It would not be an exaggeration to say that semiconductor devices have transformed human life. From computers to communications to internet and video games these devices and the technologies they have enabled have expanded human experience in a way that is unique in history. Semiconductor devices have exploited materials, physics and imaginative applications to spawn new lifestyles. Of course for the device engineer, in spite of the advances, the challenges of reaching higher frequency, lower power consumption, higher power generation etc. provide never ending excitement. Device performances are driven by new materials, scaling, and new device concepts such as bandstructure and polarization engineering. Semiconductor devices have mostly relied on Si but increasingly GaAs, InGaAs and heterostructures made from Si/SiGe, GaAs/AlGaAs etc have become important. Over the last few years one of the most exciting new entries has been the GaN based devices that provide new possibilities for lighting, displays and wireless communications