Semiconductors MCQ Quiz - Objective Question with Answer for Semiconductors - Download Free PDF

Explanation:

Positive Temperature Coefficient of Semiconductor Diode

Definition: A positive temperature coefficient (PTC) refers to the phenomenon where a parameter increases with an increase in temperature. In the context of semiconductor diodes, certain characteristics exhibit a positive temperature coefficient, which can be crucial for their operation and applications.

Correct Option Analysis:

The correct answer is:

Option 1: Reverse Leakage Current

The reverse leakage current of a semiconductor diode exhibits a positive temperature coefficient. This means that as the temperature increases, the reverse leakage current also increases. This behavior is primarily due to the following reasons:

• Thermal Generation: In a semiconductor, increasing temperature leads to an increase in the thermal generation of charge carriers. This results in a higher concentration of minority carriers, which contributes to a larger reverse leakage current.
• Energy Band Gap Reduction: The energy band gap of a semiconductor narrows as the temperature rises. This reduction facilitates easier carrier movement across the junction, further increasing the reverse leakage current.
• Carrier Mobility: While carrier mobility may decrease with increasing temperature, the dominant factor affecting reverse leakage current is the generation of additional charge carriers, which outweighs the impact of reduced mobility.

Implications:

The positive temperature coefficient of reverse leakage current has several implications:

• Thermal Runaway: In high-power applications, excessive reverse leakage current can lead to thermal runaway, where increasing temperature causes further current increase, potentially damaging the diode.
• Device Reliability: Understanding and mitigating the effects of reverse leakage current are essential for ensuring the reliability and longevity of semiconductor devices.
• Design Considerations: Engineers must account for the temperature-dependent behavior of reverse leakage current when designing circuits involving semiconductor diodes.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Reverse Breakdown Voltage

The reverse breakdown voltage of a semiconductor diode generally exhibits a negative temperature coefficient. As temperature increases, the breakdown voltage decreases due to enhanced carrier generation and tunneling effects. This behavior is opposite to what is described by a positive temperature coefficient, making this option incorrect.

Option 3: Forward Voltage

The forward voltage of a semiconductor diode typically decreases with an increase in temperature. This is because higher temperatures reduce the barrier potential of the p-n junction, allowing carriers to move more freely. Hence, the forward voltage exhibits a negative temperature coefficient, not a positive one. Therefore, this option is incorrect.

Option 4: None of the Above

This option is incorrect because, as explained earlier, the reverse leakage current of a semiconductor diode does exhibit a positive temperature coefficient. Selecting "None of the Above" would disregard this established behavior.

Conclusion:

The reverse leakage current of a semiconductor diode is the parameter that exhibits a positive temperature coefficient. This characteristic is crucial for understanding the thermal behavior of diodes and ensuring proper design and application in electronic circuits. Other parameters, such as reverse breakdown voltage and forward voltage, do not exhibit a positive temperature coefficient and instead demonstrate opposite behaviors in response to temperature changes.

Explanation:

Given:

Energy gap (E_g) for different materials:

 Insulator: E_g > 3 eV

Semiconductor: E_g = 0.2 eV to 0.3 eV

Metals: E_g ≈ 0 eV

For an insulator, the energy gap is large, generally greater than 3 eV, which prevents electron transition from the valence band to the conduction band, making electrical conduction nearly impossible.

For semiconductors, the band gap is much smaller (0.2 eV - 0.3 eV), allowing limited conduction under certain conditions. In metals, the band gap is nearly zero, allowing free movement of electrons.

∴ The forbidden energy gap for an insulator is typically around 5 eV.

Explanation:

In an n-type semiconductor at 0 K, the Fermi level lies just below the donor level. At absolute zero, electrons in the donor impurity levels are fully occupied and contribute to conduction by moving to the conduction band. Since these donor levels are slightly below the conduction band, the Fermi level is positioned near the donor level, reflecting the highest energy state that electrons can occupy. This placement ensures that most electrons are in the donor levels, and the conduction band remains mostly empty at 0 K, with the Fermi level lying below the donor energy level.

Thus, option '1' is correct.

Explanation:

The intrinsic carrier concentration of silicon () is .

Phosphorus (P) is a donor impurity, contributing free electrons, which will make the material n-type.

Boron (B) is an acceptor impurity, contributing holes, which would make the material p-type if it were the only dopant.

Given:

• Phosphorus doping concentration: 
• Boron doping concentration: .

To determine the resulting type of doping:

• Phosphorus (n-type) adds electrons to the conduction band.
• Boron (p-type) adds holes to the valence band.

We need to compare the concentrations of donors (P) and acceptors (B):

There are more boron atoms  than phosphorus atoms .

Since the acceptor concentration (Boron) is higher than the donor concentration (Phosphorus), the material will be p-type, dominated by holes from Boron.

Thus, option '3' is correct.

