ion of holes from the base into the emitter region, since the potential barrier in the valence band is higher than in the conduction band. Unlike BJT technology, this allows a high doping density to be used in the base, reducing the base resistance while maintaining gain. The HBT improves on the BJT in that it can process signals of very high frequen- cies, up to several hundred GHz. It is commonly used in modern ultra-fast electronic circuits, RF systems, and in applications requiring high-power efficiency, such as RF power amplifiers in cellular phones. Introduction and Background xci account for special events, including planned events such as sports games and unplanned events such as natural disasters. For smart devices, coverage requirements include global roaming and support for services on moving platforms such as cars, planes, high-speed trains, and mass transit. 5G will continue to improve the user and operational experience in these areas. Leveraging recent advances in storage/memory technologies, communication/connectivity, computing, big-data analytics, artificial intelligence (AI), machine learning (ML), machine vision, and other related areas will enable the fruition of immersive technologies such as augmented and virtual reality. The use of virtual reality is expected to go beyond early adopters such as interactive gaming to enhancing cyber-physical and social experiences such as conversing with family, acquaintances, business meeting, or the disabled indivi- duals. The AR/VR technologies would hypothetically allow walking around a street where everyone is talking in a different language, and interacting with those people in the same language in a fully immersive experience. The growing number of drones, robots, and other self-driving vehicles taking cameras to places that humans could never imagine reaching would allow us to collect new content from attractive points of view around the globe. Ultimately, AR/VR will provide the most personal experience with the closest screen, pro- viding the most 