It's interesting that he started with that topic. As opposed to voltage, current , RC circuits, etc. He's got quite a few topics, like emitter followers and op amps. But I think he's a bit below the level of what people need in order to create useful PCB level electronics. I think something like this would be so much more useful if it was tied to an existing book on the subject, like for instance "The Art of Electronics". Which actually doesn't cover device physics but start with just transistor and diode models presented as facts.