Processing, Characterization and Modeling of AlGaN/GaN HEMTs

Doctoral thesis, 2006

III-Nitrides electronic properties make them currently the materials of choice for high-power high-frequency applications. Their wide bandgaps, high breakdown fields, the high electron peak and saturation velocities combined with the large conduction band offset and the high electron mobility observed in AlGaN/GaN heterostructures enable excellent microwave power performance of AlGaN/GaN high electron mobility transistors (HEMTs). These devices have demonstrated a tenfold increase in output power densities at microwave frequencies compared to standard III-V technologies, opening great perspective for high power microwave electronics. III-Nitride technology has just emerged in the last decade; it is yet neither mature nor completely understood. This thesis deals with the fabrication, characterization, modeling of the RF performance of AlGaN/GaN HEMTs. The physical and electrical properties of the III-Nitrides are described before the operation of these transistors is reviewed. Advanced processing issues are discussed, before presenting the results achieved on this type of transistors. RF modeling of Schottky diodes and transistors are also presented prior to the demonstration of the enhancing performance of circuit demonstrators using such device. A complete fabrication process for the realization of such transistors has been developed and is described in details. The development of a simple and effective method for obtaining very low ohmic contacts (0.23 Ω•mm), its characterization and understanding are major achievement. Further development of the in-house process enabled the fabrication of high power transistors HEMTs without field-plate demonstrating a continuous wave output power density of 5 W/mm and 9.7 W/mm at 3 GHz on sapphire and SiC respectively A new method for the characterization of microwave Schottky diodes was developed as well as a non linear large signal model for AlGaN/GaN HEMTs. Demonstrating the possibilities of AlGaN/GaN HEMTs for high linearity mixers, the world’s first resistive mixer based on AlGaN/GaN transistors was fabricated using in-house components, exhibiting a minimum conversion loss of 7.3 dB, an IIP3 of 36 dBm for a LO power of 30 dBm.