Development of an integrated optical E-Probe for GaN power transistor reliability analysis

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AlGaN/GaN high electron mobility transistors (HEMT) are a key technology currently envisioned for future radar, satellite and communication applications, ranging from civilian to military use. Although the performance of AlGaN/GaN HEMTs presently reaches power levels up to 40W/mm at frequencies as high as 2-10 GHz, i.e., a spectacular performance enabling disruptive changes for many system applications, long-term reliability of AlGaN/GaN HEMTs is still a serious issue, not only in the UK and Europe, but also in the USA and Japan. There are several key factors affecting AlGaN/GaN HEMT reliability resulting in a variety of different failure mechanisms, including trap generation, metal migration and others. These are accelerated by: (i) device temperature, (ii) local stresses / strains (converse piezo-electric and thermal), (iii) high electric fields. Knowledge of these parameters is essential for reliability testing, in particular, for accelerated lifetime testing to predict mean time to failure (MTTF). The CDTR in Bristol developed and pioneered Raman thermography, to probe temperature and stress/strain with sub-micron spatial and nano-second time resolution in the only a few micron size active device area of AlGaN/GaN HEMTs, but there is presently no non-invasive probe available for experimentally quantifying electric field strength and its lateral distribution in particular when operating devices at high voltages. Therefore presently only simulation can be used to estimate electric field strength. The key aim of this research project is to develop, test and employ a non-invasive novel optical probe (E-probe) to quantify electric field strength and its lateral distribution in the device channel of AlGaN/GaN HEMTs, and to integrate it into Raman thermography, to enable simultaneous electric field, temperature and stress analysis of AlGaN/GaN HEMTs, to develop a unique and highly beneficial analysis technique for AlGaN/GaN HEMT reliability research. Experiments on degrading / stressing of devices to probe the resulting changes in the electric field strength and its distribution will be performed for state-of-the-art reliability research. Charge carrier traps generated during stressing change electric field strength which we expect to be able to probe directly here for the first time. Carrier trapping times range from milliseconds to seconds. We aim, a higher-risk component of this project, to also developing the ability to probing time-dependent changes in the electric field with carrier trapping / detrapping in the devices.