Now finals week, I want to give a mention to two YouTube authors whose videos have been quite helpful to me in ECE 255 and its companion course, ECE 208.
eneradi has many videos on MOSFETs and whatnot. Whoever eneradi is, he explains things well. I have been particularly pleased with the mathematical explanations of the formulae I've used, since my textbook has rarely disclosed such information.
booksofscience has some videos on similar topics. I found his videos on PN junctions especially useful when my 255 course was starting.