This book is the fourth volume in the New Era Electronics lecture notes series, a compilation of volumes defining the important concepts tied to the electronics transition happening in the 21st century.The lectures in this volume are about the underlying physics that makes semiconductor devices possible. The treatment is physical and intuitive -- the text is descriptive, not heavily mathematical. The lectures are designed to be broadly accessible to students in science or engineering and to working engineers. They present an electrical engineering perspective, but those in other fields may find them a useful introduction to the approach that has guided the development of semiconductor technology for more than 75 years.For those who use semiconductor devices, these lectures provide an understanding of the physics that underlies their operation. For those developing semiconductor technologies these lectures provide a starting point for diving deeper into the physics, chemistry, and materials science relevant to semiconductors. Those who have taken advanced courses will see how specific topics fit into a broader framework.