SUMMARY

Possessing a wide band gap and large break down field, gallium nitride (GaN) is of extreme interest for a host of high power, high frequency applications including next generation cellular base stations, advanced military radar, and WiMAX networks. Much of this interest stems directly from the continued development of AlGaN/GaN high electron mobility transistors (HEMT) which are capable of operating at large power densities (>30 W/mm) and switching speeds (>160 GHz). The same electric fields and current densities which allow for this performance, however, also elicit acute device heating which contributes to an array of degradation mechanisms limiting both performance and reliability. While the link between the thermal response of GaN devices and their reliability is clearly established, the measurement of junction temperature presents a host of challenges. In addition, an adequate description of the phonon-phonon and phonon-carrier dynamics giving rise to this intense heating has yet to be presented. In response, this study establishes a new implementation of Raman spectroscopy to examine the thermal behavior of GaN devices. First, the linewidth (FWHM) of the Stokes signal is utilized to measure the operating temperature of an HEMT independent to the influences of operational stress. Second, a new method utilizing multiple aspects of the Raman spectrum is then developed to measure the evolution of this stress in order to probe the full thermomechanical response. Through acquisition of the entirety of this response, correlation between observed device degradation and the mechanisms responsible may then be obtained. Third, the linewidth is again implemented to examine the role of free carriers on the lifetimes of the optical phonons. Analysis of these lifetimes aids in the formation of a qualitative description of the heat carrying dynamics while also providing information needed in the continued development of electro-thermal device simulations.