We ended the last discussion with the creation of a crystalline lattice due to covalent bonding between atoms. It was stated that at any temperature above absolute zero (0 o K), some of the electrons obtain enough energy to become free of the bonds that held them to their parent atom. Figure 3.1 of your text has been modified to illustrate this process and is presented to the right. The free electron created by the breaking of the covalent bond is shown by the symbol n (you can remember this as electrons having n egative charge) and the hole left in the absence of an electron is shown by the symbol p (likewise, missing an electron in a bond may be considered a p ositive charge). Once the bonds have been broken and electron and hole pairs (EHP) have been created, these positive and negative carriers are free to move throughout the lattice and participate in the conduction process (this is coming in a few more sections, hang on).
More importantly for this discussion is the fact that there is a range of allowed energies for bound electrons (or holes when the electrons are released from the bonds) and a separate range of allowed energies for free electrons. These ranges form bands of allowed energies where bound electrons (and holes) are found in the valence band, free electrons are found in the conduction band, with a forbidden band in between that represents a range of energies where no electrons may be found. Every solid has a unique band structure that defines its electrical and optical characteristics.
For our purposes, the width of the forbidden band (also referred to as the energy gap, bandgap or forbidden gap) or equivalently, the energy it takes for an electron to move from the valence band to the conduction band, is what defines a solid as an insulator, conductor, or semiconductor. As illustrated in Figure 3.3 of your text (reproduced below), insulators have a very large forbidden band. This indicates that very few electrons will possess sufficient energy to jump the bandgap and, therefore, there will be few free carriers in the material. The opposite extreme is that of a conductor (or metal). In this case, forbidden band is very small, or the valence and conduction bands may overlap as shown in the figure below. In either case,

there is no forbidden band, or it is negligible, and there are a large number of free carriers in the material.
In between these two extremes is what makes things so interesting! At temperatures of absolute zero, all electrons in a semiconductor are tied to their bonds and the material looks like an insulator. However, as previously mentioned, at any temperature above 0 o K, some of the electrons obtain enough energy to break their bonds, jump the bandgap to the conduction band, and leave holes in the valence band. This is because of the relatively small forbidden gap of semiconductors, as illustrated for a generic semiconductor in Figure 3.3 and for specific materials in Figures 3.4 and 3.5 of your text.
It is generally considered that room temperature (25 o C), along with some modifications to elemental materials that we’re going to discuss in the next segment, allows semiconductors to exhibit the necessary conductive behaviors for microelectronic devices.
A more “techno-speak” way of putting this is that we’re going to look at what happens when intrinsic semiconductors are turned into extrinsic
semiconductors.