Temperature distributions in III-V microwave power transistors using spatially resolved photoluminescence mapping

This paper describes a new method for the mapping of local temperatures in the active region of high power III-V semiconductor transistors for microwave applications. The measurement technique involves scanning a focused laser beam at the surface of a chip inside its package, while the photoluminescence (PL) spectra produced are recorded sequentially for each position of the laser beam. The local temperature is deduced from the corresponding wavelength shift of the PL peak, which represents changes in the band-gap due to heating. Results are shown both for field effect type transistors (pseudomorphic high electron mobility transistors-PHEMTs-in the GaAs/Ga_1-xAl_xAs/Ga_1-yIn_yAs system) and for bipolar type transistors (heterojunction bipolar transistors-HBTs-in the GaAs/Ga_1-xIn_xP system). A spatial resolution of 1 μm and an accuracy in the temperature determination of ±1°C are demonstrated, especially for the PHEMTs. Finally, procedures are proposed to implement the information on local operating temperatures provided by this method into thermal resistance calculations