Methods for Characterization and Optimization of AlGaN/GaN HEMT Surface Passivation

Licentiatavhandling, 2009

The Gallium Nitride based high electron mobility transistor is rapidly improving and is emerging as a viable alternative for use in high power applications operating in the microwave frequency domain. The suitability of the AlGaN/GaN material system is due to the possibility to grow epitaxial heterostructures that provide; high electron mobility, high electron saturation velocity, high carrier density and a high dielectric breakdown field. This thesis concerns the optimization of the HEMT fabrication process based on an understanding of the physical mechanisms behind the operation of the device. Optimization of a manufacturing process relies on identification of the process steps that have the largest e®ect on the device operation. A simple and fast process for manufacturing large area HEMT devices has been developed. The process is used to find critical processing steps of the HEMT process. For example, it is shown that the ohmic contact annealing step is responsible for a reduction of charge carrier density. The decrease is material dependent and between 10% and 40% of the electrons are lost after annealing, resulting in an increase of sheet resistance of 5% to 40%. The process is also used to show that the annealing step is also responsible for a more than two orders of magnitude larger buffer leakage. The process can also be used to swiftly characterize a heterostructure material and test its suitability for HEMT fabrication. The importance of the surface passivation for stable device operation is demonstrated and methods to optimize the passivation process is presented. Metal insulator semiconductor capacitor structures are studied to measure the density of states at the passivation-semiconductor interface. The density of interface states is shown be between 4*10^12 cm^-2 and 1.5*10^13 cm-2, depending on method of passivation deposition. The Low Pressure Chemical Vapor Deposition process has the lowest interface state density.