A new configuration of nematic liquid crystal thermography with applications to GaN-based devices

In this paper, a new configuration of nematic liquid crystal thermography (NLCT) that uses laser illumination to improve the contrast of thermal images on device chips is reported. The wavelength is selected so that the laser light is not absorbed by the device under measurement. The most important application of this new configuration is to measure the junction temperature of light-emitting devices (LEDs). Traditional NLCT cannot be used on LEDs, because the light emitted by the device overwhelms the light reflected from the nematic liquid crystal (NLC) layer coated on the device surface. Thus, the temperature information carried by the reflected light is lost. Using laser illumination and a color filter, this difficult measurement has been made a reality. This new configuration is extended to studying the thermal performance of heterojunction field effect transistors (HFETs) built on AlGaN/GaN structure grown on sapphire and SiC substrates, respectively. This technique is nondestructive and can be performed in real time during device operation. It has a submicrometer spatial resolution and a ±1°C temperature accuracy. The thermal resistance of HFETs and LEDs has been measured with this method. The new measurement configuration is a valuable tool for studying the thermal performance of GaN-based as well as other semiconductor devices. It provides the device engineers and physicists additional information to improve device structures and performance.