Intrinsic Semiconductor MCQ Quiz - Objective Question with Answer for Intrinsic Semiconductor - Download Free PDF

Concept:

• The effect of temperature on a semiconductor influences the number of charge carriers in the conduction band.
• As temperature increases, more electrons gain sufficient energy to jump from the valence band to the conduction band, increasing electron concentration (n_e).
• Since electrical conductivity (σ) is proportional to n_e, an increase in n_e leads to an increase in conductivity, thereby decreasing the resistance.

Calculation:

σ = n_e e μ

R ∝ 1 / σ

• Conductivity formula:
• Resistance is inversely proportional to conductivity:
• Since n_e increases with temperature, σ increases, and R decreases.

Explanation of Options:

• Option 1: Incorrect. Both n_e and resistance do not decrease together.
• Option 2: Incorrect. While n_e increases, resistance decreases, not increases.
• Option 3 (Correct): The correct relationship is that n_e increases and resistance decreases.
• Option 4: Incorrect. A decrease in n_e is incorrect since heating a semiconductor excites more electrons into conduction.

Correct Answer:

The correct answer is n_e increases, and resistance decreases.

Concept:

Intrinsic Semiconductor:

• In an intrinsic semiconductor, the number of free electrons (n_e) is equal to the number of holes (n_h) because every electron that is excited to the conduction band leaves behind a hole in the valence band.
• Thus, for an intrinsic semiconductor, the relation between the number of holes and the number of electrons per unit volume is:
  • n_h = n_e
• This is a characteristic feature of intrinsic semiconductors where the carrier concentration of electrons and holes is the same.

Calculation:

For an intrinsic semiconductor, the number of electrons in the conduction band (n_e) is equal to the number of holes (n_h) in the valence band.

⇒ n_h = n_e

∴ The number of holes per unit volume is equal to the number of electrons per unit volume in an intrinsic semiconductor.

The correct answer is - 4.5 × 10³ 

Key Points

• Pentavalent atoms
  • Pentavalent atoms are donors that provide extra electrons in the semiconductor, increasing the number of electrons.
• Electron-Hole Pair Relationship
  • In a doped semiconductor, the product of the electron concentration (n) and hole concentration (p) remains constant and equals the square of the intrinsic carrier concentration (nᵢ).
  • n * p = nᵢ²
• Calculations
  • Given: nᵢ = 1.5 × 10¹⁶ m⁻³
  • Doping concentration of pentavalent atoms = 1 in 10⁵ silicon atoms
  • Total silicon atoms = 5 × 10²⁸ atoms m⁻³
  • Donor concentration (N₅) = (5 × 10²⁸) / (10⁵) = 5 × 10²³ m⁻³
  • n ≈ N₅ = 5 × 10²³ m⁻³
  • p = (nᵢ²) / n = (1.5 × 10¹⁶)² / (5 × 10²³) = 4.5 × 10⁸ m⁻³

Additional Information

• Intrinsic Semiconductor
  • An intrinsic semiconductor is a pure semiconductor without any significant dopant atoms present.
  • Carrier concentration in intrinsic semiconductors is denoted as nᵢ.
• Extrinsic Semiconductor
  • When doped with donors, the semiconductor becomes n-type, increasing the number of electrons.
  • When doped with acceptors, the semiconductor becomes p-type, increasing the number of holes.
• Doping Process
  • Doping involves adding impurity atoms to the semiconductor to modify its electrical properties.
  • The type and concentration of dopants determine whether the semiconductor is n-type or p-type.

Concept:

Calculation of Electrons and Holes in a Doped Si Crystal:

A pure silicon (Si) crystal has a given concentration of atoms and is doped with a pentavalent impurity (arsenic). The doping creates extra electrons (from the pentavalent impurity), and the crystal's electrical properties are altered.

In intrinsic semiconductors, the number of electrons in the conduction band (n_i) is equal to the number of holes in the valence band. However, doping introduces additional electrons, and the number of holes is reduced.

