Semiconductor Electronics: Materials, Devices and Simple Circuits

Question 1

What were the basic building blocks of electronic circuits before the discovery of transistors in 1948?

Answer 1

Vacuum tubes, also known as valves

Question 2

Describe the structure of a vacuum diode.

Answer 2

A vacuum diode consists of two electrodes: an anode (plate) and a cathode.

Question 3

How many electrodes does a triode have, and what are they?

Answer 3

A triode has three electrodes: cathode, plate, and grid.

Question 4

Why is vacuum necessary in the inter-electrode space of a vacuum tube?

Answer 4

Vacuum is required to prevent electrons from losing their energy on collision with air molecules in their path.

Question 5

What is the direction of electron flow in a vacuum tube, and why are these devices referred to as valves?

Answer 5

Electrons flow only from the cathode to the anode in a vacuum tube, hence they are referred to as valves.

Question 6

What are some drawbacks of vacuum tube devices compared to semiconductor devices?

Answer 6

Vacuum tube devices are bulky, consume high power, operate at high voltages, and have limited life and low reliability.

Question 7

What was realized in the 1930s regarding semiconductor materials and their junctions?

Answer 7

It was realized that some solid-state semiconductors and their junctions offer the possibility of controlling the number and direction of flow of charge carriers through them.

Question 8

How do semiconductor devices differ from vacuum tube devices in terms of the flow of charge carriers?

Answer 8

In semiconductor devices, the supply and flow of charge carriers are within the solid itself, whereas in vacuum tubes, mobile electrons are obtained from a heated cathode and made to flow in an evacuated space.

Question 9

What are some advantages of semiconductor devices over vacuum tube devices?

Answer 9

Semiconductor devices are small in size, consume low power, operate at low voltages, and have long life and high reliability.

Question 10

What example is given of a device that works on the principle of vacuum tubes but is being replaced by solid-state electronics?

Answer 10

Cathode Ray Tubes (CRT), used in television and computer monitors, are being replaced by Liquid Crystal Display (LCD) monitors with supporting solid-state electronics.

Question 11

How are solids classified based on conductivity?

Answer 11

Solids are classified into metals, semiconductors, and insulators based on their relative values of electrical conductivity (s) or resistivity (r).

Question 12

What are the characteristics of metals in terms of resistivity and conductivity?

Answer 12

Metals possess very low resistivity (or high conductivity), with resistivity values typically ranging from 10^(-2) to 10^(-8) W m and conductivity values from 10^2 to 10^8 S m^(-1).

Question 13

Describe semiconductors in terms of resistivity and conductivity.

Answer 13

Semiconductors have resistivity or conductivity intermediate to metals and insulators, with resistivity values typically ranging from 10^(-5) to 10^6 W m and conductivity values from 10^5 to 10^(-6) S m^(-1).

Question 14

How are insulators characterized in terms of resistivity and conductivity?

Answer 14

Insulators have high resistivity (or low conductivity), with resistivity values typically ranging from 10^(11) to 10^(19) W m and conductivity values from 10^(-11) to 10^(-19) S m^(-1).

Question 15

What types of semiconductors are discussed in the chapter?

Answer 15

The chapter discusses elemental semiconductors (such as Si and Ge) and compound semiconductors, including inorganic (e.g., CdS, GaAs) and organic examples (e.g., anthracene, doped pthalocyanines).

Question 16

What are the main types of semiconductor devices discussed in the chapter?

Answer 16

Most semiconductor devices discussed are based on elemental semiconductors like Si or Ge and compound inorganic semiconductors. However, some devices using organic semiconductors and semiconducting polymers have been developed since the 1990s.

Question 17

What are energy bands in solids?

Answer 17

Energy bands in solids are continuous ranges of energy levels formed by the collective behavior of electrons in a crystal lattice, including the valence band (containing valence electrons) and the conduction band (above the valence band).

Question 18

Describe the concept of the energy band gap.

Answer 18

The energy band gap is the energy difference between the top of the valence band and the bottom of the conduction band. It determines the electrical properties of a material, with larger gaps leading to insulating behavior and smaller gaps allowing for semiconduction or conduction.

Question 19

How do metals, insulators, and semiconductors differ in terms of their energy band structures?

Answer 19

Metals have overlapping valence and conduction bands or partially filled conduction bands, allowing for easy electron movement and high conductivity. Insulators have a large energy band gap, preventing electron movement and leading to high resistivity. Semiconductors have a small band gap, allowing for some electron movement at room temperature.

Question 20

What is the lattice structure of Ge and Si?

Answer 20

The lattice structure of Ge and Si is diamond-like, where each atom is surrounded by four nearest neighbors.

Question 21

How many valence electrons do Si and Ge have?

Answer 21

Si and Ge have four valence electrons each.

Question 22

What happens to each valence electron of a Si or Ge atom in its crystalline structure?

Answer 22

Each valence electron of a Si or Ge atom tends to share one electron with each of its four nearest neighbor atoms and take a share of one electron from each such neighbor, forming covalent bonds or valence bonds.

