N-Type Semiconductor

• To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity atoms are added;
• These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb);
• As illustrated in Figure 1–17, each pentavalent atom (antimony, in this case) forms covalent bonds with four adjacent silicon atoms;

• Four of the antimony atom’s valence electrons are used to form the covalent bonds with silicon atoms, leaving one extra electron;
• This extra electron becomes a conduction electron because it is not involved in bonding;
• Because the pentavalent atom gives up an electron, it is often called a donor atom;
• The number of conduction electrons can be carefully controlled by the number of impurity atoms added to the silicon;
• A conduction electron created by this doping process does not leave a hole in the valence band because it is in excess of the number required to fill the valence band;

Majority and Minority Carriers

• Since most of the current carriers are electrons, silicon (or germanium) doped with pentavalent atoms is an n-type semiconductor (the n stands for the negative charge on an electron);
• The electrons are called the majority carriers in n-type material;
• Although the majority of current carriers in n-type material are electrons, there are also a few holes that are created when electron-hole pairs are thermally generated;
• These holes are not produced by the addition of the pentavalent impurity atoms;
• Holes in an n-type material are called minority carriers;