List of Figures

• 2.1. Down link rate for mobile communication systems.
• 2.2. $g_\ensuremath {\mathrm {m}}$ vs. $V_\ensuremath {\mathrm {th}}$ of GaN HEMTs featuring different techniques.
• 2.3. Cut-off frequency of GaN HEMTs over time.
• 2.4. Cut-off frequency $\times$ gate length of GaN HEMTs.
• 2.5. Power density vs. frequency of GaN HEMTs.
• 2.6. Breakdown voltage $V_\ensuremath {\mathrm {BR}}$ of GaN HEMTs over time.
• 2.7. Cut-off frequency $\times$ breakdown voltage of GaN HEMTs.
• 2.8. Cut-off frequency of GaAs mHEMTs over time.
• 2.9. Current gain (measured and simulated) of GaN HBTs over time.
• 3.1. Low-field electron mobility as a function of carrier concentration in GaN:
  Comparison of the MC simulation results and experimental data.
• 3.2. Low-field electron mobility as a function of lattice temperature in GaN at carrier concentration of 10$^{17}$ cm$^{-3}$.
• 3.3. Electron drift velocity versus electric field in wurtzite GaN: Comparison
  of MC simulation results and experimental data I.
• 3.4. Electron drift velocity versus electric field in wurtzite GaN: Comparison
  of MC simulation results and experimental data II.
• 3.5. Low-field hole mobility as a function of carrier concentration in GaN.
• 3.6. Low-field hole mobility as a function of lattice temperature.
• 3.7. Hole drift velocity versus electric field.
• 3.8. Low-field electron mobility as a function of carrier concentration in InN:
  Comparison of the MC simulation results and experimental data.
• 3.9. Illustration of the scattering rates in our simulation for wurtzite InN as a function of carrier concentration at 300 K.
• 3.10. Low-field electron mobility as a function of carrier concentration in InN:
  Comparison of the MC simulation results with different setups.
• 3.11. Drift velocity versus electric field in wurtzite InN: Comparison of MC simulation results.
• 3.12. Drift velocity versus electric field in wurtzite InN: MC simulation results with different parameter setups.
• 3.13. Illustration of the scattering rates in our simulation for wurtzite InN as a function of electric field.
• 3.14. Drift velocity versus electric field in wurtzite AlN: Comparison of MC simulation results.
• 4.1. GaN thermal conductivity as a function of temperature.
• 4.2. AlN thermal conductivity as a function of temperature.
• 4.3. AlGaN thermal conductivity as a function of Al content.
• 4.4. InGaN thermal conductivity as a function of In content.
• 4.5. InAlN thermal conductivity as a function of In content.
• 4.6. GaN specific heat as a function of lattice temperature.
• 4.7. InN specific heat as a function of lattice temperature.
• 4.8. Material composition dependence of the band gap of In$_x$Al$_{1-x}$N.
• 4.9. Band alignment of InN, GaN, and AlN at room temperature.
• 4.10. Electron mobility versus concentration in GaN.
• 4.11. Electron mobility versus concentration in InN.
• 4.12. Electron mobility versus concentration in AlN.
• 4.13. Hole mobility versus concentration in GaN.
• 4.14. Electron mobility versus temperature in bulk GaN.
• 4.15. Electron mobility versus temperature in 2DEG GaN.
• 4.16. GaN electron drift velocity versus electric field: simulations with different mobility models compared to MC simulation results and experimental data.
• 4.17. AlN electron drift velocity versus electric field: simulations with different mobility models compared to MC simulation results.
• 4.18. InN electron drift velocity versus electric field: simulations with different mobility models compared to MC simulation results.
• 4.19. GaN valley occupancy as a function of the electric field.
• 4.20. Piezoelectric and spontaneous polarization-induced charge $\sigma /\mathrm {q}(P_\ensuremath {\mathrm {SP}}+P_\ensuremath {\mathrm {PE}})$.
• 4.21. Voltages and currents at a two-port network.
• 4.22. Incident and reflected waves at a two-port network.
• 4.23. Equivalent circuit for a HEMT.
• 4.24. Extrinsic parasitic elements.
• 5.1. SEM image of two HEMTs (IAF Freiburg).
• 5.2. Schematic layer structure.
