# Class 12th Physics – 14 Semiconductor Electronic Materials MCQs

#### When a p-n junction is formed, the process of diffusion of charge carriers happens due to ____.

Correct! Wrong!

In an n-type semiconductor, the concentration of electrons is more compared to the concentration of holes.
In a p-type semiconductor, the concentration of holes is more than the concentration of electrons.
During the formation of p-n junction, and due to the concentration gradients across the p and n sides, holes diffuse from p-side to n-side and electrons diffuse from n-side to p-side. This motion of charges carriers gives rise to diffusion current across the junction.
Hence, when a p-n junction is formed, the process of diffusion of charge carriers happens due to concentration gradient

#### Which of the following statement is true ____.

Correct! Wrong!

Option A is wrong because an IC will have many active devices (like diode, transistor) and also passive devices like resistors and capacitors.
Option B is wrong because an operational amplifier is a linear IC.
Option C is right because NOR is negative OR. If any one of the inputs is 1, the output is 0. If both the inputs are low, the output is high.
Option D is wrong because digital signals take discrete values 0 or 1, which can vary with time.

#### In an intrinsic semiconductor (at T > 0) electrons move from ____.

Correct! Wrong!

Thermal energy at temperatures higher than 0 K excites some electrons from the valence band to the conduction band. These thermally excited electrons partially occupy the conduction band. The electrons move from the valence band to the conduction band and leave an equal number of holes in the valence band.

#### Why diamond behaves like an insulator?

Correct! Wrong!

In the energy band diagram of diamond, there is an large energy gap = 5.54 ev due to which no electron can go from valence band to conduction band

#### Under the influence of an electric field, the current produced in an intrinsic semiconductor is equal to ____.

Correct! Wrong!

A free electron produced under an applied electric field moves completely independently as conduction electron and gives rise to an electron current, Ie under an applied electric field. A hole thus produced moves towards negative potential of the electric field, thus giving the hole current Ih. The total current, I is thus the sum of the electron current Ie and the hole current Ih.
I = Ie + Ih

#### Si or Ge doped with a donor impurity produces ____.

Correct! Wrong!

A donor impurity is a pentavalent dopant which donates one extra electron for conduction. Such doping will therefore increase the number of conduction electrons than the number of holes. Thus, Si or Ge doped with a donor impurity produces an n - type semiconductor whose majority carriers are electrons.

#### The process of diffusion during the formation of a p-n junction causes formation of a layer of ____ .

Correct! Wrong!

When an electron diffuses from the n-side to the p-side, it leaves behind an ionized donor on n-side. This ionised donor (which is a positive charge) is immobile, as it is bonded to the surrounding atoms. As the electrons continue to diffuse from n to p, a layer of positive charge (or positive space-charge region) on the n-side of the junction is formed. In the same way, when a hole diffuses from p to n due to the concentration gradient, it leaves behind an ionised acceptor (negative charge) which is immobile. As the holes continue to diffuse, a layer of negative charge (or negative space-charge region) on the p-side of the junction develops.

#### The temperature coefficient of resistance of semiconductors is ___ .

Correct! Wrong!

A positive temperature coefficient means that the resistance of the given material increases with increase in temperature. Thus, metals typically have positive temperature coefficient of resistance. A negative temperature coefficient means that the resistance of the given material decreases with increase in temperature. Semiconductor materials, like Si and Ge, typically have negative temperature coefficients of resistance.

#### Which of the following statements is not true ____

Correct! Wrong!

Option D is the wrong statement, because, electron energy in the same orbit will be the same. As we move from lower orbits to higher orbits, the energy of the electron increases.
Option A is true because, the energy band gap in an insulator is very high. It is not possible to thermally excite electrons to move from the valence band to the conduction band.
Option B is true for Si and Ge atoms.
Option C is also true. In case of metals, there will be large number of electrons for conduction. This is because of electrons in the partially filled valence band and conduction band or due to the overlapping of valence band and conduction band.

#### Choose the true statement from the following ____.

Correct! Wrong!

Diamond is an insulator. So, it will have higher resistivity than copper, which is a metal. So, option C is the right choice.
As silicon is a semiconductor, the conductivity increases with increase in temperature. So, option A is wrong.
Insulators have zero conductivity. So, option B is wrong.
Silicon is a semiconductor and aluminium is a metal. So, conductivity of Aluminium is more than that of Silicon. Hence, option D is wrong.

#### The depletion layer of a p-n junction consists of ____.

Correct! Wrong!

In a p-n junction, when an electron diffuses from the n-side to p-side, it leaves behind an ionised donor on the n-side. This donor (positive charge) is immobile, as it is bonded to the surrounding atoms. Similarly, when a hole diffuses from p to n due to the concentration gradient, it leaves behind an ionised acceptor (negative charge) which is immobile. Therefore, the depletion layer of a p-n junction consists of immobile ionized donors and acceptors.

#### In case of insulators, the valence band is ____and the conduction band is ____.

Correct! Wrong!

In case of insulators, a large band gap Eg exists (Eg> 3 eV).
There are no electrons in the conduction band, and therefore no electrical conduction is possible. The energy gap is so large that electrons cannot be excited from the valence band to the conduction band by thermal excitation.

#### The thickness of the depletion region of a p-n junction is about one tenth of a ____ .

Correct! Wrong!

The depletion of a p-n junction is the space charge region (+ve charge on the n-side and -ve charge on the p-side). The width of this region will be comparable to a tenth of a micrometer. The other units given above are very much larger than the dimension of the entire p-n junction itself.

#### Identify the true statement ____.

Correct! Wrong!

