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This thesis presents theoretical and experimental investigation of wider bandgap AlGaN channels to achieve superior gain linearity and output power density in III-Nitride transistors.GaN high electron mobility transistors (HEMTs) exhibit high saturation velocity and large breakdown field, resulting in unprecedented power densities at microwave frequencies. However, their cutoff frequency and gain reduce significantly as the gate bias or current density increase, causing non-linear behavior and soft gain compression at peak efficiencies. This phenomenon is shown to be related to the sheet density dependence of velocity in HEMTs. Velocity-field measurements are carried out on unique test structures as a function of sheet charge density, which revealed strong density dependence of saturation velocity. To realize constant velocity profile as a function of gate bias, polarization graded field-effect transistors (PolFETs) with engineered charge and capacitance profiles are discussed. Constant cutoff frequency and maximum oscillation frequency over wide gate-bias and output current range are achieved in highly-scaled PolFETs, indicative of enhanced gain linearity.AlN with extremely large bandgap of 6.2 eV can withstand significantly higher breakdown field than GaN channels, which could enable higher voltage, as well as higher charge density for the same device dimensions. To realize superior breakdown voltage and current density, AlGaN channels with high Al-content are investigated. Theoretical calculation of the low-field electron mobility in AlGaN channel HEMTs, as well as its implication on the high-field transport are discussed. A major limiting factor in the development of AlGaN channel transistors, thus far, has been high-resistance ohmic contacts. Contact layers with compositional grading (i.e. electron affinity grading) are shown to mitigate this issue significantly. Specific contact resistance of 2x10-6 ohm.cm2, and average lateral breakdown field of 2 MV/cm are attained by 1st generation Al0.75Ga0.25N MOSFETs grown by molecular beam epitaxy (MBE). Considerable improvement in channel mobility and current density is obtained in 2nd generation Al0.7Ga0.3N MOSFETs grown by metal-organic chemical vapor deposition (MOCVD). Field-plated structures are demonstrated with current density of 0.5 A/mm and an average lateral breakdown field up to 3.6 MV/cm. The advantages of the two growth techniques are analyzed and compared, leading to a hybrid device design with MBE-grown graded contact layer and MOCVD-grown channel layer. Finally, the 3rd generation of high Al-content AlGaN PolFET is presented, which could potentially enable superior linearity and output power performance in III-Nitrides for advanced applications.