• 5.3. Comparison of measured and simulated transfer characteristics (Device A).
• 5.4. Comparison of measured and simulated output characteristics (Device A).
• 5.5. Comparison of measured and simulated transfer characteristics (Device B).
• 5.6. Comparison of measured and simulated output characteristics (Device B).
• 5.7. Comparison of measured and simulated transfer characteristics (Device C).
• 5.8. Comparison of measured and simulated output characteristics (Device C).
• 5.9. Comparison of measured and simulated cut-off frequency (Device C).
• 5.10. Comparison of measured and simulated S-parameters (Device C).
• 5.11. Simulated electron temperature and velocity along the channel.
• 5.12. Calibrated transfer characteristics (lines) versus experimental data (symbols) for $L_\ensuremath {\mathrm {g}}=0.25 \mu$m HEMT.
• 5.13. Calibrated output characteristics versus experimental data (symbola) for $L_\ensuremath {\mathrm {g}}=0.25 \mu$m HEMT at 425 K. Dot-dashed line - without self-heating, solid lines - with self-heating.
• 5.14. Lattice temperature in the calibration device, $V_\ensuremath {\mathrm {GS}}$=2 V, $V_\ensuremath {\mathrm {DS}}$=20 V.
• 5.15. Predicted transfer characteristics (lines) compared to measured data (symbols) for $L_\ensuremath {\mathrm {g}}=0.5 \mu$m HEMT.
• 5.16. Predicted output characteristics versus experimental data for $L_\ensuremath {\mathrm {g}}=0.5 \mu$m HEMT at 425 K.
• 5.17. Current gain $\vert h_{21}\vert$ for $L_\ensuremath {\mathrm {g}}=0.25 \mu$m HEMT, experimental data (solid lines) versus simulation (dashed lines).
• 5.18. Simulated cut-off frequency $f_\ensuremath {\mathrm {t}}$ (lines) compared to measurements (symbols) for $L_\ensuremath {\mathrm {g}}=0.25 \mu$m HEMT.
• 5.19. S-parameters for the $L_\ensuremath {\mathrm {g}}=0.25 \mu$m device at 300 K.
• 5.20. S-parameters for the $L_\ensuremath {\mathrm {g}}=0.25 \mu$m device at 425 K.
• 5.21. S-parameters for the $L_\ensuremath {\mathrm {g}}=0.5 \mu$m device at 300 K.
• 5.22. S-parameters for the $L_\ensuremath {\mathrm {g}}=0.5 \mu$m device at 425 K.
• 5.23. Simulated maximum $I_\ensuremath {\mathrm {D}}$ ( $V_\ensuremath {\mathrm {DS}}$=7 V) as a function of ambient temperature normalized to 300 K values.
• 5.24. Simulated maximum $f_\ensuremath {\mathrm {t}}$ ( $V_\ensuremath {\mathrm {DS}}$=7 V) as a function of ambient temperature normalized to 300 K values.
• 5.25. Drain current and transconductance at $V_\ensuremath {\mathrm {DS}}$=7 V: measured data compared to simulation.
• 5.26. Electron velocity along the channel [cm/s].
• 5.27. Electric field along the channel [V/cm].
• 5.28. $\Delta v$n (scaled) along the channel [cm$^{-2}$s$^{-1}$].
• 5.29. Transconductance g $_\ensuremath {\mathrm {m}}$ versus gate voltage $V_\ensuremath {\mathrm {GS}}$ for five devices with different source-gate length ( $V_\ensuremath {\mathrm {DS}}$=7 V).
• 5.30. Change of electron velocity $\Delta v$ along the channel at $V_\ensuremath {\mathrm {GS}}$ -1 V and 1 V for two devices with different $L_\ensuremath {\mathrm {SG}}$.
• 5.31. A schematic layer structure of single heterojunction AlGaN/GaN HEMTs with field plates.
• 5.32. Transfer characteristics for different polarization charge densities at the AlGaN/GaN heterojunctions.
• 5.33. Comparison of measured (solid lines) and simulated (dashed lines) output characteristics of $L_\ensuremath {\mathrm {g}}$= $L_\ensuremath {\mathrm {FP}}$=600 nm HEMTs.
• 5.34. Comparison of measured (symbols) and simulated (lines) transfer characteristics of HEMTs with and without field plate.
