How a free electron is generated in a semiconductor Why the

How a free electron is generated in a semiconductor? Why the number of free electrons and holes are not always the same?

2- What are valence band, conduction band, and forbidden gap in an energy band diagram? Explain how insulators, semiconductors, and conductors are classified based on their energy band diagram.

3- Silicon and germanium are two semiconductors with different atomic number and bandgap energy. Verify the fact that the semiconductor with the larger atomic number has smaller bandgap energy and larger intrinsic carrier density ( ). Justify your observation.

4- What is an n-type semiconductor? What type of impurities are added to form an n-type semiconductor?

5- What is mobility? Explain why free electrons have higher mobility than holes in silicon.

6- By increasing temperature, how does change? Why?

7- What is drift current? Why both free electrons and holes are considered in calculating drift

current.

8- What is diffusion current? Assume ( ) = h > 0 find the diffusion current for holes.

9- In an experiment, that you saw in a video clip in the class, by exposing a piece of semiconductor

to light its resistance dropped. Explain why this happened.
10- In a semiconductor, two types of impurity atoms are added. The acceptor atom density is 1018

/cm3 and the donor atom density is 1015 /cm3. If the intrinsic carrier density is 1012/cm3, find the density of holes and electrons in the semiconductor.

Solution

1. A free electron will generate when force is applied for an atom . Then by this force one electron will leave valence shell of the atom . In this process free electron will generate.

Actually holes means vacant places . Electrons will occupy holes . As Electrons are more in any conductors so no of Electrons is not equal to holes.

2 valence band is upper band .

Conduction band is lower band , in conduction band free Electrons will more .

When we given force free Electrons will acquire energy and will come from conduction band to valence band .

Forbidden band is middle band between conduction band and valence band

In insulators forbidden band is very very large and Electrons cannot come to valence band from conduction band .

In conductors forbidden band is very small so when we given small energy also Electrons will easily come to valence band

In semiconductor forbidden band is medium .

4 semiconductor is of two types intrinsic and extrinsic semiconductor .

In intrinsic semiconductor no of Electrons is equal to no of holes .

In extrinsic semiconductor we will add impurities

Ntype we will add phosphorus as phosphorus is having 5 valence electrons and combine with silicon with 4 valence electrons ,

So 4 plus 5 = 9 valence electrons .

Actually an atom is stable if it is having 8 valence electrons .

So in Ntype one electron. Is more . So Ntype is having more no of Electrons .

Ptype will have more no of holes .

5 mobility :- The mass of an electron is significantly less than that of a hole therefore making it more mobile. And yes, holes are considered to have mass, effectively