Hello, I just watched this video, where Dr Brattain talk about semiconductor physics and I thought about what I learnt in my solid state physics course, namely band theory. I have several questions : 1)The explanation in terms of chemical bonds given by Dr Brattain (around 10mins) seems a lot more intuitive than the band theory so why did physicists developed a new model (band theory is more recent , right?). I remembered that in the law of mass-action, the constant K' isn't actually constant, it depends on the temperature to 3/2. But was it the only motivation. 2) Is the gap between valence and conduction bands the energy needed to break the chemical bonds ? I hope it's clear, and that someone will be able to provide answers.