A comprehensive study of AlGaAs/GaAs beryllium-and carbon-doped base heterojunction bipolar transistor structures subjected to rapid thermal processingWe present a phenomenological model for the lifetime prediction of GaAs based heterojunction bipolar transistors ͑HBTs͒. Both thermal activation process and recombination enhanced defect generation process were considered phenomenologically. At high stress temperatures, the lifetime of the transistor is determined by the thermal activation process and is independent of the stress current. But at low stress temperatures, the device's lifetime is controlled by the recombination enhanced defect generation process and is dependent on the stress current. The model has been used to fit experimental data, and very reasonable agreement was obtained. The model is valid for all stress temperatures and stress currents. It provides a good guideline for projecting HBTs lifetimes using accelerated stress test.

Stator-rotor systems are commonly used in many different types of turbomachinery applications to supply an air for secondary air flows. Commercial CFD codes with variety of turbulence models are widely used in order to estimate the amount of flow supplied by the preswirl stator-rotor system. CFD investigations can provide detailed information about the local flow field which is extremely difficult to obtain from rotating rig due to the measurement limitations in rotating frame, however the accuracy of CFD needs to be investigated by conducting experiments. In this study the purpose is to evaluate how accurate CFD simulations with different turbulence models can predict the flow rate supplied by the system. An experimental rig composed of a stationary preswirler, a rotating disk with an internal flow path and a stator-rotor cavity with a rim seal was used in this study. Air is supplied to the stator from the ambient due to the suction provided by the rotor which can rotate at up to 3100 rpm. Incoming air first flows through annular preswirl guide vanes located inside the stator then discharges into the stator-rotor cavity located downstream of the preswirl guide vanes. Some fraction of the flow induced into the rotor by the help of inlet guides which are attached to the rotor face and angled to match the flow angle in rotating frame. Remaining part of the flow passes through rim seal and discharges out to the ambient. Two experimental cases, one with preswirl guide vanes without endwall contouring and the other with endwall contouring were been investigated at 3100 rpm. Mass flow rate at the inlet was 14.6% higher for the case with endwall contoured configuration compared to the case without endwall contouring. For both of the cases approximately 90% of the inlet flow was purged through rim seal while remaining 10% flows through the radial rotor disk passages. CFD analysis of the rotating rig were conducted using commercial code STAR CCM+. Turbulence models of k-ε, k-ω, Reynolds stress (RST) and Spalart-Allmaras were used and the mass flow rate drawn into the system was compared with experiments. The mass flow rate into the rig from experimental measurements was 7.4% higher compared to the best CFD prediction given by RST Linear. Among all turbulence models k-w was the worst performer by predicting mass flow 13% lower compared to the experimental value. Different sub-options of these turbulence models were also investigated. This study provided significant information for preswirl stator-rotor system designers in terms of the amount of flow rate that can be obtained and how well can it be predicted by CFD.