 JEE Mains Previous Years Questions with Solutions

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1

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 $Si$ and $Ge$ but small in $C.$
B
The number of free conduction electrons is significant in $C$ but small in $Si$ and $Ge.$
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

Explanation

$Si$ and $Ge$ are semiconductors but $C$ is an insulator. Also, the conductivity of $Si$ and $Ge$ is more than $C$ because the valence electrons of $Si, Ge$ and $C$ lie in third, fourth and second orbit respectively.
2

AIEEE 2006

If the lattice constant of this semiconductor is decreased, then which of the following is correct? 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

Explanation

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 $\left( {Eg} \right)$, 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.
3

AIEEE 2006

In the following, which one of the diodes is reverse biased?
A B C D Explanation

$p$-side connected to low potential and $n-$ side is connected to high potential.
4

AIEEE 2006

The circuit has two oppositively connected ideal diodes in parallel. What is the current following in the circuit? A
$1.71$ $A$
B
$2.00$ $A$
C
$2.31$ $A$
D
$1.33$ $A$

Explanation

${D_2}$ is forward biased whereas ${D_1}$ is reversed biased.

So effective resistance of the circuit

$R = 4 + 2 = 6\Omega$

$\therefore$ $i = {{12} \over 6} = 2\,A$