Advanced topics

Question 1

What is the Hall effect?

Answer 1

used extensively in the semiconductor
industry to measure the electrical properties of
semiconductors at various stages in the fabrication of
semiconductor devices

Question 2

What is the hall effect used to measure? (3)

Answer 2

(1) Type of charge carrier: if the material is n-type (electron
majority charge carrier) or p-type (hole majority charge
carrier)
(2) The density of the charge carriers
(3) The mobility of the charge carriers (in conjunction with a
conductivity measurement)

Question 3

What is the Lorentz force?

Answer 3

The Lorentz force, Fm, is the force on the
moving charge carriers (current I) produced by
the magnetic field, B, and it separates the
charge carriers to create a voltage, VH.

Question 4

What does the lorentz force do? (2)

Answer 4

• When a current-carrying conductor is placed into a magnetic field, a voltage will be
  generated perpendicular to both the current and the field.
• When a perpendicular magnetic field is present. A Lorentz force is exerted on the electron. The Lorentz force tends to move the electron in perpendicular direction to
  both current and Magnetic Field. Charge is separated producing a voltage. The
  voltage is developed across the semiconductor perpendicular to the direction of the
  current.

Question 5

What is a laser?

Answer 5

The laser generates light waves that are in phase

(coherent) and that travel parallel to one another

Question 6

What does laser stand for?

Answer 6

- Light
• Amplification by
• Stimulated
• Emission of
• Radiation

Question 7

How does a ruby laser work? (3)

Answer 7

• pump electrons in the lasing material to excited
  states
• Direct electron decay transitions — produce incoherent light

Stimulated Emission
– The generation of one photon
by the decay transition of an
electron, induces the emission
of other photons that are all in
phase with one another.
– This cascading effect produces
an intense burst of coherent
light.

Question 8

What is stimulated emission in the operation of the ruby laser? (2)

Answer 8

Stimulated Emission
– The generation of one photon
by the decay transition of an
electron, induces the emission
of other photons that are all in
phase with one another.
– This cascading effect produces
an intense burst of coherent
light.

Question 9

What is nanotechnology?

Answer 9

The technology associated with the solid state devices that have
dimensions small enough (<100nm) so that the wave nature of electrons becomes
important.

Question 10

What is epitaxial layer growth?

Answer 10

In epitaxial growth the component
atoms are directed to the surface of
single crystal substrate wafer.
The substrate is hot and the atoms
have enough thermal energy to
find their minimum energy
configuration and so crystalline
epitaxial layers form on the surface
of the substrate.

Question 11

What is MBE growth?

Answer 11

MBE has a hot rotating single crystal wafer – the substrate – in a vacuum chamber -ultra
high vacuum. The substrate is exposed to atoms coming from hot sources of atomic vapour.
The vapour is directed onto the substrate and the amount of vapour from each atomic source
is controlled by a mechanic shutter. The crystal structure of the epitaxial (very thin) layer
than forms of the substrate surface is monitored by the diffraction pattern formed by low
energy electron diffraction. The layers grow at around 1 micron per hour. Very high precision
growth possible down to one monolayer.

Question 12

What is MOCVD growth?

Answer 12

A chemical reaction breaks down metal –
organic gases close to the surface of the hot
substrate and there is no high vacuum.

Question 13

What are advantages and disadvantages of MOCVD compared to MBE?

Answer 13

Advantages
Faster growth several microns per hour –
PV solar cells can be grown this way

Disadvantages
Difficult to monitor growth with an
electron beam because there is no vacuum
Not as abrupt interfaces between layers
Very toxic and/or explosive gases