Self-Heating Profile in an AlGaN/GaN Heterojunction Field-Effect Transistor Studied by Ultraviolet and Visible Micro-Raman Spectroscopy

Direct measurements of self-heating in gallium nitride (GaN) transistor using ultraviolet (UV) and visible micro-Raman spectroscopy are reported. The material stack was grown on silicon substrates and consists of an AlN nucleation, AlGaN transition, GaN buffer, and AlGaN barrier layers. Phonon shifts are used to estimate the temperature rise. UV measurements probe the current-carrying 2-D electron gas (2-DEG) in the GaN near the interface with the barrier region. Visible micro-Raman measurements provide an average temperature rise through GaN, AlN, and substrate near its interface with AlN. Together, these measurements provide a temperature depth profile. Under identical drive conditions, the increase in temperature from UV micro-Raman is approximately twice what is obtained from the visible measurements, reaching as high as 350°C above ambient temperature at input power of 7.8 W/mm. The temperature depth profile is simulated using finite-element analysis. We find that the temperature dependence of the thermal conductivity of GaN is important to incorporate in these simulations due to the large temperature rise in the 2-DEG region. A thermal boundary resistance of 1× 10^-8~K·m² /W is obtained from the combined simulation and experimental results.