Technology and characterization of InAlN/GaN FETs

In this work the lattice matched In0.17Al0. 83N/GaN heterostructure was investigated. The work started by establishing the main features of this heterostructure in a HEMT configuration through the investigation of the DC, small signal and large signal characteristics of planar HEMTs. The study included heterostructures with variable barrier thicknesses, but still thinner than what is conventionally used in AlGaN/GaN HEMTs. Using these heterostructure a barrier scaling study combined with static simulations revealed an initially low surface potential, which is combined with the high polarization discontinuity of this heterostructure, thus yielding HEMTs with high current densities and high aspect ratios. Current densities above 1 A/mm and up to 2.4 A/mm could be achieved in this work and the HEMTs demonstrated high cut-off frequencies, scaling with the gate length, up to an ft= 61 GHz and fmax= 112 GHz for 0.1 μm long gates. In addition, the high thermal stability of the heterostructure was established through a series of storage and operating tests at temperatures above 500 °C and up to 1000 °C. This allowed the optimization of a thermal oxidation process to reduce the HEMT gate leakage but most importantly as an efficient preparation for surface passivation. The oxidation process was investigated by using actual HEMT structures, with the corresponding DC, small signal and large signal analysis, and also dedicated structure to investigate the effect of localizing the oxidation process around the gate area, which is the area of interest for power operation. This passivation scheme yielded a lag free device at 4 GHz with output power density of 11.6 W/mm at a drain voltage of 20 Volts, so far the highest reported for this heterostructure. In addition, relying on the initial experiments indicating the heterostructure stability, nanocrystalline Diamond overgrowth experiments were conducted for efficient heat extraction from the device, as was evaluated using thermal simulations. The Diamond deposition conditions were optimized to yield films with high thermal conductivity reaching 1000 W/mK in the vertical direction of heat flow toward the position where the heat sink would be placed. These conditions were restricted to deposition temperatures above 700 °C, with bias enhanced nucleation also conducted above 700 °C, with slow deposition rate of around 0.1 μm/hour. A series of experiments were conducted to verify the heterostructure stability under these conditions by fabricating devices after growth and complete removal of the overgrown Diamond film. After establishing the heterostructure stability, a series of experiments progressed toward obtaining a fully Diamond coated HEMT by growth experiments on gateless HEMTs and the subsequent optimization of the ohmic contact, and the growth on gateless HEMTs with different passivation and the subsequent optimization of the passivation deposition process, to reach finally a fully processed Diamond coated HEMT with 1 μm Diamond film thickness. This was done using two different Diamond nucleation techniques, namely the bias enhanced nucleation and the nanoparticle seeding. The first Diamond overgrown GaN HEMTs operating at frequencies above 1 GHz were demonstrated, keeping a relatively high current density above 1 A/mm. The optimization of this