Besides silicon, group IVa of the periodic system contains also the isovalent elements carbon, germanium, tin, and lead. Carbon, the lightest of them, is an omnipresent impurity introduced often unintentionally. On the other hand, as discussed in Section 4.1, it is used for a variety of applications from stress compensation to proximity gettering. The properties of germanium in silicon are summarized in Section 4.2. Unfortunately, the space would not have sufficed for a presentation of silicon-germanium materials in general. Tin, the last isovalent impurity considered here, has already limited technical applications. However, it was studied to investigate basic diffusion processes in silicon. The main results will be presented in Section 4.3.