It is well known that GaN HEMTs are excellent candidates for high-speed, high-power applications because of the large band gap and relatively high electron velocity of GaN-based materials. The emerging N-polar HEMTs have been shown to be an attractive alternative to developing high-frequency GaN HEMTs with the following advantages over Ga-polar HEMTs: (a) the existence of a natural back barrier increases electron confinement, (b) very low contact resistances can be achieved to the 2DEG, and (c) aggressive scaling of both the gate-barrier and channel thicknesses while maintaining high charge in the channel from the polarization electrostatics of the back-barrier is possible, because of the independence of charge inducing layer and gating barrier layer.

To achieve high current-gain (fT) and maximum oscillation frequencies (fmax), the gate length has been aggressively scaled down to 20-nm. For the sub 100-nm gate length HEMTs, scaling down the channel-thickness (tch) in GaN/(In,Al,Ga)N high-electron-mobility-transistors(HEMTs) is essential to eliminating short-channel effects. However, this scaling can degrade both charge density (ns) and mobility (micro), thereby reducing channel-conductivity. The epitaxial design and growth of ultra-scaled N-polar GaN/(Al, In, Ga)N HEMTs has not been explored yet for achieving high frequency and RF power performance. The object of this work is to develop epitaxial structures with scaled channel thickness (≤ 10-nm) and high channel conductivity, as well as low gate leakage and high breakdown voltage, using metalorganic chemical vapor deposition.

A combinational back barrier design with both AlGaN and InAlN materials was proposed to retain high channel conductivity for ultra-scaled channel thicknesses. With the back barrier design of AlN/InAlN/AlGaN, sheet resistance of ~230 ohm/sqr; and mobility ~1400 cm2/V-s were obtained for devices with 5-nm-thick channel. The fabricated sub-100nm T-gate HEMTs (By Dr. Dan Denninghoff) exhibited Ion of 4A/mm, peak gm of 1887 mS/mm, fT and fmax of 201GHz, 406GHz respectively. Furthermore, the back barrier design space for achieving high channel conductivity with tch<5-nm was investigated. By implementing indium compositional-grading (from In0.18Al0.82Nto AlN), low sheet-resistance of 329 ohm/sqr; was demonstrated for a HEMT with 3-nm-thick-channel.

The second main part of this thesis is to solve the very negative pinch-off voltage and high gate leakage problems in the SiNx capped first generation N-polar InAlN HEMTs. A thin InAlN cap layer on top of the GaN channel layer was employed for reducing gate leakage and improving breakdown voltage. Using this technique, devices with breakdown voltages up to 500 V were obtained. In addition, microwave power result for N-polar InAlN/GaN HEMTs was obtained for the first time. The stability of InAlN cap layer and AlGaN cap layer was compared under high field stress. A gate-degradation was observed in the devices with InAlN cap. In contrast, the devices with AlGaN cap layer exhibited better stability under high field stress. The behavior of vertical channel scaling with AlGaN cap layer was then investigated. 3-nm-thick channel with high Al composition AlGaN cap was developed for sub 100-nm gate RF power HEMTs.

Additionally, the atomic structure and Ga-unintentional incorporation of AlN interlayer were studied. The growth strategy for high Al-content AlN was discussed.