Semiconductor Meterials

Semiconductors Materials

• We are acquainted with conducting and insulating materials. Conducting materials are good conductors of electricity and are characterized by a large electrical conductivity and small Electrical resistivity. Examples of good conductors are gold, silver, copper, aluminum etc. 

• Insulating materials are characterized by poor electrical conductivity and are used to block current from flowing where it is not to pass. Examples of insulators are mica, porcelain, glass, quartz, rubber, bakelite etc.

• There is another group of materials known as semiconductors having electricalo Conductivities intermediate between metals and insulators.

• The resistivity of various semiconductor materials lies in a very wide range from 10-Q-m to about 0.5 ohm m whereas the resistivities of conductors and insulators are of the order of 10m and 10 Q-m respectively.

• It is significant that the conductivity of semiconducting materials can be varied over orders of magnitude by changes in temperature, optical excitation and impurity content.

• This variability of electrical properties makes the semiconductor materials natural choices for electronic-device investigations.

• Semiconductor materials are found in column IV and neighboring columns of the periodic table as shown in Table 1.1.1.

• The column IV semiconductors, silicon and germanium are called elemental semiconductors because they are composed of single species of atoms. In addition to the elemental materials, compound of column III and column V atoms, as well as certain combinations from II and VI, build up the inter-metallic or compound semiconductors.

Table 1.1.1: Common semiconductor material

(a) The portion of the periodic table where semiconductors occur

B

C

AI SI P

Zn Ga Ge As Se

S

Cd

In

Sn

Sb Te

Table 1.1.1(b): Elemental and compound semiconductors

Elemental

IV compounds

III-V compounds

AIP

II-VI compounds

SIC

AIAS

AISH

ZnSe

ZnTe

Si

Ge

GaP

Cas

CdSe

GaSb

InP

InAs

InSb

• The resistance of conductor increases with the increase in temperature and resistance of insulator remains almost constant upto certain temperature. The resistance of semiconductor decreases with the increase in temperature as shown in Fig. 1.1.1. This implies that the temperature co-efficient of resistance of semiconductors is negative. .

• Semiconductors can be used as a insulator at very low temperature and as a conductor at

high temperature,

• The resistivity of semiconductors decreases in active state i.e. under illumination, under application of electric field. The current flows in the semiconductor under the application of electric field is not proportional to voltage like conductors which obeys Ohms's law.

• But in semiconductors current increases by far more than the applied voltage. This implies that semiconductors are non-linear resistors.

• The electronic and optical properties of semiconductor changes considerably when even minute amounts of certain other substances called the impurities, are added to them.