Draw two lines that each spreading out to two lines as in the lecture notes (semiconductor) . The spreading out is because the atoms are close together. The upper band in the conduction band and the lower band is the valence band if you draw a vertical line specifying the atomic separation. Once how the band exist is understood , then you need to know whether they are filled . The upper conduction band is not filled but the lower valence band is completely filled in semiconductor. Therefore only if electron is promoted to the CB leaving a hole in the VB can there be conduction by both charge carriers. Electron move in CB and hole (also electron moving)in the VB.
The band formation cannot be explained in terms of electron and holes . The band unique dependent on atomic number. It will be filled completely or partially depending on material. Semiconductor is filled VB and empty CB until electrons promoted thus leaving holes. Electron and holes go hand in hand. If someone gets up from a seat , people can move to fill it but we say seat move when actually people move. So do not be too particular above which is which.
