'Pure semiconductors, by themselves, are not particularly usefully.' Should be: 'Pure semiconductors, by themselves, are not particularly useful.' I'm sure this isn't what you want in terms of help in proofreading, but this subject fundamentally over my head. OTOH, it's very interesting. So far this beats all of the basic electricity/electronics books I've seen. Using complex numbers to calculate impedance makes it much easier to understand. This post is more about saying thank you very much than anything else.