The concentration of electrons and holes can be calculated using the formula:

n = n_i + N_d, where N_d is the donor concentration.

p = n_i² / n, where p is the hole concentration.

Calculation:

Given,

Number of atoms in the Si crystal = 5 × 10²⁸ atoms m⁻³

Pentavalent dopant concentration (As) = 1 ppm (1 ppm = 10⁻⁶)

n₁ = 1.5 × 10¹⁶ m⁻³ (intrinsic carrier concentration)

⇒ N_d = 1 ppm × 5 × 10²⁸ atoms m⁻³

⇒ N_d = 5 × 10²² m⁻³

Next, calculate the number of electrons:

⇒ n = n_i + N_d

⇒ n = 1.5 × 10¹⁶ m⁻³ + 5 × 10²² m⁻³

⇒ n ≈ 5 × 10²² m⁻³

Finally, calculate the number of holes:

⇒ p = n_i² / n

⇒ p = (1.5 × 10¹⁶)² / 5 × 10²²

⇒ p ≈ 4.5 × 10⁹ m⁻³

∴ The number of electrons is approximately 5 × 10²² m⁻³, and the number of holes is approximately 4.5 × 10⁹ m⁻³.

Concept:

• The effect of temperature on a semiconductor influences the number of charge carriers in the conduction band.
• As temperature increases, more electrons gain sufficient energy to jump from the valence band to the conduction band, increasing electron concentration (n_e).
• Since electrical conductivity (σ) is proportional to n_e, an increase in n_e leads to an increase in conductivity, thereby decreasing the resistance.

Calculation:

σ = n_e e μ

R ∝ 1 / σ

• Conductivity formula:
• Resistance is inversely proportional to conductivity:
• Since n_e increases with temperature, σ increases, and R decreases.

Explanation of Options:

• Option 1: Incorrect. Both n_e and resistance do not decrease together.
• Option 2: Incorrect. While n_e increases, resistance decreases, not increases.
• Option 3 (Correct): The correct relationship is that n_e increases and resistance decreases.
• Option 4: Incorrect. A decrease in n_e is incorrect since heating a semiconductor excites more electrons into conduction.

Correct Answer:

The correct answer is n_e increases, and resistance decreases.

CONCEPT:

• Semiconductors: The materials that have a conductivity between conductors and insulators are called semiconductors.
  • Even though carbon lies in the same group of the periodic table as germanium and silicon, it is not a pure or an intrinsic semiconductor.

EXPLANATION:

• When the external voltage becomes greater than the value of the potential barrier for the diode the potential barriers opposition will be overcome and the current will start to flow. This is the conventional current.
• The potential barrier is approx 0.3 volts for germanium and 0.7 volts for silicon while for the carbon it is 7eV.
• This is far much higher for Carbon to make it a semiconductor that has lower forbidden energy gaps (Si and Ge).
• This is why Carbon is not a semiconductor.
• So the correct answer is option 1.

​Additional Information

The correct answer is option 4) i.e. 0 K

CONCEPT:

• Intrinsic semiconductor: An intrinsic semiconductor is a pure semiconductor without any dopant species present in it.
  •  Since there are no impurities present in it, the number of free electrons equals the number of holes in an intrinsic semiconductor.
• Energy band gap: It is the distance between the valence band and conduction band of material.
  • Electrons in the conduction band are responsible for current.
  • The gap between the valence band and conduction band represents the minimum energy required by an electron in the valence band to jump to the conduction band.

EXPLANATION:

• In an intrinsic semiconductor, conduction occurs only when the electrons in the valence band can move to the conduction band.
• To cross over the energy gap between the valence band and conduction band, the electrons will require some amount of energy.
• Hence, at higher temperatures, the electrons will gain energy and jump into the conduction band.
• 0 K is also known as absolute zero. It is the temperature at which the particles in a substance are motionless.
• At 0 K, the electrons will not have the energy to move into the conduction band.
• Thus, there is no conductivity at 0 K in intrinsic semiconductors and it behaves as an insulator.

