Tham khảo sách 'chemistry part 36, julia burdge,2e (2009)', khoa học tự nhiên, hoá học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 890 CHAPTER 23 Metallurgy and the Chemistry of Metals Figure a Silicon crystal doped with phosphorus b Silicon crystal doped with boron. Note the formation of a negative center in a and a positive center in b . for example when a trace amount of boron or phosphorus is added to solid silicon. Only about five out of every7 million Si atoms are replaced by B or P atoms. The structure of solid silicon is similar to that of diamond that is each Si atom is covalently bonded to four other Si atoms. Phosphorus Ne 3v23 r has one more valence electron than silicon Ne 3s23p2 so there is a valence electron left over after four of them are used to form covalent bonds with silicon Figure . This extra electron can be removed from the phosphorus atom by applying a voltage across the solid. The free electron can move through the structure and function as a conduction electron. Impurities of this type are known as donor impurities because they provide conduction electrons. Solids containing donor impurities are called n-type semiconductors where n stands for negative the charge of the extra electron . The opposite effect occurs if boron is added to silicon. A boron atom has three valence electrons 1522s 22p one less than silicon. Thus for every boron atom in the silicon crystal there is a single vacancy in a bonding orbital. It is possible though to excite a valence electron from a nearby Si into this vacant orbital. A vacancy created at that Si atom can then be filled by an electron from a neighboring Si atom and so on. In this manner electrons can move through the crystal in one direction while the vacancies or positive holes move in the opposite direction and the solid becomes an electrical conductor. Impurities that are electron deficient are called acceptor impurities. Semiconductors that contain acceptor impurities are called p-type semiconductors where p stands for positive. In both the p-type and -type semiconductors the energy gap between the valence band and .
