As a first example of the effect of impurities, assume that a small concentration of group V atoms is added to the host Si crystal at the manufacturing stage. These atoms will substitute for Si atoms in the lattice. Since they have five rather than four valence electrons, one electron for each impurity atom is unused in the bonding. At 0 K the spare electron remains in the vicinity of its parent atom by virtue of Coulombic attraction to the one remaining uncompensated proton in the nucleus of the impurity. The single, unbonded electron effectively orbits the impurity in much the same way that an electron orbits an isolated hydrogen nucleus. This is shown in schematic form in figure 7; it is easily shown that the radius of the electron orbit is large, extending over many unit cells. As a consequence, the binding energy is low and only a small amount of "external" energy is required to liberate the electron from its parent impurity atom. Therefore, the electron becomes a free, conduction band electron at relatively low temperatures as depicted in figure 8. However, this time note that the release of the electron by the impurity does NOT result in the generation of a hole. The impurity, called a donor , becomes ionised (positive charge) but is locked into the lattice and unable to move. Nonetheless, it can still effect the motion of electrons when an electric field is applied to the semiconductor and a current flows. This will be discussed in more detail later in the course. Generally, it can be assumed that the electron concentration at room temperature in n -type semiconductor is just equal to the total concentration of all donors since the overwhelming majority will be ionised;
and the concentration of donors which remain neutral (un-ionised) at 300 K , N 0 D = 0.
Figure 7 : Silicon at T = 0K containing a trace concentration of group V impurity atoms. There are no free charges so the crystal is still an insulator.
Figure 8: Silicon at T > 0K, with the group V impurities ionised and free electrons available for conduction.