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In this thesis, we have investigated and overcome the major limiting factors of intrinsic and parasitic parameters in III-Nitride high electron mobility transistors for high frequency performance with a combined study of simulations and experimental work. The high frequency RF performance of these transistors is severely degraded when short-channel effects, and parasitic resistances are present. To investigate short-channel effects, first, we have developed a simulation model for Ga-polar and N-polar HEMT structures, and found the main reasons for these degradations are drain-induced barrier-lowering and space-charge-limited current injection. To mitigate this, a strong electrostatic back-barrier structure from N-polar orientation is suggested. Secondly, the effect of quantum displacement in GaN HEMTs were investigated using both simulation and experimental measurements. It was discovered that the quantum displacement in a highly scaled device can give more than 2x change in the gate-source capacitance between two different orientations. Therefore it is imperative that the device design for such highly scaled devices consider quantum displacement effects in order to avoid short-channel effects. Another limiting factor from extrinsic elements is the parasitic resistances, especially from contact resistance. To achieve low-resistance non-alloyed Ohmic contacts, we developed two new process technologies that included an inserted graphene layer between metal and AlGaN, and a graded n+ AlGaN Ohmic layer. Both approaches utilized the current path where no barrier exists. In this work, we set the record low contact resistance of 0.049 Ohm mm for Ga-polar technology using the graded AlGaN scheme. The most crucial factor for improving high frequency performance for III-Nitride HEMTs is the saturation of the effective electron velocity. We have modeled the velocity saturation in GaN channel based on LO phonon emission, and explain the phenomena of rapidly decreasing behavior transconductance. This model was also applied to 2-D device simulation where it was possible to obtain the DC and RF characteristics matching to the experimental results. To overcome the fast reduction in gm and fT, we have introduced a polarization graded channel in AlGaN/GaN HEMT and graded AlGaN HFET to tailor the charge profile, and demonstrated a flat gm profile for the first time in field-effect-transistor structure. The high frequency performance of this engineered channel was measured from a highly scaled graded AlGaN HFET with advanced process technology including contact layer regrowth and e-beam lithography. Although the flat gm improved the linearity of fT and fmax, they do not stay flat due to an increasing capacitance profile in a regular 2 X 50 micrometre device. With further scaling of the device width, we obtained an increasing gm profile which can compensate the increment of capacitance, and finally we demonstrated flat fT and fmax profile which has been unseen in any field-effect-transistors. All these results shown in this thesis can contribute to further improvements in high frequency and high power performance of III-Nitride HEMTs for the future RF application beyond mm-Wave frequency |
