Quantum well devices for microwave and millimeter wave oscillator applications

The analysis, design, fabrication, and testing of a QWITT (quantum well injection transit time) diode oscillator and self-oscillating mixer are presented.; Small signal and large signal models for the QWITT device that relate physical device parameters to the dc terminal characteristics and predicted rf performance were developed. These models were then used to provide optimum device dimensions to maximize rf performance at a desired frequency of operation. The large signal device model was also used to design a planar QWITT diode oscillator.; Initially, GaAs/AlAs resonant tunneling diode structures were grown by MBE and devices with good room temperature negative differential resistance (NDR) characteristics were fabricated. A number of QWITT devices were then fabricated and planar and waveguide oscillator circuits were designed to test these devices at frequencies between 1-35 GHz. The rf performance of QWITT devices was shown to be significantly better than resonant tunneling diodes. An output power of 1 mW, corresponding to an output power density of 3.5-5 kW/cm{dollar}sp2{dollar} in the frequency range of 5-8 GHz has been obtained from a planar QWITT oscillator. This is the highest output power obtained from any quantum well oscillator at any frequency and is approximately 5 times higher power and 2-3 higher output power density than reported in the literature for a comparable frequency. This result also represents the first planar circuit implementation of a quantum well oscillator. By comparison, the cw output power density obtained from an IMPATT diode at these frequencies is 10-30 kW/cm{dollar}sp2{dollar}. Good qualitative agreement between the dc and rf characteristics of QWITT devices and theoretical predictions based on small signal and large signal analyses has been achieved. We also present results on improving dc-to-rf conversion efficiency by optimizing the design of the drift region in the device through the use of a doping spike. By optimizing the doping concentration of the spike, an increase in efficiency from 3% to 5% has been obtained, without compromising the output power at X-band.; We have demonstrated that self-oscillating QWITT diode mixers have the ability to produce conversion gain in both waveguide and planar circuits. A maximum conversion gain of 10 dB was obtained over a narrow band width of 10-20 MHz at X-band frequencies. If broad band operation (around 100-200 MHz) is desired, then this may be achieved with an average conversion loss of about 3-5 dB. To the best of our knowledge, this is the first report of conversion gain obtained from a self-oscillating mixer using any quantum well device and is also the highest conversion gain reported for a self-oscillating mixer circuit using any semiconductor device.