Properties of Superconductors:

• Perfect Diamagnetism: Superconductors exhibit perfect diamagnetism, which is known as the Meissner effect. This means that when a material transitions to its superconducting state, it will expel all magnetic fields from its interior, making it a perfect diamagnet. This is a defining characteristic of superconductors.
• Ionic Conductivity: Superconductors do not exhibit ionic conductivity. Ionic conductivity is a property of materials where ions move through the lattice, which is typical for electrolytes and some solid-state ionic conductors, but not for superconductors. Superconductors conduct electricity through Cooper pairs (pairs of electrons) rather than ions.
• Magnetic Field Causes an Increase in Transition Temperature: This statement is not true for superconductors. In fact, the presence of a magnetic field generally reduces the transition temperature (the temperature below which a material becomes superconducting). Strong magnetic fields can even destroy superconductivity.
• Loss of Superconductivity in Long Circular Wire by Large Currents: Superconductors can lose their superconducting state if the current through them exceeds a certain critical value. This is because large currents can generate magnetic fields and heat, which can disrupt the Cooper pairs and destroy superconductivity.

Given the above properties:

• Property (1) Perfect diamagnetism is true for superconductors.
• Property (2) Ionic conductivity is not a property of superconductors.
• Property (3) Magnetic field causes an increase in transition temperature is not true for superconductors.
• Property (4) Loss of superconductivity in long circular wire by large currents is true for superconductors.

Therefore, the correct properties exhibited by a superconductor are:

• (1) Perfect diamagnetism
• (4) Loss of superconductivity in long circular wire by large currents

So, the correct answer is Option 3: 1, 3 and 4.

Mistake Points

The question is asking about the number of valence electrons, and not about the valency of the atom. Since a valence electron is the number of outer shell electrons that is associated with an atom, Phosphorous will have 5, and Silicon will have 4 valence electrons.

The correct answer is 5 and 4.

Explanation:

• A valence electron is an outer shell electron that is associated with an atom.
• These electrons can participate in the formation of a chemical bond.

Key Points

• Silicon has two electrons in its first shell, eight electrons in the second shell, and four (4) electrons in the third shell.
• Since the electrons in the third shell are the outermost electrons, silicon has four valence electrons.
• Phosphorus having an atomic number 15 is a pentavalent element, which means it has 5 valence electrons in its outermost shell.

Property of Semiconductors:

• Semiconductors are the materials that have a conductivity between conductors (generally metals) and non-conductors or insulators (such as ceramics).
• Semiconductors can be compounds such as gallium arsenide or pure elements, such as germanium or silicon.
• Semiconductors have an almost empty conduction band and an almost filled valence band.
• In a semiconductor, the mobility of electrons is higher than that of the holes.
• Its Resistivity lies between 10^-5 to 10⁶ Ωm
• Conductivity lies between 10⁵ to 10^-6 mho/m
• The temperature coefficient of resistance for semiconductors is Negative.
• The current Flow in the semiconductor is mainly due to both electrons and holes.

• The avalanche breakdown is a phenomenon in which there is an increase in the number of free electrons beyond the rated capacity of the diode; This results in the flow of heavy current through the diode in reverse biased condition
• Avalanche breakdown occurs in lightly doped diode
• Zener breakdown mainly occurs because of a high electric field; When the high electric field is applied across the PN junction diode, then the electrons start flowing across the PN-junction; Consequently, develops little current in the reverse bias
• The Zener breakdown occurs in heavily doped diodes

 

Important difference between Avalanche and Zener breakdown:

Avalanche Breakdown	Zener Breakdown
Lightly doped diode	Heavily doped diode
High reverse potential	Low reverse potential
Junction is destroyed	The junction is not destroyed
A weak electric field is produced	A strong electric field is produced
Occurs at high reverse potential	Occurs at low reverse potential

Effect of temperature on resistance of different material:

Conductor: When the temperature of conducting material increases, the resistance of that particular material increases.

Insulator: When the temperature of conducting material increases, the resistance of that particular material decreases.

Semiconductor: When the temperature of semiconducting material increases, the resistance of that particular material decreases. 

A negative coefficient for a material means that its resistance decreases with an increase in temperature. Hence, pure semiconductor materials (silicon and germanium) typically have negative temperature coefficients of resistance.

Concept:

The correct answer is an option [3]

• The combining capacity of an atom of an element to form a chemical bond is called its valency.
• The outermost electron shell of an atom is called the valence shell.
• The electrons present in the outermost shell of an atom are called valence electrons.
• The valence electron of an atom takes part in a chemical reaction because they have more energy than all the inner electrons.
• The valency of an element is
  • Equal to the number of valence electrons
  • Equal to the number of electrons required to complete eight electrons in the valence shell.
• Valency of a metal=No. of Valence electrons
• Valency Of a non-metal=8-No. of valence electrons
• For Ex:
  • Sodium(Z=11) Electronic Configuration=2,8,1
    • Valency=1
  • Magnesium(Z=2) Electronic Configuration=2,8,2
    • Valency=2
  • Chlorine(Z=17) Electronic Configuration=2,8,7
    • Valency=8-7=1
  • Oxygen= 8 Electronic Configuration=2,6
    • Valency=8-6=2

Carrier concentrations depends on the energy gap in the intrinsic semiconductor.

In the case of Ge and Si the intrinsic carrier concentration of Ge is higher than that of Si because the energy gap in Ge is smaller than that of Si.

The semiconductor in which impurities are added is called extrinsic semiconductor. When the impurities are added to the intrinsic semiconductor, it becomes an extrinsic semiconductor. The process of adding impurities to the semiconductor is called doping. The impurity added to extrinsic semiconductors is of the order of 1 in million

At low temperature, the energy of the valance band electron is not enough to jump in the conduction band. Hence there will be very few free-electrons available for conduction. Hence at low temperature, the resistance of semiconductors at low temperatures is high.