Background Semiconductor Physics and Quantum Mechanics

Question 1

When an atom loses energy and drops to a lower energy level, what is the energy of the photon produced equal to?

Answer 1

The difference between the two energy states of the atom.

Question 2

Why do different elements produce different colours when their atoms are excited?

Answer 2

Atoms of different elements have different energy levels that correspond to photons of different wavelength.

Question 3

What happens to the energy levels in an atom when many are grouped to form a solid?

Answer 3

The energy levels are spread out into a series of bands of levels.

Question 4

If N atoms make up a solid then there are N states in each band. Why can each band hold a maximum of 2N electrons?

Answer 4

In each state the electron can have either spin up or spin down, accounting for the factor of 2.

Question 5

Why can only bands which are partly occupied allow electrical conduction?

Answer 5

Electrons can more through the solid if there are some unoccupied states within that band.

Question 6

What is the structure of the bands (how full they are) for an element with an odd atomic number Z?

Answer 6

A number of bands completely filled followed by one band that is half filled.

Question 7

Why is Aluminium (Z=13) a good conductor of electricity?

Answer 7

The 7th band is half filled. It has a large number of electrons and unoccupied states, making it easy for these electrons to move.

Question 8

Why is Silicon (Z=14) an insulator?

Answer 8

The first 7 bands are completely full, all other bands are empty.

Question 9

What distinguishes an insulator from a semiconductor?

Answer 9

The energy difference between the top most filled band and the next empty band.

Question 10

What can happen (in a semiconductor) if the band gap is reasonable small?

Answer 10

It is possible for electrons to be excited from the filled band into the empty band by thermal energy.

Question 11

What are the two important bands in a semiconductor and where are they located?

Answer 11

The valence band - the topmost normally filled band. The conduction band - the next band which is normally unoccupied.

Question 12

How does semiconductor electrical conduction occur?

Answer 12

Through the motion of electrons in the conduction band and the remaining electrons in the valence band.

Question 13

What happens to the conduction in a semiconductor as the temperature increases?

Answer 13

More electrons and holes are excited so the semiconductor conducts electricity better.

Question 14

Why is the conductivity of a semiconductor much smaller than that of a metal?

Answer 14

Even at room temperature, the number of electrons in the conduction band is much smaller than the number in the half filled band of a conductor.

Question 15

What is n-type doping?

Answer 15

A method of increasing the number of electrons in the conduction band by adding impurities that have extra valence electrons (n = negative).

Question 16

What is p-type doping?

Answer 16

A method of increasing the number of holes in the valence band by adding impurities with less valence electrons (p = positive).

Question 17

What is the basis of a photo detector or solar cell?

Answer 17

A photon enters a semiconductor and excites an electron into the conduction band. This electron and hole is extracted into an electrical circuit and can be detected.

Question 18

How is the potential energy due the electrostatic interaction with ions incorporated into calculations?

Answer 18

The mass of the electron is altered to include the effect of potential energy. This is known as the effective mass, m*.

Question 19

What is a quantum well?

Answer 19

A thin semiconductor sheet that restricts motion in the third direction, perpendicular to the plane of motion, effectively trapping electrons and holes in the sheet.

Question 20

What free motion is allowed in a quantum wire and a quantum dot?

Answer 20

Quantum wire - 1 dimension. Quantum dot - no free motion is allowed.