Semiconductor MCQ | Semiconductor Question and Answer with explanation

Answer.3. Free electrons and holes

Explanation:-

In an intrinsic semiconductor, even at room temperature, electron-hole pairs are created. When an electric field is applied across an intrinsic semiconductor, the current conduction takes place due to free electrons and holes Under the influence of electric field, total current through the semiconductor is the sum of currents due to free electrons and holes.

Though the total current inside the semiconductor is due to free electrons and holes, the current in the external wire is fully by electrons. 

Answer.1. 1

Explanation:-

A perfect semiconductor with no impurities or lattice defects is called an intrinsic semiconductor. The generation of a hole in an intrinsic semiconductor is accomplished by breaking a covalent bond of the crystal producing a free electron and a vacancy in the lattice. Since free electrons and holes are created in pairs, the number of free electrons in the conduction band is always equal to the number of holes in the valence band i.e the ratio of the electron-hole pair is equal to 1.

.

Answer.4. N-type semiconductor

Explanation:-

N-type semiconductors are created by doping an intrinsic semiconductor with donor impurities. In an n-type semiconductor, the Fermi energy level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band.

As shown in the figure, each of the four out of five valency electrons of impurity says of Arsenic enters into covalent bonds with Germanium, while the fifth valence electron is set free to move from one atom to the other. The impurity is called donor impurity as it donates an electron and the crystal is called N-type semiconductor. A small amount of Arsenic (impurity) injects billions of free electrons into Germanium thus increasing its conductivity enormously. In N-type semiconductor, the major carriers of charge are the electrons and holes are minority carriers. This is because when donor atoms are added to a semiconductor, the extra free electrons give the semiconductor a greater number of free electrons than it would normally have And, unlike, the electrons that are freed because of thermal agitation, donor electrons do not produce holes. As a result, the current carriers in a semiconductor doped with pentavalent impurities are primarily negative electrons.

The impurity atom has five valence electrons. After donating one electron, it is left with + I excess charge. It then becomes a positively charged immobile ion. It is immobile because it is held tightly in the crystal by the four covalent bonds.

It is important to understand that in N-type semiconductors, although electrons (negative charges) are the majority carriers, but the semiconductor doped with impurity remains electrically neutral. Free electrons and holes are generated in pairs due to thermal energy and negative charge of electrons donated by impurity atoms is exactly balanced by the positive charge of the immobile ions. 

Answer.2. Decrease

Explanation:-

Intrinsic conductivity:- Semiconductors are conductive beyond a certain temperature level as valence electrons are released from their chemical bonds with increasing temperatures and thus reach the conduction band (intrinsic conductivity). They become conduction electrons that are able to move freely through the crystal lattice (i.e. electron conduction).

On the other hand, also the resulting hole inside the valence band can move through the semiconductor material since a neighboring electron can advance to the hole. Holes thus contribute equally to conductivity (hole conduction). Since every free electron creates a hole within undisturbed pure semiconductor crystals both types of charge carriers equally exist.

Intrinsic conductivity is counteracted by recombination, namely the recombination of a free electron and a positive hole. Despite this recombination, the number of holes and free electrons remains equal since at a certain temperature level always the same number of electron-hole-pairs are fined as recombining. For every temperature, there thus exists an equilibrium state with a certain number of free holes and free electrons. The number of free electron-hole-pairs increases with rising temperature.

As the temperature is decreased to 100 K, therefore, less number of the electron-hole pair will be generated hence the conductivity of the semiconductor will be low.

Answer.1. Increase

Explanation:-

As the temperature is increased, therefore, more number of the electron-hole pair will be generated hence the conductivity of the semiconductor will be increased.

Note:- For more information check question number 74

Answer.3. Free Electrons

Explanation:-

The mobility is proportional to the carrier relaxation time and inversely proportional to the carrier effective mass. The electron mobility is often greater than hole mobility because quite often the electron effective mass is smaller than hole effective mass. The electron and hole effective masses in Si are 0.26 and 0. 38m_e, respectively, suggesting higher electron mobility than hole mobility in Si (in Si, µn ≈1400 cm²/V s and µ_p ≈ 500 cm²/V. 

The relaxation times are often of the same order of magnitude for electrons and holes, and therefore, they do not make too much difference. In order to increase the speed of a device, one has to choose materials with small electron and hole effective masses and long relaxation times, i.e. where the electrons and holes do not have to experience too much collision on crystal imperfections, impurities, etc.  

Answer.4. Temperature

Explanation:-

The materials which have resistivities lying between those of an insulator and a conductor are known as semiconductors. At absolute zero. pure and perfect crystals of the semiconductors are non-conducting, their resistivity approaching the resistivity of an insulator. They can be made conducting by adding impurities, and due to thermal, agitation, lattice defects etc. The resistivity of a semiconductor depends upon the temperature and it decreases with rising of temperature; consequently, a semiconductor crystal becomes conducting even at room temperature. At room temperature, their resistivity lies in the range 10² to 10⁹ ohm-cm and is thus intermediate between the resistivity of a good conductor (10^-6 ohm-cm) and an insulator (10¹⁴ to 10²² ohm-cm). At very low temperature a semiconductor behaves as an insulator.

Answer.2. 0 eV

Explanation:-

For a conductor, valence band and conduction band overlap so that forbidden energy gap is 0 eV.

Answer.3. Perfect insulator

Explanation:-

At 0 K an intrinsic semiconductor behaves as an insulator. The crystal structure of Si (or Ge) is a tetrahedron with an atom at each vertex. Each Si atom in the crystal contributes four valence electrons so that the atom is tetravalent. The inert ionic core of the Si atom carries a positive charge of +4e. The binding forces between neighboring atoms result from the fact that each Si atom shares its four valence electrons, which are tightly bound to the nucleus, with the four neighboring atoms forming four covalent bonds. The covalent bond is a fairly strong bond, requiring a fair amount of energy to release an electron from it. At a very low temperature ( 0K), the intrinsic Si approaches the ideal structure, and the crystal behaves as an insulator since no free charge carriers are available for conduction to occur.

The band structure of the intrinsic semiconductor at 0K is shown in Figure. The conduction band is totally unoccupied and the valence band has completely filled The absence of electrons in the conduction band does not allow current to flow under the influence of an electric field. Therefore, they are insulators at low temperatures.

Answer.4. Decrease with the increase in temperature

Explanation:-

The average velocity with which free electrons get drifted in a conductor under the influence of an electric field applied across the conductor is called drift velocity.

The drift velocity of electrons in a conductor decreases with the increase in temperature of the conductor. It is because the resistance of a conductor increases with the increase in temperature.