Laser And Semiconductor

Question 1

Stimulated absorption: How does an atom go from ground state to excited state?

Answer 1

When an atom absorbs a proton with energy equivalent to E2-E1, the electron moves from ground state E1 to a higher energy state E2.

Question 2

What is spontaneous emission?

Answer 2

An atom can only stay in excited state for a very short period of time and after which, it would transit to a lower energy state, emitting a photon with energy matching that of the energy difference between the excited state and lower energy state.

Question 3

Photons emitted through spontaneous emission are _____, meaning they have different direction, phase and plane of polarisation.

Answer 3

Incoherent

Question 4

What are the two possibilities when a photon with energy level matching the energy difference between two energy states is incident on an excited atom?

Answer 4

1. It will excite another electron from the ground state to a higher energy state.
2. It will interact with the excited atom and cause the atom to return to ground state, emitting a second photon. Both the incident proton and that proton will be emitted coherently.

Question 5

How does the metastable state affect the atom’s duration in the excited state?

Answer 5

It increases the duration in excited state.

Question 6

What is stimulated emission?

Answer 6

It is when the emission of photons from. Excited atoms is triggered by an incident or incoming photon.

Question 7

Describe the energy of emitted and incident photons in spontaneous emission.

Answer 7

The energy of the incident photon is equal to the energy difference between the upper and lower level, and the emitted photon has the same phase, energy, frequency, polarisation and direction of travel as the incident proton.

Question 8

Why is population needed for stimulated emission to be dominant?

Answer 8

With more atoms in the higher excited energy state, the probability that an incident photon will cause stimulated emission is higher.

Question 9

What is population inversion and how is it achieved?

Answer 9

It is when more atoms are in the higher excited energy state than lower excited energy states.
It is achieved by supplying energy to the medium such that most atoms or molecules are placed in The excited state.
Supplies of energy: high voltage discharge, optical source e.g. Flash lamp

Question 10

Why is population inversion not enough for spontaneous emission?

Answer 10

Excited states have short lifetimes and release their excess energy in a short time by spontaneous emissions.

Question 11

Why is population inversion required for stimulated emission?

Answer 11

It is to ensure the rate of stimulated emission is greater than the rate of simulated absorption.

Question 12

Why does the excited system have to be in metastable state to produce a laser?

Answer 12

It is to ensure that population inversion can be established and maintained so that stimulated emission is more likely to occur before spontaneous emission.

Question 13

Why does the system to produce laser light have to be enclosed by 2 mirrors, where one is fully rejecting while the other is partially reflecting?

Answer 13

This is to Ensure the photons are kept in the system Long enough to enable them to stimulate further stimulated emissions from exited atoms.
The photons that escape through the partially reflecting mirror forms the laser.

Question 14

Name the properties of laser light

Answer 14

1. It is coherent. (Photons are in same phase, polarisation plane and direction since)
2. It is monochromatic(same frequency)
3. If us small divergence in angle and is unidirectional.

Question 15

Why is laser light coherent?

Answer 15

A single stimulated emission results in the production of a photon having the same phase and wavelength as the incident photon.

Question 16

Why do stimulated emissions produce photons with the same energy?

Answer 16

The incident photon interacts with the excited atom causing the atom to return to ground state. Hence, the second photon emitted must have the same energy as the difference in the higher and lower energy level.

Question 17

Why do lasers have higher intensity than ordinary sources?

Answer 17

A laser channels all its energy to one wavelength and in one direction while ordinary sources spread their energy over a wide range of wavelengths and emit in different directions.
For the particular wavelength of the laser, the power per unit area of the laser beam is much greater than that for the same wave length emitted by an ordinary sources

Question 18

Why are energy bands formed in solids instead of discrete energy levels?

Answer 18

As atoms are close together in a solid compared to has, the original atomic energy levels of the valence electrons interacts with one another and shift upwards or downwards by varying amounts.

The energy levels combine to form energy bands as they are closely spaced in the bands.

Question 19

Explain how Pauli’s exclusion principle leads to the shifting of energy levels of valence electrons.

Answer 19

It states that no two electrons can be in the same quantum state. Hence, due to interactions between energy levels, they energy levels begin to shift.

Question 20

What’s is the conduction band?

Answer 20

It is the upper band of allowed energy states. Energy is required for electrons to get there from the valence band so there are few electrons there.

Question 21

Why can electrons only conduct electricity at the conduction band?

Answer 21

Electrons in the conduction band are free to move about the crystal.

Question 22

What is the valence band?

Answer 22

It is the lower band of allowed energy states and the highest energy level an electron can posses at absolute zero temperature

Question 23

Electrons tend to fill the lowest available energy states. When electrons absorb energy when temperature rises or when light is introduced, they _____.

Answer 23

Leave the valence band to rise up the conduction band.

Question 24

When electrons gain enough energy, greater than the band energy, it’s conductivity increases because_____.

Answer 24

It can now go to many unoccupied energy states in the presence of an E-field.

