Development of silicon carbide IMPATT oscillators

Impact-Ionization-Avalanche-Transit-Time (IMPATT) diodes have been used for RF power generation at frequencies between 10 GHz to 300 GHz. Under pulsed conditions, the peak output power of an IMPATT diode at a given frequency is limited by its underlying material properties. Because of the high breakdown field and high electron saturation velocity of silicon carbide (SiC), the peak power capability of a SiC IMPATT diode is expected to be 400x and 350x higher than silicon and gallium arsenide IMPATT diodes, respectively. To explore the advantageous properties of SiC, 4H-SiC pulsed IMPATT oscillators were developed in this project. In this thesis, issues in design, simulation, fabrication and characterization of these devices will be discussed. X-band and Ka-band IMPATT diodes with single-drift regions and high-low (loping profiles were designed. Large signal numerical simulations were performed in MEDICI. For both Ka-band and X-band devices, simulations predicted a, maximum RF power over 1 kW, with the corresponding power efficiency greater than 15% In practical operation, actual output power and efficiency will be lower due to the existence of parasitic resistances and capacitances. Despite the good thermal properties of SiC, the high breakdown voltage of SiC diodes and the high current density required for IMPATT operation result in a large heat dissipation that prohibits continuous-wave operation. When operated in pulsed mode low (≤1%) duty cycles and sub-microsecond pulse lengths have to be used. Prototype devices were built and packaged. DC testing showed that sustainable avalanche breakdown was achieved on both X-band and Ka-band diodes. RF testing was performed in waveguide cavities with either reduced-height or top-hat geometry. The first, SiC IMPATT oscillator, which operated at X-band (7–9GHz), has been successfully demonstrated