### Physics - Electronic Devices

#### Question - 1

The correct relation for the cohesive energy is

• A cohersive energy = free energy of atoms - crystal energy
• B cohersive energy = free energy of atoms + crystal energy
• C cohersive energy = $\frac { free\quad energy\quad of\quad atoms }{ crystal\quad energy }$
• D cohersive energy = free energy of atoms x crystal energy

#### Question - 2

In an unbaised p-n junction, holes diffuse from the p-region to n-region because

• A free electrons in the n-region attract them
• B they move across the junction by the potential difference
• C hole concentration in p-region is more as compared to n-region
• D all the above

#### Question - 3

A spectrum of silicon is to be made p-type semiconductor. For this one atom of indium, on an average, is doped in 5 x ${ 10 }^{ 7 }$silicon atoms. If the number density of silicon is 5 x ${ 10 }^{ 28 }$atoms/${ m }^{ 2 }$, then the number of acceptor atoms per ${ cm }^{ 3 }$will be

• A 2.5 x${ 10 }^{ 30 }$
• B 1.0 x ${ 10 }^{ 13 }$
• C 1.0 x ${ 10 }^{ 15 }$
• D 2.5 x ${ 10 }^{ 36 }$

#### Question - 4

In the ration of the concentration of electrons and of holes in a semiconductor is 7/5 and the ration of currents is 7/4, then what isthe ration oftheir drift velocities?

• A 4/7
• B 5/8
• C 4/5
• D 5/4

#### Question - 5

The conductivity of a semiconductor increases with increase in temperature because

• A number density of free current carriers increase
• B relaxation time increases
• C both number density of carriers and relaxation time increase
• D number density of current carriers increases, relaxation time decreases but effect of decerese in relaxation time is much less than increase in number density

#### Question - 6

The ratio of electron and hole currents in a semiconductor is $\frac { 5 }{ 4 }$ and the ration of drift velocities of electrons and holes is $\frac { 7 }{ 4 }$, then the ration of concentrations of electrons and holes will be

• A $\frac { 25 }{ 49 }$
• B $\frac { 49 }{ 25 }$
• C $\frac { 7 }{ 5 }$
• D $\frac { 5 }{ 7 }$

#### Question - 7

The resistivity of an n-type extrinsic semiconductor is 0.25 $\Omega$-m. If the electron mobility is 8.25 ${ m }^{ 2 }$/v-s, then the concentration of donor atoms will be (in ${ m }^{ -3 }$)

• A 3.0 x $10^{ 16 }$
• B 3.0 x $10^{ 17 }$
• C 3.0 x $10^{ 18 }$
• D 3.0 x $10^{ 19 }$

#### Question - 8

Assume that the number of hole- electron pairs in am intrinsic semiconductor is proportional to $e^{ -\triangle E/2kT }$. Here , $\triangle E$ = energy. gap and k = 8.62 x ${ 10 }^{ -5 }$eV/K. The ration of electron-hole pairs at 300 k and 400 k, is

• A ${ e }^{ -5.31 }$
• B ${ e }^{ +5 }$
• C e
• D ${ e }^{ +3 }$

#### Question - 9

THe contribution in the total current flowing through a semiconductor due to electrons and holes are $\frac { 3 }{ 4 }$ and $\frac { 1 }{ 4 }$. If the drit velocity of the electron is $\frac { 5 }{ 2 }$ times that of holes at this temperature, then the ratio of concentration of electrons and holes is

• A 6 : 5
• B 5 : 6
• C 3 : 2
• D 2 : 3

#### Question - 10

The number of densities of electrons and holes in pure silicon at ${ 27 }^{ 0 }$C are equal and its value is 1.5 x ${ 10 }^{ 16 }$${ m }^{ -3 }$. On doping with indium, the hole density increase to 4.5 x ${ 10 }^{ 27 }$${ m }^{ -3 }$. The electron density in doped silicon will be

• A 50 x ${ 10 }^{ 9 }$${ m }^{ -3 }$
• B 5 x ${ 10 }^{ 9 }$${ m }^{ -3 }$
• C ${ 10 }^{ 8 }$${ m }^{ -3 }$
• D ${ 10 }^{ 7 }$${ m }^{ -3 }$     