Question 23

What are covalent bonds in the context of Si and Ge atoms?

Answer 23

Covalent bonds are formed when each Si or Ge atom shares one of its four valence electrons with each of its four nearest neighbor atoms and takes a share of one electron from each such neighbor.

Question 24

What is the significance of the shared electron pairs in covalent bonds?

Answer 24

The shared electron pairs in covalent bonds hold the atoms together strongly, forming a stable structure.

Question 25

Describe the 2-dimensional representation of the Si or Ge structure and its significance.

Answer 25

The 2-dimensional representation overemphasizes the covalent bond and shows an idealized picture where no bonds are broken. This representation is significant at low temperatures.

Question 26

How does an increase in temperature affect the covalent bonds in a crystalline lattice?

Answer 26

As the temperature increases, more thermal energy becomes available to the electrons, causing some of them to break away and contribute to conduction.

Question 27

What is the effect of thermal energy on the crystalline lattice at high temperatures?

Answer 27

Thermal energy ionizes a few atoms in the lattice, creating vacancies in the bonds, which are referred to as holes.

Question 28

What is an intrinsic semiconductor?

Answer 28

An intrinsic semiconductor is a semiconductor where the number of free electrons (ne) is equal to the number of holes (nh), denoted by ni, at equilibrium.

Question 29

How do holes move in a semiconductor?

Answer 29

Holes move when an electron from a covalent bond jumps to a vacant site, effectively shifting the hole to the new site. This movement occurs independently of the motion of free electrons.

Question 30

What is the total current in a semiconductor?

Answer 30

The total current (I) in a semiconductor is the sum of the electron current (Ie) and the hole current (Ih).

Question 31

What is recombination in a semiconductor?

Answer 31

Recombination in a semiconductor occurs when electrons collide with holes, leading to the neutralization of charge carriers. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers.

Question 32

How does an intrinsic semiconductor behave at T = 0 K?

Answer 32

At T = 0 K, an intrinsic semiconductor behaves like an insulator, with all electrons in the valence band.

Question 33

What effect does thermal energy have on an intrinsic semiconductor at T > 0 K?

Answer 33

At T > 0 K, thermal energy excites some electrons from the valence band to the conduction band, partially occupying the conduction band and creating holes in the valence band.

Question 34

What determines the conductivity of an intrinsic semiconductor?

Answer 34

The conductivity of an intrinsic semiconductor is determined by its temperature, but at room temperature, it is very low due to the lack of significant charge carriers.

Question 35

How can the conductivity of intrinsic semiconductors be improved?

Answer 35

The conductivity of intrinsic semiconductors can be improved by doping them with suitable impurities, which increases their conductivity manifold.

Question 36

What is the purpose of doping in semiconductors?

Answer 36

Doping is done to modify the electrical properties of semiconductors by intentionally introducing impurity atoms into the crystal lattice.

Question 37

What are dopants, and what role do they play in semiconductor doping?

Answer 37

Dopants are the impurity atoms deliberately added to semiconductors. They alter the conductivity of the semiconductor by either donating or accepting charge carriers.

Question 38

How do dopants affect the lattice structure of semiconductors?

Answer 38

Dopants do not significantly distort the original semiconductor lattice; they occupy only a few of the original semiconductor atom sites, maintaining the integrity of the crystal structure.

Question 39

What are the two types of dopants commonly used in semiconductor doping?

Answer 39

The two types of dopants used in semiconductor doping are pentavalent (valency 5), such as Arsenic, Antimony, and Phosphorous, and trivalent (valency 3), such as Indium, Boron, and Aluminium.

Question 40

Explain how pentavalent dopants affect the conductivity of semiconductors.

Answer 40

Pentavalent dopants, such as Arsenic or Phosphorous, donate an extra electron for conduction, making them donor impurities, which significantly increases the number of free electrons in the semiconductor.

Question 41

Describe the conductivity of n-type semiconductors.

Answer 41

N-type semiconductors are doped with pentavalent impurities, where the number of conduction electrons greatly exceeds the number of holes, making electrons the majority carriers and holes the minority carriers.

Question 42

How does doping with trivalent impurities affect the conductivity of semiconductors?

Answer 42

Doping with trivalent impurities, such as Boron or Aluminium, creates p-type semiconductors, where holes become the majority carriers and electrons become the minority carriers.

Question 43

What is the significance of the energy band structure in extrinsic semiconductors?

Answer 43

Doping in extrinsic semiconductors introduces additional energy states due to donor or acceptor impurities, which affect the conductivity and electronic properties of the material.

Question 44

Explain the concept of charge neutrality in doped semiconductors.

Answer 44

Doped semiconductors maintain overall charge neutrality, where the additional charge carriers introduced by doping are balanced by the ionized cores in the lattice.

Question 45

How do extrinsic semiconductors indirectly reduce the intrinsic concentration of minority carriers?

