JEE Main Semiconductor PYQ

11. (AIEEE 2007)

Carbon, silicon and germanium have four valence electrons each. At room temperature which one of the following statements is most appropriate ?

A. The number of free electrons for conduction is significant only in $S i$ and $G e$ but small in $C .$

B. The number of free conduction electrons is significant in $C$ but small in $S i$ and $G e .$

C. The number of free conduction electrons is negligibly small in all the three.

D. The number of free electrons for conduction is significant in all the three

The Correct Answer is Option (A)

$S i$ and $G e$ are semiconductors but $C$ is an insulator. Also, the conductivity of $S i$ and $G e$ is more than $C$ because the valence electrons of $S i, G e$ and $C$ lie in third, fourth and second orbit respectively.

12. (AIEEE 2006)

If the ratio of the concentration of electrons to that of holes in a semiconductor is $\frac{7}{5}$ and the ratio of currents is $\frac{7}{4},$ then what is the ratio of their drift velocities?

A. $\frac{5}{8}$

B. $\frac{4}{5}$

C. $\frac{5}{4}$

D. $\frac{4}{7}$

The Correct Answer is Option (C)

$\frac{I_{e}}{I_{h}} = \frac{n_{e} e A v_{e}}{n_{h} e A v_{h}}$

$\Rightarrow \frac{7}{4} = \frac{7}{5} \times \frac{v_{e}}{v_{h}}$

$\Rightarrow \frac{v_{e}}{v_{h}} = \frac{5}{4}$

13. ( AIEEE 2006)

A solid which is not transparent to visible light and whose conductivity increases with temperature is formed by

A. Ionic bonding

B. Covalent bonding

C. Vander Waals bonding

D. Metallic bonding

The Correct Answer is Option (D)

Van der Waal's bonding is attributed to the attractive forces between molecules of a liquid. The conductivity of semiconductors (covalent bonding) and insulators (ionic bonding) increases with increase in temperature while that of metals (metallic bonding) decreases.

14. (AIEEE 2006)

If the lattice constant of this semiconductor is decreased, then which of the following is correct? AIEEE 2006 Physics - Semiconductor Question 159 English

A. All $E_{c}, E_{g}, E_{v}$ increase

B. $E_{c}$ and $E_{v}$ increase, but $E_{g}$ decreases

C. $E_{c}$ and $E_{v}$ decrease, but $E_{g}$ increases

D. All $E_{c}, E_{g}, E_{v}$ decrease

The Correct Answer is Option (C)

A crystal structure is composed of a unit cell, a set of atoms arranged in a particular way; which is periodically repeated in three dimensions on a lattice. The spacing between unit cells in various directions is called its lattice parameters or constants. Increasing these lattice constants will increase or widen the band-gap $(E g)$ , which means more energy would be required by electrons to reach the conduction band from the valence band. automatically $E_{c}$ and $E_{v}$ decreases.

15. ( AIEEE 2005)

The electrical conductivity of a semiconductor increases when electromagnetic radiation of wavelength shorter than $2480$ $n m$ is incident on it. The band gap in $(e V)$ for the semiconductor is

A. $2.5$ $e V$

B. $1.1$ $e V$

C. $0.7$ $e V$

D. $0.5$ $e V$

The Correct Answer is Option (D)

Band gap $=$ energy of photon of wavelength $2480$ $n m .$ So,

$Δ E = \frac{h c}{λ}$

$= (\frac{6.63 \times 10^{- 34} \times 3 \times 10^{8}}{2480 \times 10^{- 9}}) \times \frac{1}{1.6 \times 10^{- 19}} e V$

$= 0.5 e V$

Semiconductor Electronics