Lecture 12: Electrical Properties

Question 1

What determines the electronic properties of a material?

Answer 1

The electronic properties depend on electrical conductivity, the identity of the charge carriers, and the crystal structure.

Question 2

Define electrical conductivity.

Answer 2

Electrical conductivity (σ) is a measure of a material’s ability to conduct an electric current, calculated by
𝜎=𝑛𝑒𝜇, where 𝑛 is the number of charge carriers, 𝑒 is the charge on each carrier, and 𝜇 is their mobility.

Question 3

What are the characteristics of a conductor?

Answer 3

Conductors have a significant portion of delocalized valence electrons that can freely move throughout the structure, facilitating electrical conductivity.

Question 4

Explain the difference between intrinsic and extrinsic semiconductors.

Answer 4

Intrinsic semiconductors have electrical conductivity based on pure material properties, while extrinsic semiconductors have conductivity enhanced by doping with impurities.

Question 5

What is the band gap in semiconductors?

Answer 5

The band gap is the energy difference between the valence band and the conduction band; its size determines a material’s electrical conductivity and optical properties.

Question 6

Define photoconductivity.

Answer 6

Photoconductivity is a phenomenon where a material’s conductivity increases upon exposure to electromagnetic radiation, as electrons are excited from the valence band to the conduction band.

Question 7

What is the significance of the band gap of CdS in solar cells?

Answer 7

CdS has a band gap of 2.5 eV, allowing it to absorb blue visible light, making it useful in solar cells for converting sunlight into electricity.

Question 8

Describe the role of phonons in electrical conductivity.

Answer 8

Phonons, or lattice vibrations, interact with electrons, which can lead to scattering and resistance, affecting a material’s electrical conductivity.

Question 9

What is a dielectric material?

Answer 9

Dielectric materials are insulators that can be polarized by an electric field, which makes them important in applications like capacitors.

Question 10

Explain the Lever Rule in the context of phase diagrams.

Answer 10

The Lever Rule is a tool used to determine the proportions of different phases in a binary phase diagram based on the overall composition of the system.

Question 11

What does the term ‘band bending’ refer to?

Answer 11

Band bending occurs when there is a spatial variation in the band edges within a semiconductor, often due to external influences like electric fields or junctions.

Question 12

Define a p-n junction.

Answer 12

A p-n junction is a boundary or interface between p-type and n-type semiconductors, crucial for semiconductor-based electronic devices.

Question 13

What are the electrical properties of p-type and n-type semiconductors?

Answer 13

P-type semiconductors have an abundance of holes (positive charge carriers), while n-type semiconductors have an excess of electrons (negative charge carriers).

Question 14

How does doping affect the properties of semiconductors?

Answer 14

Doping introduces impurities into a semiconductor, which can provide extra electrons or holes to enhance conductivity.

Question 15

What is an LED and how does it work?

Answer 15

An LED (Light Emitting Diode) is a semiconductor device that emits light when electric current flows through it, due to electron-hole recombination releasing energy in the form of photons.

Question 16

Describe how the band gap influences LED color.

Answer 16

The color of the LED light depends on the band gap energy; different materials with different band gaps emit photons of varying energies, corresponding to different colors.

Question 17

What is the difference between direct and indirect band gaps?

Answer 17

In direct band gap semiconductors, electron transitions between the valence and conduction bands occur vertically in momentum space, facilitating efficient light emission. In indirect band gaps, these transitions also involve a change in momentum, requiring a phonon to conserve momentum, which is less efficient for light emission.

Question 18

How do transistors amplify electrical signals?

Answer 18

Transistors control a large current between the output terminals by a smaller current or voltage at the input terminal, allowing them to act as amplifiers.

Question 19

What is a MOSFET?

Answer 19

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of transistor that uses an electric field to control the conductivity of a channel in a semiconductor material.

Question 20

Explain the function of CMOS technology in digital circuits.

Answer 20

CMOS (Complementary Metal-Oxide-Semiconductor) technology uses a pair of p-type and n-type MOSFETs to create logic gates that are highly effective in digital circuits due to low power consumption and high speed.

Question 21

What role do semiconductors play in computers?

Answer 21

Semiconductors form the foundation of modern electronics, including transistors, diodes, and integrated circuits, which are essential components of computers.

Question 22

Describe the concept of ferroelectric materials.

Answer 22

Ferroelectric materials have spontaneous electrical polarization that can be reversed by the application of an external electric field, similar to ferromagnetism in magnetic materials.

Question 23

How do ion conductors differ from electronic conductors?

Answer 23

Ion conductors allow ions to move freely through the material, which is crucial in applications like batteries and fuel cells, whereas electronic conductors allow electrons to move freely.

Question 24

What is piezoelectricity?

Answer 24

Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress.

Question 25

What are the applications of solid-state batteries?

Answer 25

Solid-state batteries are used in applications requiring high safety, long life, and stability over a wide temperature range, such as in automotive and medical devices.

Question 26

How does a fuel cell generate electricity?

Answer 26

A fuel cell generates electricity through the electrochemical reaction of hydrogen and oxygen, producing water and releasing energy.

Question 27

Explain how a lithium-ion battery functions.

Answer 27

In a lithium-ion battery, lithium ions move from the anode to the cathode through an electrolyte during discharge and back when charging, storing and releasing energy.

Question 28

What is the significance of yttrium substitution in zirconia?

Answer 28

Yttrium substitution in zirconia stabilizes the crystal structure at high temperatures, making it useful as an oxygen ion conductor in solid oxide fuel cells and oxygen sensors.

Question 29

What is the function of the anode in a lead-acid battery?

Answer 29

In a lead-acid battery, the anode undergoes oxidation, releasing electrons that flow through the external circuit to the cathode.

Question 30

Describe the properties of a hydrogen/oxygen fuel cell.

Answer 30

A hydrogen/oxygen fuel cell combines hydrogen and oxygen to produce electricity, with water as the only byproduct, offering a clean energy alternative.