Dopant energy levels

If the empty energy levels of the dopant are located just over the occupied band of an intrinsic semiconductor, the dopant may serve as an electron acceptor for the electrons from the occupied band (thus introducing its own conduction band), we have a p-type semiconductor, (Fig. 9.13b). If the dopant energy levels are occupied and located just under the conduction band, the dopant may serve as a n-type semiconductor (Fig. 9.13c). 

The excitation can occur either from the valence band to the conduction band to create an electron-hole pair intrinsic excitation) or from a discrete crystal-defect (dopant) energy level to either band to create a conduction electron or hole... 

The energy-band structure and DOS offer information on band gap, atomic energy level and constitution, defect or dopant energy-level position, as well as composition. Thus, we can judge whether this system is a metal, a semiconductor. 

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

For a triplet emissive guest (phosphorescent) dopant, the triplet energy level of the host normally should be higher than that of the guest. 

Instead of considering how the incorporation of a dopant ion perturbs the electronic structure of the crystal, we will face the problem of understanding the optical features of a center by considering the energy levels of the dopant free ion (i.e., out of the crystal) and its local environment. In particular, we shall start by considering the energy levels of the dopant free ion and how these levels are affected by the presence of the next nearest neighbors in the lattice (the environment). In such a way, we can practically reduce our system to a one-body problem. 

---