Antimony is a pentavalent impurity, (i.e.) a donor. So, the resulting semiconductor is n - type. Hence, "A" is true.
The conductivity increases with increase in temperature in case of a semiconductor. So, option "B" is wrong.
Option "C" is wrong since majority carriers are electrons in an n- type semiconductor.
Aluminium is a trivalent impurity. So, doping with it will result in a p - type semi conductor. Hence, option D is wrong.

#### The drift current in a p-n junction flows____.

Correct! Wrong!

A p-n junction formation creates a positive space-charge region on n-side of the junction and negative space charge region on p-side of the junction. Thus, an electric field directed from positive charge towards negative charge develops. Due to this field, an electron on p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side. This motion of charge carriers due to the electric field is called drift, and the current opposite to the direction of the diffusion current is called is called the drift current. As the direction of flow of electrons is from p-side to n-side, the direction of flow of current is from the n-side to the p-side.

#### The barrier potential opposes the movement of ____ .

Correct! Wrong!

The process of diffusion accounts for the loss of electrons from the n-region and the gain of electrons by the p-region. This causes a difference of potential across the junction of the two regions. The polarity of this potential is such as to oppose further flow of carriers, so that a condition of equilibrium exists. The n-material is thus positive (due to loss of electron) relative to the p-material (due to gain of electron). Since this potential tends to prevent the movement of electrons from the n-region into the p-region, it is called as barrier potential.

#### In a p-type semiconductor, the concentration of electrons is ____ .

Correct! Wrong!

In p-type semiconductors, holes are the majority carriers and electrons are minority carriers. The concentration of electrons will therefore, be lower than that of holes. So, option "A" is right. As doping increases the number of holes in a p-type semiconductor, the hole concentration cannot be equal to the intrinsic carrier concentration of a pure crystal. Hence, option "D" is wrong.

#### An n - type semiconductor ____.

Correct! Wrong!

The doping process introduces additional charge carriers. But, the semiconductor crystal maintains an overall charge neutrality (i.e.) it is electrically neutral (neither positively charged, nor negatively charged). This is because the charge of additional charge carriers is equal and opposite to that of ionized cores in the lattice.

#### If the interatomic spacing becomes equal to actual crystal lattice spacing then at absolute zero ____.

Correct! Wrong!

At absolute zero for interatomic spacing becoming equal to actual crystal lattice spacing distance the 4N filled energy levels get separated from 4N unfilled energy levels.

#### At equilibrium, in an unbiased p-n junction, the net current is ____

Correct! Wrong!

During the initial formation of p-n junction, the diffusion current is large and drift current is small. As the diffusion process continues, the space-charge regions on either side of the junction extend, thus increasing the electric field strength increases the drift current. The process continues until the diffusion current equals the drift current. Now the p-n junction is formed. The loss of electrons from the n-region and the gain of electrons by the p-region causes a difference of potential across the junction of the two regions. The polarity of this potential is such that it will oppose further flow of carriers and therefore, a condition of equilibrium exists. Thus, in a p-n junction under equilibrium, the net current is zero.

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

Correct! Wrong!

The energy gap of Si and Ge are small when compared with C at room temperature. Therefore conduction electrons are significant in Si, and Ge rather than in C.

#### If equal number of Antimony and Phosphorous atoms are added to a silicon crystal, then ____.

Correct! Wrong!

Antimony and Phosphorous are both pentavalent dopants. Therefore, doping with these impurities will make the silicon to become an n - type semiconductor. (Pentavalent dopants increase the number of conduction electrons, which are majority carriers of an n - type semiconductor).

#### A zener diode is characterised by ____ .

Correct! Wrong!

Zener diode is fabricated by heavily doping both the p and n regions of the junction. Due to this, the depletion region formed is very thin and the electric field at the junction is extremely high, even for a small reverse bias voltage. As electric field is inversely proportional to the depletion layer width, options "B" and "C" are wrong.

#### When the p-n junction is reverse biased, ____.

Correct! Wrong!

When a diode is reverse biased, the n-side is positive and p-side is negative. The direction of applied voltage is same as the direction of barrier potential. Therefore, the barrier height increases and the depletion region widen.

#### Integrated circuits are produced on a piece of semiconductor crystal using a process called ____.

Correct! Wrong!

Integrated circuits are produced on a piece of semiconductor crystal or a chip using a process called Photolithography.

#### At the temperature T = 0 K, Silicon behaves as a (n) ____.

Correct! Wrong!

Silicon is an intrinsic semiconductor. So, at absolute temperature, (i.e) T = 0 K, it will behave like behave like an insulator, due to absence of free electrons in the conduction band. Thermal energy at temperatures higher than 0 K is responsible for excitation of electrons from the valence band to the conduction band.

#### A small strip of silver and another of silicon are cooled from room temperature to 80 K. The conductivity of silver____ and that of silicon ____.

Correct! Wrong!

With a decrease in temperature (as given by 'cooled from room temperature to 80 K'), the conductivity of silver increases (as silver is a metal). Silicon is a semiconductor. So, its conductivity will decrease with decrease in temperature.

#### The probability of electrons to be found in the conduction band of an intrinsic semiconductor at a finite temperature.

Correct! Wrong!

As band gap increases electrons require increasing energy values to jump from valence band to conduction band, therefore the availability of electrons decreases exponentially with increasing band gap.

#### Materials A, B and C fall under the three different classifications of materials, based on resistivity. 'A' has a resistivity greater than that of B, but less than that of C. A, B and C are ____ respectively.

Correct! Wrong!

Metals, semiconductors and insulators are the major classification of materials. Since material A has resistivity greater than material B, but less than that of C, "A" is a semiconductor, B is a metal and C is an insulator.

#### Some properties of semiconductor are ____.

Correct! Wrong!

A semiconductor has negative temperature coefficient with crystalline structure formed due to covalent bond.

Class 12th Physics - 14 Semiconductor Electronic Materials MCQs
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