13. Displays

Question 1

What are radiometric units?

Answer 1

They describe the physical properties of electromagnetic radiation

Question 2

What are photometric units?

Answer 2

They characterise the light and colour sensation by the human eye

Question 3

What is the name of the spectrum that gives the response of the human eye to various wavelengths of light, and what does it look like?

Answer 3

The photopic spectral luminous efficiency curve

It looks like a parabolic curve centred at 555nm (a green, where the human eye is most sensitive) and magnitude of 1.

Question 4

What is the gamut of a device?

Answer 4

The portion of the colour space that can be represented, or reproduced.

Question 5

What is the principle behind FEDs(Field emission displays)?

Answer 5

FEDscanbethoughtofflatcathode‐raytubes(CRTs).Electronsare emittedfromthesurfaceofacathodematerialandareaccelerated invacuumtowardsananodeviaanelectricfield.Theelectron energyexcitesaluminescentphosphorlayer,whichupondeexcitation, emits light

Question 6

How does the electron emitter work in FEDs?

Answer 6

In FEDs electron emission is driven byhighelectricalfields.Atveryhigh fieldsthepotentialenergyoutsidetheemitter(typically metal)will bendsufficientlytoallowelectronstotunnelthroughthebarrierandescapeintothevacuum.

1. apply field
2. barrier gets lowered
3. e- can tunnel through barrier when distance gets small enough

Question 7

What does the probability of tunnelling depend on?

Answer 7

The effective barrier height and the width of the barrier

Question 8

How can geometry help with field emission?

Answer 8

Field emission depends exponentially on E, so it can be enhanced by the cathode having a sharp tip where the field is maximum.

Question 9

Name 3 properties that make multi-walled carbon nanotubes ideal for field emission?

Answer 9

• Highest current carrying capacity of any material, due to very high threshold for electromigration, due to its strong covalent bonds
• Small diameter, and very large aspect ratio, meaning high field enhancement around its tip
• High electrical and thermal conductivities

Question 10

What is Liquid Crystal?

Answer 10

Liquid crystals (LCs) are a state of matter which has properties between those of conventional liquids and those of solid crystals (mesophase). They are characterised by anisotropy of properties without the existence of a 3D crystal lattice, due to the tendency of the molecules to order.

Question 11

Why are semiconductors with a band gap >3 eV transparent?

Answer 11

Semiconductors have a band gap. That is a gap between the energy levels occupied by the electrons in the crystal lattice and the empty energy levels that an electron can be promoted to. That means if light has wavelength that is too long (corresponding to photons with energy below that of the band gap) it will behave like a transparent medium. For shorter wavelengths (higher energy photons) light can be absorbed promoting electrons to the higher energy states.
This works because photons can only be absorbed as a whole quantum of energy (to first order). Therefore if the energy of the photon is not enough to promote an electron into the next band, it simply does not get absorbed. Notice that it is a band, so once the energy threshold is reached, photons of higher energy will be absorbed, with the energy in excess of the band edge adding to the kinetic energy of the electron in the upper band. The upper band is generically called the conduction band, while the lower band (that is completely filled with electrons) is called the valence band.

Question 12

What is Birefringence?

Answer 12

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.

Question 13

How does the LC cell act as a light valve?

Answer 13

The cell has polarising filters on each side, which are perpendicular to each other thus no light would pass through regularly. When the cell is turned on however, the use of LC birefringence and the Nematic liquid crystal phase allows the polarisation of the light to be rotated to pass through the second polarising filter.

Question 14

Give examples of the class of materials that are used as transparent conductors

Answer 14

Conducting oxides such as ZnO, In2O3, SnO2

Question 15

What is the typical dopant for transparent semiconductors and what is it’s purpose?

Answer 15

Tin (Sn) and it is used to move the fermi level into the conduction band. This gives materials such as ITO (Indium tin oxide) which is transparent and conductive,

Question 16

What is active matrix pixel control?

Answer 16

Each pixel is controlled by its own thin film transistor.

Active matrix is a type of addressing scheme used in flat panel displays. In this method of switching individual elements (pixels), each pixel is attached to a transistor (thin film transistor) and capacitor actively maintaining the pixel state while other pixels are being addressed, in contrast with the older passive matrix technology in which each pixel must maintain its state passively, without being driven by circuitry.

Question 17

What are the benefits of the new discovery of amorphous oxides for use in the thin film transistors (TFT) in AM LCDs?

Answer 17

They have much higher electron mobility than the old a-Si technology due to the fact that they have metal s-like orbitals, which also have the benefit of no directional dependency. This is important as LCDs are becoming so large that high mobility is required to control all the pixels at the desired faster frame rates.

Question 18

What are 4 advantages of OLED technology compared to LCD?

Answer 18

• Simple fabrication process
• Superior viewing performance
• Excellent operating characteristics
• Good form factor

They can have deep black levels, be thinner, lighter and faster, higher contrast ratio and wider viewing angle

Question 19

Why is a double heterojunction structure utilised for OLEDs rather than a single layer?

Answer 19

Since for single layer structure, the active region can spread towards the electrodes, where emission is quenched and electrons and holes recombine non-radiatively (without emission of a photon of the energy difference). A double heterojunction is used to maximise electron and hole injection and to pose a barrier for carriers to reach the electrodes.

Question 20

What are the name of the 3 regions between the electrodes in a double heterojunction OLED?

Answer 20

ETL- electron transport layer
EML- emission layer
HTL- hole transport layer

Question 21

What is the main downside with OLED technology?

Answer 21

The limited lifetime of organic materials, where blue luminescent material degrades quicker, hindering the display colour representation.

Question 22

What are quantum dot LEDs?

Answer 22

A quantum dot display is a display device that uses quantum dots (QD), semiconductor nanocrystals which can produce pure monochromatic red, green, and blue light.

Question 23

Name 4 benefits of quantum dots for display technology over organic luminescent materials:

Answer 23

QDs are better due to their inherent luminescent properties, such as:

• Narrow spectral emission bandwidths (v pure colours)
• High photoluminescence quantum efficiency (therefore lower power consumption)
• Good stability
• Controllable bandgap