Silicon and Germanium

• The atomic structures of silicon and germanium are compared in Figure 1–9;

• Silicon is used in diodes, transistors, integrated circuits, and other semiconductor devices;
• Notice that both silicon and germanium have the characteristic four valence electrons;
• The valence electrons in germanium are in the fourth shell while those in silicon are in the third shell, closer to the nucleus;
• This means that the germanium valence electrons are at higher energy levels than those in silicon and, therefore, require a smaller amount of additional energy to escape from the atom;
• This property makes germanium more unstable at high temperatures and results in excessive reverse current;
• This is why silicon is a more widely used semiconductive material;

Covalent Bonds

• Figure 1–10 shows how each silicon atom positions itself with four adjacent silicon atoms to form a silicon crystal;

• A silicon (Si) atom with its four valence electrons shares an electron with each of its four neighbors;
• This effectively creates eight shared valence electrons for each atom and produces a state of chemical stability;
• Also, this sharing of valence electrons produces the covalent bonds that hold the atoms together;
• Each valence electron is attracted equally by the two adjacent atoms which share it;
• Covalent bonding in an intrinsic silicon crystal is shown in Figure 1–11;

• An intrinsic crystal is one that has no impurities;
• Covalent bonding for germanium is similar because it also has four valence electrons;