CONCEPT:

• The material which is not a good conductor or a good insulator is called a semiconductor.
• For example: Silicon, Germanium, etc.
• The charge carriers which are present in more quantity in a semiconductor compared to other particles are called the majority charge carrier.
• The impurity atoms added are called dopants and semiconductors doped with the impurity atoms are called extrinsic or doped semiconductors.
• The pure form of semiconductor is called an intrinsic semiconductor.

Intrinsic Semiconductors:

• A pure semiconductor is called an intrinsic semiconductor. It has thermally generated current carriers.
• They have four electrons in the outermost orbit of the atom and atoms are held together by a covalent bond.
• Fermi energy level lies at the center of the conduction band (C.B) and valence band (V.B) in an intrinsic semiconductor.
• Because of fewer charge carriers at room temperature, intrinsic semiconductors have low conductivity so they have no practical use.

EXPLANATION:

• An ideal, perfectly pure semiconductor (with no impurities) is called an intrinsic semiconductor.
• At absolute zero temperature valence band is full of electrons and the conduction band is empty, hence there are no free electrons in the conduction band and holes in the valence band.
• The charge carrier concentration is zero. Hence intrinsic semiconductor behaves like an insulator
• Hence, at zero temperature the intrinsic semiconductor behaves like an insulator. The correct option is 3.

CONCEPT:

• The material which is not a good conductor or a good insulator is called a semiconductor.
• For example: Silicon, Germanium, etc.
• The charge carriers which are present in more quantity in a semiconductor compared to other particles are called the majority charge carrier.
• The impurity atoms added are called dopants and semiconductors doped with the impurity atoms are called extrinsic or doped semiconductors.
• The pure form of semiconductor is called an intrinsic semiconductor.

Intrinsic Semiconductors:

• A pure semiconductor is called an intrinsic semiconductor. It has thermally generated current carriers.
• They have four electrons in the outermost orbit of the atom and atoms are held together by a covalent bond.
• Fermi energy level lies at the center of the conduction band (C.B) and valence band (V.B) in an intrinsic semiconductor.
• Because of fewer charge carriers at room temperature, intrinsic semiconductors have low conductivity so they have no practical use.

EXPLANATION

• An ideal, perfectly pure semiconductor (with no impurities) is called an intrinsic semiconductor.
• At absolute zero temperature valence band is full of electrons and the conduction band is empty, hence there are no free electrons in the conduction band and holes in the valence band.
• The charge carrier concentration is zero. Hence intrinsic semiconductor behaves like an insulator
• Hence, at zero temperature the intrinsic semiconductor behaves like an insulator. The correct option is 3.

CONCEPT:

• N-type extrinsic semiconductor: It is the type of extrinsic semiconductor where the dopant atoms are capable of providing extra conduction electrons to the host material (e.g. phosphorus in silicon) is called as N-type semiconductor
  • This creates an excess of negative (n-type) electron charge carriers.

Difference between N-type and P-type semiconductor:

Sl. No.	N-type semiconductor	P-type semiconductor
1.	When pentavalent impurity atoms like As, Sb, etc are added in the intrinsic semiconductor, we get an n-type semiconductor.	When trivalent impurity atoms like gallium, indium, etc are added to the intrinsic semiconductor, we get a p-type semiconductor.
2.	Majority carriers are electrons and minorities are holes.	Majority carriers are holes and minority carriers are electrons.
3.	The number density of electrons >> number density of holes.	The number density of holes >> number density of electrons.
4.	The donor level lies closer to the conduction band.	Accepts level lies closer to the valence band.
5.	Examples: Phosphorus (P), Arsenic (As), Antimony (Sb).	Examples: Boron (B), Gallium (Ga), Indium(In), Aluminium(Al)

EXPLANATION:

• In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. 
  • A common dopant for n-type silicon is phosphorus or arsenic.

The correct option is 1.