Answer 45

Extrinsic semiconductors, by adding a large number of majority carriers, increase the likelihood of recombination with minority carriers, effectively reducing their concentration.

Question 46

What is the relationship between electron and hole concentration in a semiconductor in thermal equilibrium?

Answer 46

The product of electron and hole concentrations in a semiconductor in thermal equilibrium is equal to the square of the intrinsic carrier concentration.

Question 47

How does the energy gap between the conduction and valence bands influence the resistivity of semiconductors?

Answer 47

The energy gap between the conduction and valence bands determines the resistivity of semiconductors, with smaller energy gaps corresponding to higher conductivity.

Question 48

What is a p-n junction and why is it significant in semiconductor devices?

Answer 48

A p-n junction is a boundary between a p-type semiconductor and an n-type semiconductor, crucial in various semiconductor devices like diodes and transistors for controlling the flow of current.

Question 49

How is a p-n junction formed?

Answer 49

A p-n junction is formed by combining a p-type semiconductor with an n-type semiconductor, creating a metallurgical junction between them through processes like diffusion and drift.

Question 50

Describe the diffusion process during the formation of a p-n junction.

Answer 50

During diffusion, holes from the p-side diffuse to the n-side, while electrons from the n-side diffuse to the p-side, creating a diffusion current across the junction.

Question 51

What happens to the charges left behind during diffusion in a p-n junction?

Answer 51

Charges left behind during diffusion result in the formation of a positive space-charge region on the n-side and a negative space-charge region on the p-side, collectively known as the depletion region.

Question 52

Explain the concept of drift in a p-n junction.

Answer 52

Drift occurs due to the electric field within the depletion region, causing electrons on the p-side to move towards the n-side and holes on the n-side to move towards the p-side, leading to a drift current opposite to the diffusion current.

Question 53

How does the thickness of the depletion region change during the formation of a p-n junction?

Answer 53

The depletion region initially forms with a small thickness but extends as diffusion continues, increasing the electric field strength and hence the drift current until equilibrium is reached.

Question 54

What is the condition of a p-n junction under equilibrium?

Answer 54

Under equilibrium, there is no net current flow across the p-n junction, with the loss and gain of electrons balancing each other, resulting in a barrier potential opposing further flow of carriers.

Question 55

What is the significance of the barrier potential in a p-n junction?

Answer 55

The barrier potential in a p-n junction prevents the movement of electrons from the n-region to the p-region, maintaining equilibrium and controlling the flow of current in semiconductor devices.

Question 56

What is a semiconductor diode, and how is it represented symbolically?

Answer 56

A semiconductor diode is a p-n junction with metallic contacts at its ends. Symbolically, it’s represented as a triangle with an arrow pointing from the p-side to the n-side.

Question 57

Describe the behavior of a p-n junction diode under forward bias.

Answer 57

Under forward bias, an external voltage is applied with the positive terminal to the p-side and the negative terminal to the n-side. This reduces the barrier height, allowing minority carriers to cross the junction, leading to current flow.

Question 58

What happens to the depletion region and barrier height when a diode is under forward bias?

Answer 58

The depletion region decreases, and the barrier height reduces due to the applied voltage being opposite to the built-in potential.

Question 59

Explain the process of minority carrier injection in a diode under forward bias.

Answer 59

Electrons from the n-side and holes from the p-side cross the depletion region, leading to minority carrier injection. This increases the minority carrier concentration significantly near the junction.

Question 60

How does diffusion contribute to current flow in a diode under forward bias?

Answer 60

Injected electrons and holes diffuse from the junction to the other end of their respective sides, creating a concentration gradient that results in current flow.

Question 61

What is the behavior of a p-n junction diode under reverse bias?

Answer 61

Under reverse bias, the applied voltage widens the depletion region, increasing the barrier height and suppressing current flow.

Question 62

Describe the mechanism of drift current in a diode under reverse bias.

Answer 62

Drift current occurs due to the motion of carriers from their minority side to their majority side across the junction, resulting in a low but steady current flow.

Question 63

What is the breakdown voltage in a diode, and what happens when this voltage is exceeded?

Answer 63

The breakdown voltage is the critical reverse bias voltage where the diode’s reverse current sharply increases. Exceeding this voltage can lead to overheating and destruction of the diode.

Question 64

How are the V-I characteristics of a diode studied, and what do they indicate?

Answer 64

The V-I characteristics are studied by varying the applied voltage and measuring the resulting current. This yields a graph showing the relationship between voltage and current, revealing the diode’s behavior under different biases.

Question 65

What is the threshold voltage, and how does it differ for different types of diodes?

Answer 65

The threshold voltage, also known as the cut-in voltage, is the voltage at which the diode begins to conduct significantly. It’s approximately 0.2V for germanium diodes and 0.7V for silicon diodes.

Question 66

Explain the concept of dynamic resistance in diodes.

Answer 66

Dynamic resistance is defined as the ratio of small changes in voltage to small changes in current in a diode, indicating how the diode’s resistance varies with applied voltage.