12. Applications Flashcards Preview

4C3 Advanced Materials > 12. Applications > Flashcards

Flashcards in 12. Applications Deck (41)
Loading flashcards...
1
Q

Where is the fermi level relative to the band gap for insulators?

A

In the middle of the band gap

2
Q

What is Poole-Frenkel conduction?

A

Poole–Frenkel conduction is a means by which an electrical insulator can conduct electricity.
Electrons can move (slowly) through an insulator by the following method. The electrons are generally trapped in localized states (loosely speaking, they are “stuck” to a single atom, and not free to move around the crystal). Occasionally, random thermal fluctuations will give that electron enough energy to get out of its localized state, and move to the conduction band. Once there, the electron can move through the crystal, for a brief amount of time, before relaxing into another localized state (in other words, “sticking” to a different atom). The Poole–Frenkel effect describes how, in a large electric field, the electron doesn’t need as much thermal energy to get into the conduction band (because part of this energy comes from being pulled by the electric field), so it does not need as large a thermal fluctuation and will be able to move more frequently.

3
Q

Name mechanisms through which insulators can conduct

A
  • Poole-Frenkel conduction (large electric field can move e- into the conduction band
  • Tunnelling (e- can tunnel through a sufficiently small barrier)
  • Hot carrier injection (carrier gains enough KE under a high field that there’s a probability of thermal excitation over the barrier, or to tunnel through the barrier)
4
Q

How does flash memory work?

A

Flash memory consists of an FET with an extra, electrically isolated floating gate between the external/control gate and the channel. The floating gate stores charge, by electrons tunnelling across the tunnel oxide, to be stored in the floating gate. The charge shifts the threshold (switch-on) voltage of the FET controlled by the control gate to more positive voltages (less negative). This charge can be retained for years. The charge is emptied through the oxide through application of a large reverse field.

5
Q

What is SRAM?

A

Static random-access memory (static RAM or SRAM) is a type of semiconductor memory that uses bistable latching circuitry (flip-flop) to store each bit. SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.

6
Q

What is DRAM?

A

Dynamic random-access memory (DRAM) is a type of random access semiconductor memory that stores each bit of data in a separate tiny capacitor within an integrated circuit. The capacitor can either be charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1. The electric charge on the capacitors slowly leaks off, so without intervention the data on the chip would soon be lost. To prevent this, DRAM requires an external memory refresh circuit which periodically rewrites the data in the capacitors, restoring them to their original charge. This refresh process is the defining characteristic of dynamic random-access memory, in contrast to static random-access memory (SRAM) which does not require data to be refreshed.

7
Q

What is the difference between SRAM and DRAM?

A

SRAM does not need to be a periodic refresh of its memory, where as DRAM does

8
Q

What is the name for the law which describes the exponential increase in transistor numbers on a chip?

A

Moore’s law

9
Q

What is the common method for transistor scaling?

A

Constant field scaling
It ensures all materials run at the same drive field, so not breakdown will occur due to increasing fields and electron velocity is the same.

10
Q

What limits the device structures and thus number of transistors that can be put on a chip (from a manufacturing standpoint)? (and describe this process)

A

It is limited by photolithography, a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask/optical mask to a photosensitive chemical photoresist on the substrate. A series of chemical treatments then either etches the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist.

11
Q

What places a limit on the resolution limit of photolithography?

A

The limit is set by the ‘Young’s slits’ resolution of light:
d = λ / NA, where λ is the lights wavelength and NA is the numerical aperture of the lens. Therefore smaller wavelength light and greater numerical aperture (using lenses) increases the resolution of photolithography.

12
Q

What is the current method for introducing dopants into a material, and what is a downside to this?

A

Ion implantation - allows the dopants to be placed in precisely defined patterns and depths, as depth is controlled by the ion energy. Downside is that ion bombardment creates damage + vacancies.

13
Q

How can defects/vacancies by removed from a semiconductor?

A

By annealing, called ‘Dopant Activation’. After implantation heat up to ~1000C for 5s and the vacancies and interstitials move out to the edge of the sample and annihilate.

14
Q

What is the limit of SiO2 as a gate oxide?

A

AS the channel gets smaller, the gate oxide thickness also gets smaller, and it is now at a value where the oxide is thin enough to allow direct quantum mechanical tunnelling of electrons across it, so it is not an insulator any more. This leads to an unacceptably high gate leakage current and thus excessive static power dissipation in the device.

15
Q

How does gate leakage current scale with decreasing oxide thickness?

A

the current increases exponentially

16
Q

What value for a oxide determines how good of an insulator it will be for the gate?

A

Having a higher dielectric constant ε

17
Q

Describe the principle behind an LED

A

An LED in its basic form is a p-n junction, with electrons and holes injected from opposite electrodes. They recombine at the junction to give out light/photons.

18
Q

Define the fermi level

A

The energy level up to which the electron states are all filled.

