Advanced polarization-based design of aluminum gallium nitride/gallium nitride HEMTs

During the past few years, enormous progress has been made in the development of GaN-based devices. Due to high breakdown field, high sheet charge density and high electron saturation velocity, GaN-based HEMTs have great potential for high frequency high power applications. Extensive research has being carried out on the material growth and the device structure. This dissertation focuses on the efforts to develop novel epitaxial structures to improve the electron mobility and suppress the dispersion without surface passivation. Relying on the utilization of strategic band engineering and polarization charge, unpassivated high power GaN-based HEMTs with minimal dispersion have been demonstrated.; The application of AlN in GaN-based HEMT is discussed. AlN is a binary material, thereby alloy disorder scattering is eliminated which improves the electron mobility. The high polarization field in AlN also results in high carrier concentration. Low sheet resistance is observed in AlN/GaN heterostructures. The incorporation of a thin AlN layer in an AlGaN/GaN HEMT is investigated, resulting in an AlGaN/AlN/GaN structure. Due to the absence of alloy disorder scattering, and the reduction of wavefunction penetration into AlGaN, the electron mobility is improved. Carrier concentration is also improved slightly due to the high polarization effect of AlN. The DC and RF performances are presented.; Dispersion at different temperatures is presented. Increased dispersion is observed at lower temperature. Hopping conduction and de-trapping/band conduction models are discussed.; The concept of a thick GaN cap on top of an AlGaN/GaN HEMT is proposed to reduce dispersion at epitaxial level without passivation. This approach utilizes a thick cap layer to increase the distance between the channel and surface, thereby screening the surface potential fluctuations. A GaN/AlGaN/GaN heterostructure is investigated. Dispersion is suppressed without passivation. In order to decrease the leakage current and increase the breakdown voltage, several variations of device structures are discussed. By employing a SiO 2 insulating layer, lowering Si doping sheet density and utilizing a thick graded AlGaN cap layer, leakage current is reduced and breakdown voltage is increased. These improvements resulted in a record output power density of 12W/mm at 10GHz for GaN-based HEMTs without passivation.