# be2

## What are conductors,insulators and semiconductors?

Conductors: The materials which permits the flow of charge carriers with higher conductivity are called conductors. In conductor, the valance band and conduction band overlaps with out any energy gap(EG). The conductivity of a conductor is normally  greater than 103. The examples of conductors are gold, silver, copper etc.

Insulators: The materials which blocks the flow of electrons or which offers higher resistivity to the flow of electrons are called insulators. The energy  gap between valency and conduction bad in the insulators is greater than 1eV. The conductivity of insulator is less than 10-7 and the examples of insulators are rubber, glass, Teflon, mica etc.

Semiconductors: The materials which are partially conductive and whose conductivity can be controlled are called semiconductors. These are most useful materials in electronic circuits. The energy gap Eg between valency and conduction bad of semiconductors are almost equal to 1eV. Silicon and Germanium are the best examples of semiconductors.

#### What are the advantages of silicon over germanium?

• Easily available in nature.
• Cost is low compared to germanium
• Higher temperature operating ranges compared to germanium.
• Wider band gap than germinum
• Less noisy compared to germanium
• The material properties are accurately controlled

#### What are the advantages and disadvantages of semiconductor over other devices?

• These are smaller in size
• Long life compared to vacuum tubes.
• Operated on low DC power
• Accuracy is high compared to vacuum tubes
• Noise is less
• Warm up is not needed in semiconductors.

• Cannot withstand for high power.
• Frequency range of operation is low.
• produces less output power.
• Accuracy changes with the temperature.
• Low ambient temperature.

#### What is a mobility of a charge carrier?

The mobility of a charge carrier is the velocity per unit electri field. It is enoted as μ and its units are m2/v-sec.

$\mu&space;=&space;\frac{v}{\varepsilon&space;}$

Note: As the temperature increases the mobility of charge carrier decreases because of the random motion of charge carriers.

#### What is mass action law and law of electrical neutrality?

Mass Action law:  It states that in an intrinsic semiconductor the product of free electrons ‘n’ and free holes ‘p’ is constant. I.e.

np = ni2

Law of electrical neutrality: It states that when no voltage is applied to semiconductor, the magnitude of positive charge density must equals that of negative charge density.

Total positive charge density              = ND+p

Total negitive charge density              = NA+p

i.e.

ND + P=NA + n

Where ND and NA are donor and acceptor densities.

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