RF characteristics of QWITT devices and theoretical predictions based on small-signal and large-signal analyses is achieved. The device efficiency has been increased from 3% to 5% by optimizing the design of the drift region in the device through the use of a doping spike with optimized concentration, without compromising the output power at X -band. Self-oscillating QWITT diode mixers are also demonstrated at X-band in both waveguide and planar circuits. The self-oscillating mixer exhibits a conversion gain of about 10 dB in a narrow bandwidth and a conversion loss of about 5 dB if broadband operation is desired