1. The problem statement, all variables and given/known data I recently completed an experiment where I measured the transmittance of a set of samples (different semiconductors). I'm trying to understand why the transmittance was different for different samples. Only just started learning some solid-state physics. The spectrum (I think it was Voltage [y-axis] vs. Wavelength [x-axis]) The Transmittance of the surface of a material/semiconductor is its effectiveness in transmitting radiant energy. The bandgap of a semiconductor is the minimum energy required to excite an electron that is stuck in its bound state into a free state where it can participate in conduction. So the transmittance of different semiconductors is different because they have different bandgaps. Is that correct to infer that? If so, I'm wondering how different materials have different bandgaps? Is it purely because of their crystalline structure?