Carlo De Santi

In this work, we analyzed the optical degradation of 845 nm VCSELs designed for silicon photonics (SiPh) applications as a function of the oxide aperture. First, we investigated the optical degradation in relation to the stress current. The experimental results showed that devices with a larger oxide aperture exhibit better reliability. From the analysis of the degradation kinetics, we found that the current at which the highly accelerated degradation process occurs depends inversely on the aperture radius. This result was explained by showing that devices with a larger aperture have lower thermal impedance, and therefore operate at lower internal temperatures at similar bias points. This interpretation is supported by isothermal constant current stress tests, which show that when the same internal temperature is maintained during ageing, for all VCSEL geometries under study the onset of degradation occurs at the same time. The equivalent activation energy of the degradation process was found to fall between 0.43 and 0.68 eV. Our analysis demonstrated that using a larger aperture can improve device reliability.

We investigate the sustainable breakdown operation of lateral E-mode GaN high-electron mobility transistors. First, the dependence of the sustainable breakdown voltage (Vaval) on gate bias was investigated; a reduction in Vaval was observed at increased gate voltage, and ascribed to the progressively increasing sub-threshold conduction. Evidence for hole trapping under the gate was then obtained, by monitoring the threshold voltage variation during the test, and a dedicated measurement routine was used to prove the recombination of trapped electrons with holes generated by impact ionization. Finally, technological solutions to improve the sustainable breakdown characteristics of the devices are proposed and validated, through the analysis of two different process splits.