Semiconductor — Page 2 | JEE Physics

Q11

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

Correct Answer

Option B

Solution

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.

Q12

By increasing the temperature, the specific resistance of a conductor and a semiconductor

A increases for both

B decreases for both

C increases, decreases

D decreases, increases

Correct Answer

Option C

Solution

Specific resistance is resistivity which is given by

\rho = {m \over {m{e^2}\,\tau }}

where

n=no.

of free electrons per unit volume and $\tau$ $=$ average relaxation time For a conductor with rise in temperature

n

increases and $\tau$ decreases. But decrease in $\tau$ is more dominant than increase in

n

resulting an increase in the value of $\rho$ For a semiconductor with rise in temperature,

n

increases and $\tau$ decreases. But the increase in

n

is more dominant than decrease in $\tau$ resulting in decrease in the value of

\rho .

Q13

A piece of copper and another of germanium are cooled from room temperature to

77K,

the resistance of

A copper increases and germanium decreases

B each of them decreases

C each of them increases

D copper decreases and germanium increases

Correct Answer

Option D

Solution

Copper is a conductor, so its resistance decreases on decreasing temperature as thermal agitation decreases; whereas germanium is semiconductor therefore on decreasing temperature resistance increases.

Q14

When

p

-

n

junction diode is forward biased then

A both the depletion region and barrier height are reduced

B the depletion region is widened and barrier height is reduced

C the depletion region is reduced and barrier height is increased

D both the depletion region and barrier height are increased

Correct Answer

Option A

Solution

Both the depletion region and barrier height is reduced.

Q15

For a transistor amplifier in common emitter configuration for load impedance of

1k\,\Omega

\left( {{h_{fe}} = 50} \right.

and

\left. {{h_{oe}} = 25} \right)

the current gain is

A

-24.8

B

-15.7

C

-5.2

D

-48.78

Correct Answer

Option D

Solution

In common emitter configuration current gain

{A_i} = {{ - h{f_e}} \over {1 + {b_{0c}}{R_L}}}

= {{ - 50} \over {1 + 25 \times {{10}^{ - 6}} \times 1 \times {{10}^3}}}

= - 48.78

Q16

The manifestation of band structure in solids is due to

A Bohr's correspondence principle

B Pauli's exclusion principle

C Heisenberg's uncertainty principle

D Boltzmann's law

Correct Answer

Option B

Solution

Pauli's exclusion principle.

Q17

When

npn

transistor is used as an amplifer

A electrons move from collector to base

B holes move from emitter to base

C electrons move from base to collector

D holes move from base to emitter

Correct Answer

Option C

Solution

Electrons move from base to emmitter.

Q18

At absolute zero, Si acts as

A non-metal

B metal

C insulator

D none of these

Correct Answer

Option C

Solution

Pure silicon, at absolute zero, will contain all the electrons in bounded state.

The conduction band will be empty.

So there will be no free electrons (in conduction band) and holes (in valence band) due to thermal agitation.

Pure silicon will act as insulator.

Q19

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

Correct Answer

Option A

Solution

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.

Q20

A working transistor with its three legs marked

P,Q

and

R

is tested using a multi-meter. No conduction is found between

P

and

Q

. By connecting the common (negative) terminal of the multi-meter to

R

and the other (positive) terminal to

P

or

Q,

some resistance is seen on the multi-meter. Which of the following is true for the transistor?

A It is an

npn

transistor with

R

as base

B It is an

pnp

transistor with

R

as collector

C It is an

pnp

transistor with

R

as emitter

D It is an

npn

transistor with

R

as collector

Correct Answer

Option A

Solution

It is a

n

-

p

-

n

transistor with

R

as base.