• 5.35. Simulated electric field along the channel of $L_\ensuremath {\mathrm {g}}=600$ nm HEMTs with and without field plate for $V_\ensuremath {\mathrm {GS}}$=0 V and $V_\ensuremath {\mathrm {DS}}$=7 V.
• 5.36. Simulated electric field along the channel for various field plate lengths $L_\ensuremath {\mathrm {FP}}$ ( $L_\ensuremath {\mathrm {g}}=600$ nm).
• 5.37. Simulated electric field along the channel for various gate lengths ( $L_\ensuremath {\mathrm {FP}}$= $L_\ensuremath {\mathrm {g}}$).
• 5.38. Simulated electric field along the channel for various field plate lengths $L_\ensuremath {\mathrm {FP}}$ ( $L_\ensuremath {\mathrm {g}}=300$ nm).
• 5.39. Simulated electric field along the channel for various field plate lengths $L_\ensuremath {\mathrm {FP}}$ ( $L_\ensuremath {\mathrm {g}}=150$ nm).
• 5.40. Schematic layer structure of the investigated device.
• 5.41. Band alignment of the heterointerface.
• 5.42. Transfer characteristics for different values of the polarization charge density at the InAlN/GaN interface and $-10^{13}$ cm$^{-2}$ at the InAlN top surface ( $V_\ensuremath {\mathrm {DS}}$=8.0 V).
• 5.43. Transfer characteristics for different values of the total charges (polarization and traps) at the InAlN top surface and 3.3$\times$10$^{13}$ cm$^{-2}$ at the InAlN/GaN interface ( $V_\ensuremath {\mathrm {DS}}$=8.0 V).
• 5.44. Comparison of simulated transfer characteristics for different values of the thermal resistance and experimental data ( $V_\ensuremath {\mathrm {DS}}$=8.0 V).
• 5.45. Comparison of simulated output characteristics and experimental data.
• 5.46. Schematic layer structure of the recessed device.
• 5.47. Transfer characteristics at $V_\ensuremath {\mathrm {DS}}$=7 V: lines - simulation, symbols - experimental data.
• 5.48. Transconductance $g_\ensuremath {\mathrm {m}}$ at $V_\ensuremath {\mathrm {DS}}$=7 V: lines - simulation, symbols - experimental data.
• 5.49. Comparison of DHEMT output characteristics, $V_\ensuremath {\mathrm {GS}}$ stepping 0.5 V.
• 5.50. Comparison of EHEMT output characteristics, $V_\ensuremath {\mathrm {GS}}$ stepping 0.5 V.
• 5.51. Comparison of the cut-off frequency $f_\ensuremath {\mathrm {t}}$: symbols - simulation, lines - experimental data.
• 5.52. Simulated transfer characteristics for devices with different barrier thickness t $_\ensuremath {\mathrm {bar}}$ under the gate.
• 5.53. Simulated transconductance for devices with different barrier thickness t $_\ensuremath {\mathrm {bar}}$ under the gate.
• 5.54. Simulated cut-off frequency for devices with different barrier thickness t $_\ensuremath {\mathrm {bar}}$ under the gate.
• 5.55. Simulated gate-source capacitance for devices with different barrier thickness t $_\ensuremath {\mathrm {bar}}$ under the gate.
• 5.56. Schematic layer structure of the three HEMTs under investigation.
• 5.57. Comparison of simulated and measured transfer characteristics for the three devices.
• 5.58. Energy band diagrams of a HEMT with (dot-dashed line) and without (solid line) InGaN layer.
• 5.59. Simulated DC $g_\ensuremath {\mathrm {m}}$ for the three devices.
• 5.60. Output characteristics of a HEMT with non-recessed InGaN cap layer.
• 5.61. Comparison of the measured (symbols) and simulated (lines) transfer characteristics at $V_\ensuremath {\mathrm {DS}}$=5 V.
• 5.62. Simulated transfer characteristics at $V_\ensuremath {\mathrm {DS}}$=5 V for HEMTs with different gate recess depths.
• 5.63. Comparison of the measured (symbols) and simulated (lines) DC transconductance $g_\ensuremath {\mathrm {m}}$ at $V_\ensuremath {\mathrm {DS}}$=5 V.