Question 25

In what frequency does an atom absorb or emit electromagnetic radiation?

Answer 25

In frequencies that correspond to the energy differences between the allowed states since E=hf.

Question 26

Describe the characteristics of the energy bands in a conductor.

Answer 26

The conduction band is either partially filled or that the valence band and conduction bands overlap so that unoccupied energy states are less available.

Question 27

How does current flow arise in a metal?

Answer 27

When an electric field is applied across a metal, electrons accelerate, increasing their energy, occupying the unoccupied energy states that are of higher energy.

Question 28

Describe the characteristics of the bands and electrons in an insulator at absolute zero.

Answer 28

The valence band is completely filled. Since Pauli’s exclusion principle states electrons cannot occupy the same quantum state, electrons cannot move to other occupied levels or within the band (in a solid).

The conduction band is unoccupied.

Question 29

How can an insulator conduct electricity?

Answer 29

Electrons can only move by jumping across the energy gap of the valence and conduction band of as much as 5eV.
The insulator must be subjected to a very large electric field (10^10 Vm^-1) of electrons to overcome the energy gap.

Question 30

Compare intrinsic semiconductors and insulators.

Answer 30

Semiconductors have the same type of band structure as insulators (valence band is completely filled, conduction band is empty) but the energy gap is much smaller (1eV)

Question 31

How does an intrinsic semi conductor conduct electricity?

Answer 31

Thermal agitation at room temperature causes electrons to jump from the valence to conduction bands, leaving behind holes, which are vacant sites.

Holes allow nearby electrons to occupy it(hence they are charge carriers), resulting in the migration of holes in the opposite direction of electron movement. Holes behave like particles with positive charge.

In the presence of an electric field Holes move in the directionof the field while electrons move in the opposite direction.

Question 32

As temperature increases, why does the conductivity increase?

Answer 32

The number of conducting electrons increase, as more electrons are able to jump from the valence band to conduction band due to increase in energy(?)

Question 33

What is fermi energy?

Answer 33

It is the energy of the highest filled state at 0K since at 0K, all energy levels below the fermi level is filled, while those that are above are empty.

Question 34

Doping of intrinsic semicondcutors can involve _____ or ____.

Answer 34

Group V atoms (resulting in n-type semiconductors)
Group III (resulting in p-type semiconductors)

Question 35

How does doping with group V atoms result greater conductivity?

Answer 35

This electron free from the parent group V atom, just below the conduction band, jumps to the conduction band with a small amount of thermal excitation, where it behaves as a negatively charged charge carrier.

Question 36

How does doping with group III atoms result greater conductivity?

Answer 36

A small amount of thermal excitation moves electrons from the valence band into the energy band(which energy band?) of the group III atom(due to the presence of holes there) Thus, the hole can carry a current in the presence of an electric field.

Question 37

How do Group V and Group III atoms interact with the semiconductor Group IV atom?

Answer 37

Group V atoms form covalent bonds with the 4 valence electrons of the semiconductor atom while the remaining 1 electron is nearly free from its parent atom.

Group III atoms form covalent bonds using 3 of its valence electrons, leaving an electron deficiency, forming a hole.

Question 38

What are the 3 distinct regions in a p-n junction?

Answer 38

The p region, n region, anddepletion region.

Question 39

what happens at the depletion region?

Answer 39

Free electrons move by diffusion across the junction into the p type material by diffusion where they fill holes while holes diffuse into the n type material through diffusion.

Question 40

How is the potential barrier in the depletion layer created?

Answer 40

N near the depletion region becomes positively charged after the diffusion of charges and p near the depletion region becomes negatively charged, preventing the exchange of charge as the charged n and p stops the flow of charges.

The immobile charges create an electric field to keep mobile charges out, preventing further diffusion of charges unless an external voltage is applied.

Question 41

Why does the forward-biased diode have a narrower depletion region?

Answer 41

Holes and electrons flow in the direction the depletion region due to the direction of current flow as electrons are repelled by the negative terminal and attracted by the positive terminal in the n-side and p-side respectively. The movement of holes is due to the flow of electrons to the positive terminal. This causes the depletion region to narrow and the resistance of the diode to decrease.

Question 42

How does a forward biased diode arise?

Answer 42

When the positive terminal is connected to the p and negative terminal connected to n.

Question 43

Why does the reverse-biased diode have a wider depletion region?

Answer 43

The flow of current forces electrons in the n-side and holes in the p-side away from the junction, causing potential to build up until it opposes the battery potential and the depletion region to widen.

Question 44

When the reverse potential difference is too large and exceeds the breakdown voltage, what happens to the depletion layer?

Answer 44

It breaksdown and the resistance of the diode becomes zero.

Question 45

How does a smaller depletion region result in greater current flow?

Answer 45

The built in junction potential decreases so there is larger current flow.

Question 46

What is one use of a junction diode?

Answer 46

It can serve as a rectifier.