List of Figures

• 1.1. Comparison between long, middle and short range technologies.
• 1.2. Atmospheric attenuation versus millimeter wavelengths.
• 2.1. Current oscillations in a GaAs sample (J. B. Gunn)
• 2.2. Crystallographic directions in a zinc-blende crystal.
• 2.3. Simplified band structures of zinc-blende (cubic) GaAs, InP and GaN.
• 2.4. Schematic view of the average electron drift velocity as a function of the electric field E for GaAs, InP and GaN.
• 2.5. Generation of a stable dipole domain.
• . Illustration of the equal area relationship between $\mathcal{E}_R$ and $\mathcal{E}_p$.
• 2.7. Domain profile in the limit condition of zero diffusion.
• 2.8. The relationship between the domain voltage and the electric field outside the domain imposed by the space charge dynamics.
• 2.9. Energy-band diagrams for Schottky contact on n-semiconductor.
• 2.10. Principle schema of a Gunn diode with a graded AlGaAs injector.
• 2.11. DC electrical model of a graded gap injector Gunn diode.
• 2.12. Functional principles of an RTD
• 2.13. Transport process in a resonant tunneling diode
• 2.14. General two-port network
• 2.15. Current and voltage characteristics for the Transit Time mode
• 2.16. Current and voltage characteristics for the Delayed Domain mode
• 2.17. Current and voltage characteristics for the Quenched Domain mode.
• 2.18. Representation of the different operating modes of a Gunn diode.
• 2.19. Approximation of the device geometry to simplify the heat transfer problem
• 2.20. Heat-sink temperature profile for a standard Gunn diode chip.
• 2.21. Double-sided heat-sink temperature profile for a quasi-planar Gunn diode.
• 2.22. Heat flux profile for a standard Gunn diode chip.
• 2.23. Heat-flux profile for a quasi-planar Gunn diode.
• 3.1. Semiconductor bandgaps and lattice parameters
• 3.2. Bandgap energies of $Al_xGa_{1-x}As$ vs. the Al concentration.
• 3.3. Bandgaps of III-N semiconductors with wurtzite and zincoblende structure versus their lattice parameter
• 3.4. Schematic view of a MBE system.
• 3.5. Typical layer sequence of the GaAs Gunn diode with a graded gap injector.
• 3.6. Conduction band profile and local density of states plot for a Resonant Tunnelling Injector
• 3.7. Typical layer sequence of the GaAs Gunn diode with a resonant tunneling injector.
• 3.8. HRTEM image of a resonant tunneling double barrier.
• 3.9. Layer sequence of the GaN Gunn diode.
• 3.10. AFM image of the surface morphology for GaN Gunn structure
• 4.1. Operation principle of the AFM.
• 4.2. Errors in AFM characterization.
• 4.3. Hall measurement conventions.
• 4.4. Hall measurement contact configurations and layouts
• 4.5. Contact resistance test patterns. (a) TLM and (b) CLM configuration.
• 4.6. Evaluation of the contact and sheet resistances for TLM measurements.
• 4.7. Schematic view of the pulse measurement setup.
• 4.8. Vector network analyzer HP8510C (8510XF)
• 4.9. Smith Chart
• 5.1. SEM pictures of the GaAs Gunn diode mesas after ECR-RIE etching
• 5.2. SEM pictures of the GaN Gunn diode mesas after ECR-RIE etching
• 5.3. SEM pictures of the GaN nanocolumns (topdown approach)
• 5.4. SEM picture of the GaAs Gunn diode ohmic contacts.
• 5.5. SEM picture of the GaAs Gunn diode after the isolation step.
• 5.6. Schematic view of the planarization with polyimide.
• 5.7. Optical microscope picture and schematic view of the bond-pads deposition after the polyimide planarization.
• 5.8. SEM pictures of the finished GaAs Gunn diode with coplanar pads.
• 5.9. SEM pictures of the monolithic integrated GaAs Gunn diode oscillators.
• 5.10. Air-bridge fabrication using gold plating process.
• 5.11. SEM pictures of the gold airbridges on GaAs Gunn diodes.
• 6.1. Graphic analysis of the transmission line measurement
• 6.2. I-V characteristics of a graded gap injector Gunn diode
• 6.3. I-V characteristics of diodes with different Al maximum concentration in the graded barrier.
• 6.4. I-V characteristics of a graded gap injector Gunn diode for different temperatures
• 6.5. Graphical procedure to obtain the barrier height $\phi$, the effective Richardson constant $A^*$ and the saturation current Is.
• 6.6. The graded gap barrier height as function of the saturation current $Is$ for different graded gap injectors.
• 6.7. I-V characteristics of a typical resonant tunneling injector Gunn diode.
• 6.8. I-V characteristics of a typical GaN Gunn diode.
• 6.9. 100ns pulses I-V characteristics of a graded gap injector Gunn diode
• . Impedance analysis up to $50 \thickspace MHz$ of a Gunn diode without injector.
• . Impedance analysis up to $50 \thickspace MHz$ of a graded gap injector Gunn diode
• 6.12. Admittance vs. frequency for a GaAs Gunn diode without injector
• 6.13. Admittance vs. frequency for a graded gap injector GaAs Gunn diode.
• 6.14. Admittance vs. frequency for a resonant tunneling injector GaAs Gunn diode.
• 6.15. Drift velocity vs. bias voltage for a Gunn diode without injector.
• 6.16. Drift velocity vs. bias voltage for graded gap Gunn diodes with different maximum Al concentration.
• 6.17. $L$-valley occupation vs. electric field for a graded gap injector GaAs Gunn diode
• 6.18. Drift velocity vs. bias voltage for different temperatures.
• 6.19. Cross sectional view of a typical Gunn diode cavity oscillator.
• 6.20. Frequency and output power, vs. voltages for a graded gap injector Gunn diode second harmonic cavity oscillator.
• 6.21. Schematic view and equivalent circuit of an unit cell of the periodic low pass filter.
• 6.22. Simulated and measured response of the three cell, periodic low-pass filter. The unit cell length is 0.8mm.
• 6.23. Effect of additional cells on the low-pass filter roll-off. Simulated insertion loss of two and three cell. The unit cell length is 1 mm.
• 6.24. Typical Gunn diode MMIC voltage controlled oscillator layout.
• . Admittance vs. frequency for the $36\thickspace GHz$ resonant circuit.
• . Admittance vs. frequency for the $44.5\thickspace GHz$ resonant circuit.
• 6.27. Frequency and output power vs. voltage for a graded gap injector Gunn diode monolithic oscillator (type I).
• 6.28. Frequency and output power vs. voltage for a graded gap injector Gunn diode monolithic oscillator (type II).