19
Q

What would happen if the fermi level wasn’t constant throughout the material?

A

A current would flow

20
Q

What is a zero bias p-n junction?

A

The p-n Junction in which no external voltage is applied is called zero bias p-n junction.

21
Q

How can the colour of an LED be changed

A

By varying the semiconductor band gap by alloying, for example GaAs and AlAs alloys.

22
Q

How does the band gap change with increasing Al content?

A

The band gap gets larger with increasing Al content

23
Q

How are blue and green LEDs made?

A

To get green/blue LEDs, we need a wa yto get a sufficiently large bandgap, however simple increasing of Al fraction in alloying will not work as there is a transition to an indirect band gap at ~2 eV. However, if we add a Nitrogen impurity, this creates a localised impurity level in the gap, and thus a quasi-direct band gap for transitions is made between the N level and the valence band. The e- can then enter the conduction band and drop down to the N level, recombining and emitting photons.

24
Q

What are 2 features of light emitted by stimulated emission?

A

The light is coherent, and the photons are all in phase

25
Q

What does laser stand for

A

Light Amplification by Stimulated Emission of Radiation

26
Q

What are the 2 main requirements for the laser action?

A
  1. Get a high concentration of electrons in the conduction band and a high concentration of holes in the valence band
  2. Confine the light by surfaces or interfaces acting as mirrors
27
Q

What is stimulated emission?

A

Laser light is a type of stimulated emission of radiation.
Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a new photon with a phase, frequency, polarization, and direction of travel that are all identical to the photons of the incident wave. This is in contrast to spontaneous emission, which occurs at random intervals without regard to the ambient electromagnetic field.

28
Q

What is a heterojunction and how do they allow more efficient lasers to be made that work at lower temperatures?

A

A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction.

By incorporating a smaller direct band gap material like GaAs between two larger band gap layers like AlAs, carriers can be confined so that lasing can occur at room temperature with low threshold currents.

29
Q

What is the major constraint on heterojunction lasers?

A

There needs to be no defects (dangling bonds) at the interface as these can cause electrons and holes to recombine and give out heat (phonons) instead of light (photons)

30
Q

What is the process which ensures that there are no defects at the interface between semiconductor types in heterojunctions?

A

Lattice matching - this is done by making sure that each semiconductor has the same lattice constant so that they match, otherwise we will get dislocations which are another source of dangling bonds where carrier recombination will occur.

Lattice constant is very important for 3D materials due to the 3D bonds that are needed at their interfaces, unlike 2D materials which can simply be layered.

31
Q

What are the 2 main factors that determine the band gap size for semiconductors?

A
  • The bond length: Wider band gap for smaller bond length
  • The amount of ionic bonding: Wider band gap for stronger ionic bonds (greater difference in electronegativity) Atomic radii or bond length increases as we go down the periodic table, so elements in the same row have the same bond length. Therefore wider band gaps can be made for the same lattice constant by using more ionic compounds like ZnSe that are in the same row as GaAs but have stronger ionic bonding due to having a greater difference in electronegativity between the elements (Zn is less electronegative and Se is more electronegative)
32
Q

What other feature of the band structure engineering needs to be considered other than band gap size when matching materials for a heterojunction?

A

It matters how the individual valence and conduction bands line up against each other, as this can result in different types of line ups/staggered line ups or even broken gaps (where the band gaps to not overlap)

33
Q

What is the rule for where electrons and holes will situate themselves when you line up different semiconductors with different band gaps in a heterojunction?

A

Electrons sink to the lowest energy, holes rise to the highest energy.
This can be useful to guide electrons or holes into the desired layers either for recombination or to separate them, and it will increase the non-equilibrium density of the electrons and holes in the conduction and valence band respectively.

34
Q

What is a type I band line-up?

A

A smaller bandgap sits within a larger one

35
Q

What is a type II band line-up?

A

There is a staggered line up, so the smaller band gap isn’t totally within the larger one.

36
Q

What is a type II band line-up?

A

This is a broken gap and the band gaps do not overlap

37
Q

What are the benefits of group III-nitrides like GaN?

A

They have wide, direct band gaps

Their dislocations do not give ride to mid-gap defect states (like they normally would do)

38
Q

What are the 3 potential ways you could create white light from LEDs?

A
  1. Using 3 different LEDs (blue, green, red)
  2. Down converting from a blue source (the cheapest and is what is used)
  3. Down converting from a UV source (UV exposure not ideal)
39
Q

What are the two main requirements of a material that you want to create a laser from?

A

That it has a direct band gap

That it can be n and p type doped

40
Q

How do you n and p type dope a III-V compound like GaN?

A

p-type: substitute group II for the group III element

n-type: substitute group VI for the group V element

41
Q

How can recombination occur in an indirect band gap semiconductor?

A

It requires the absorption of phonons (lattice vibrations), which strongly lowers the probability and speed of recombination compared to a direct band gap semiconductor