CONCEPT:

• Intrinsic Semiconductor: The intrinsic semiconductor is a pure semiconductor.
  • Since they are pure semiconductors they have the same number of holes and electrons.
  • The conductivity of an intrinsic semiconductor is very low at room temperature.
• Extrinsic Semiconductor: We add a small amount of impurity to the pure semiconductor.
  • This increases the conductivity of the semiconductor by manifold.
  • This impure semiconductor is called an extrinsic semiconductor.

EXPLANATION:

• Option 1: The conductivity of an intrinsic semiconductor is very low at room temperature. It means very low or no current flows at room temperature.
  • So this is an incorrect statement.
• Option 2: Since intrinsic semiconductors are pure semiconductors they have the same number of holes and electrons. 
• Option 3 and 4: The intrinsic semiconductor is a pure semiconductor and an extrinsic semiconductor is a semiconductor with impurity.
  • So option 4 gives the correct statement.
• So the correct answer is option 4.

CONCEPT:

• The material which is not a good conductor or a good insulator is called the semiconductor.
• For example Silicon, Germanium, etc.

Two types of semiconductor:

1. Intrinsic semiconductors: It is an undoped semiconductor or pure semiconductor without adding any impurity
2. Non-intrinsic or extrinsic semiconductor: It is a semiconductor that is doped with a specific impurity that can deeply modify its electrical properties, making it suitable for electronic applications (diodes, transistors, etc.)

• Depending upon the impurity added non-intrinsic semiconductors can be classified into P-type and N-type of semiconductors depending on whether the semiconductor is rich in holes or electrons.

EXPLANATION:

• ​An intrinsic semiconductor is made up of only a single type of element.
• Germanium (Ge) and Silicon (Si) are the most common type of intrinsic semiconductor.
• When the temperature rises, due to collisions, some covalent bonds break and few electrons are unbounded and become free to move through the lattice, thus creating an absence in its original position (hole).​ Hence, option 3 is correct.

CONCEPT:

Semiconductor: 

• Semiconductors are the materials that have a conductivity between conductors and insulators.
  • Semiconductors are made of compounds such as gallium arsenide or pure elements, such as germanium or silicon.
• Holes and electrons are the types of charge carriers accountable for the flow of current in semiconductors. 
  • ​Holes (valence electrons) are the positively charged electric charge carrier whereas electrons are the negatively charged particles.

EXPLANATION:

• Intrinsic semiconductors are made of pure elements.
• Since no impurity is mixed in the intrinsic semiconductors, so the numbers of electrons and holes remain equal in an intrinsic semiconductor.
• Hence, option 1 is correct.

CONCEPT:

Semiconductor: 

• Semiconductors are the materials that have a conductivity between conductors and insulators.
  • Semiconductors are made of compounds such as gallium arsenide or pure elements, such as germanium or silicon.
• Holes and electrons are the types of charge carriers accountable for the flow of current in semiconductors. 
  • ​Holes (valence electrons) are the positively charged electric charge carrier whereas electrons are the negatively charged particles.

EXPLANATION:

• The resistance of semiconductor materials decreases with the increase in temperature and vice-versa.
• At absolute zero Kelvin temperature, the covalent bonds are very strong, and there are no free electrons, and the semiconductor behaves as a perfect insulator.
• Hence, option 4 is correct.

CONCEPT:

Semiconductor: 

• Semiconductors are materials that have a conductivity between conductors and insulators.
  • Semiconductors are made of compounds such as gallium arsenide or pure elements, such as germanium or silicon.
• Holes and electrons are the types of charge carriers accountable for the flow of current in semiconductors. 
  • ​Holes (valence electrons) are the positively charged electric charge carrier whereas electrons are the negatively charged particles.

EXPLANATION:

• Intrinsic semiconductors are made of pure elements.
• The mobility of electrons and holes depends on their effective masses. The effective mass of electrons is less than that of holes hence electrons have higher mobility than holes.
• Hence, option 3 is correct.