Chemistry, Third edition

12 · CHEMICAL FAMILIES

Semiconductors

Silicon and germanium are used as semiconductors. Semiconductors have electrical

resistances somewhere between those for conductors and insulators and the

electrical resistance of a semiconductor decreases as the temperature gets higher. In

metals, which are good conductors, the valence electrons are able to break free from

the individual atoms and move through the metal when a potential difference is

applied across it. In insulators, the valence electrons cannot move freely around the

material in the same way. The valence electrons in semiconductors, however, can

break free of their atoms if a little energy is applied. As the temperature rises more

valence electrons are able to break away (become delocalized) and the electrical

conduction of the substance increases. The semiconducting properties of silicon are

used in integrated circuits known as silicon chips. The element silicon contains

many silicon atoms covalently bonded to each other in tetrahedral arrangements so

that a giant molecule is built up. At very low temperatures, the electrons in the co-

valent bonds that hold the silicon atoms in place tend to stay associated with their

‘parent’ atoms. As silicon is heated, however, more of these electrons gain sufficient

energy to delocalize and the resistance of the silicon decreases.

202

Semiconductors

(i)iDiamond is a very good

insulator because the

valence electrons around

its carbon atoms tend to

stay firmly in place. Why

do the electrons in Si–Si

covalent bonds delocalize

much more easily?

(ii)In a sample of silicon, if

electrons from Si–Si

covalent bonds are set

free, what stops the

structure from

collapsing?

Exercise 12J

BOX 12.5

Doping semiconductors

The electrical properties of semiconductors

can be changed by adding small amounts of

other substances. This process is known as

doping. In n-typesemiconductors, a small

number of Group 15 element atoms, such as

arsenic, are spread out in a sample of silicon.

The arsenic atoms use four of their five

valence electrons to bond with surrounding

silicon atoms and the fifth electron is

delocalised in the structure. This makes the

substance a much better electrical conductor

than pure silicon. The name n-type is derived

from the fact that the doped material has an

excess of negative charge. This is shown in

Fig. 12.3

If small traces of a Group 13 element, such

as boron, are added to silicon then a

deficiency of electrons is created. Boron has

only three valence electrons to bond with four

surrounding silicon atoms. The incomplete

covalent bond created contains a ‘hole’

where an electron should be. A neighbouring

valence electron from a silicon atom can jump

into this hole, creating a new hole ready to be

occupied by another electron, and so on. In

this way electrons move throughout the

sample and, again, the doped substance is a

better conductor of electric current. These

type of doped semiconductors are called

p-type; the prefers to an excess of positive

charge.

Fig. 12.3Ann-type semiconductor.

Hydrolysis of Group 14 chlorides

(i)iWrite a balanced equation for the hydrolysis of SiCl 4.

(ii)In the hydrolysis of SiCl 4 , Si–Cl bonds must be broken. Why is the Si–Cl bond easier to

break than a C–Cl bond?

Exercise 12I