14. EXTRA PART 1

Question 1

What 2 things determine the field at the surface of a hard magnet?

Answer 1

Its shape and its remnant magnetisation

Question 2

What are the key features of the M-H curves for type 1 and type 2 superconductors?

Answer 2

The slope of the downwards section is equal to the susceptibility therefore = -1, and the intercepts with the x axis are the critical fields

Question 3

If the geometry of the question is ‘large’ what does this allow you to do?

Answer 3

Neglect geometric effects

Question 4

What is dD/dt inside a superconductor?

Answer 4

0, as there cannot be any electric field inside a superconductor

Question 5

What are the origins of the thermal time constant for a pyroelectric element?

Answer 5

The thermal time constant (tau t) essentially depends on how quickly absorbed heat in the element is rejected to the environment. It depends, therefore, on the heat capacity of the element, H, and on the thermal conductivity of the element to its surroundings Gt.
tau t=H/Gt

Question 6

What are the origins of the electrical time constant?

Answer 6

The electrical time constant (tau e) depends on the capacitance of the element and the amplifier. It is a measure of how quickly charge is transferred from one to the other.
tau e=Rg(Ce+Ca)

Question 7

What is the feature of pyroelectric materials that allows them to produce a charge with the change in temperature?

Answer 7

Pyroelectric crystals have a rare asymmetry due to their single polar axis. This causes their polarization to change with temperature.

Question 8

Why is a modulated source necessary for use with pyroelectric detectors?

Answer 8

As a pyroelectric element responds only to changes in infrared radiation.

Question 9

In current mode operation of a pyroelectric detector, what are we concerned about and under what conditions does this occur?

Answer 9

In current mode we are concerned about current generated by unit of input power.
This is the case at low frequency of modulation of the input flux – say in a PIR detector.

Question 10

In voltage mode operation of a pyroelectric detector, what are we concerned about and under what conditions does this occur?

Answer 10

In voltage mode we are concerned about voltage generated by unit of input power.
This is the case at high modulation frequencies as might be expected in an imaging system where the input scene is deliberately chopped.

Question 11

Is pure iron a soft or hard magnetic material?

Answer 11

soft

Question 12

Name an example of a hard magnetic material

Answer 12

SmCo

Question 13

Explain why the maximum magnetisation, Ms, obtained in a conventional ferromagnet is a materials property, whereas in a bulk superconductor it depends on
sample size.

Answer 13

The magnetisation from a conventional magnetic material is due to the alignment of the spin (both electron and orbital) associated with the atoms within the material. The number of Bohr magnetons available per volume is fundamentally limited and thus maximum Magnetisation is sample size independent. It depends on the material itself.
In a superconductor however, the Magnetisation depends on a current loop and thus depends on the size of the sample as well as the maximum critical current available.

Question 14

What is the origin of the demagnetising field Hm?

Answer 14

The Magnetisation leads to a magnetic field in the magnetic material that acts back through the material itself – hence the term demagnetisation.

Question 15

Equation for demagnetising field

Answer 15

Hm = -NM

N- demagnetising factor

Question 16

Explain how the charge displacement arises and consequently gives rise to the observed behaviour for the case of each of piezo- and pyro- electric materials.

Answer 16

Both pyroelectric and piezoelectric materials are sub-classes of dielectric materials. Their electrical properties arise from asymmetry in their crystal structure.

Question 17

Explain why, when pyroelectric or piezoelectric behaviour is required, the material of choice is, in fact, often a ferroelectric.

Answer 17

Ferroelectrics are a subset of pyroelectrics, but where their polar axis can be changed by the application of an electric field. This means they exhibit both pyro and piezo electricity, and polycrystalline materials can be used which allow for the axis for the polar properties to be defined by applying a high enough electric field in that direction.

Ferroelectrics form a subset of the set of pyroelectrics because they are polar materials in which the direction of the polar axis can be changed by the application of an electric field. As a consequence they are both pyroelectric and piezoelectric. As many of the largest pyroelectric and piezoelectric effects occur in ferroelectric materials, they have become very important technologically. Furthermore, the fact that the polar axis can be reoriented by the application of a field means that polycrystalline ferroelectrics in which the crystallographic axes of the component crystallites are randomly oriented can be made to show polar properties by applying a sufficiently large electric field (the process of “poling”).

Question 18

Why do tensor equations arise?

Answer 18

Tensor equations arise when a change in one variable in one direction causes a change in a second variable in a different direction.

Question 19

Explain why, although there are 18 components to the reduced piezoelectric coefficient, only three piezoelectric coefficients are normally of technological
interest.

Answer 19

Piezoelectric materials are usually formed in thin plates which means that large field or strain is only developed across the thickness, therefore 2 of the stress and field values fall to 0, resulting in only 3 piezoelectric coefficients being of interest

Question 20

What is a benefit of working a piezoelectric material in shear mode?

Answer 20

Charge is only generated when the it is under shear, therefore the device is insensitive to the pyroelectric effect and will not generate charge with changes in temperature

Question 21

Describe the changes in gate oxide material in MOSFETS and why it was made

Answer 21

Changed from SiO2 to a thicker layer of a high k (dielectric constant) oxide such as HfO2
Done to stop tunnelling conduction through the gate oxide

Question 22

Describe the changes in material for interconnects in MOSFETS and why it was made

Answer 22

Al changed to Cu

To increase conductivity

Question 23

Describe the changes in material for interconnect dielectric in MOSFETS and why it was made

Answer 23

Changed from SiO2 to low k (dielectric constant) oxides such as SiOF
Replacing the silicon dioxide with a low-κ dielectric of the same thickness reduces parasitic capacitance, enabling faster switching speeds and lower heat dissipation.

Question 24

Describe the changes in material for the channel in MOSFETS and why it was made

Answer 24

Changed from Si to high mobility SiGe

To increase electron mobility (how fast an electron can move in a material)

Question 25

What is passivation, and what are 2 typical passivants?

Answer 25

passivation is the removal of gap states from the gap, often by a chemical reaction with a
passivant. Typical passivants are SiO2 or hydrogen.

Question 26

Describe the nature of states in amorphous silicon (a-Si) in the different energy ranges, in terms of their origin and conduction properties, and explain how they are related to those of crystalline silicon.

Answer 26

Extended states conduct electricity at 00K, like the band states in a crystal.
Localised states of band tail are created by the disorder. They do not conduct electricity at 00K.
Conduction for these occurs by hopping from 1 state to the next (tunnelling) or by thermal
activation to the extended states.

Question 27

Name to hard magnetic materials with a high Hc

Answer 27

NdBCO and SmCo

Question 28

What are the 2 types of gap states in a-Si

Answer 28

localised (tail) states and (deep) defect states.

Question 29

What are the cause of the tail/localised states in a-Si

Answer 29

Tail state due to angular and lattice disorder