Lecture 1

Question 1

State the 3 most common applications of semiconductors

Answer 1

• Transistors
• Diodes
• Light emitting diodes (LEDs)

Question 2

In the low temperature limit, all materials are either __________ or _______.

Answer 2

Insulators
Metals

Question 3

Describe how the resistivity of a pure metal changes with temperature

Answer 3

The resistivity rapidly increases with increasing temperature.

Question 4

How does the resistivity of a semiconductor relate to that of a metal or an insulator?

Answer 4

It is an intermediate between metals and insulators at room temperature.

Question 5

Describe how the resistivity of a pure (intrinsic) semiconductor changes with temperature

Answer 5

The resistivity decreases rapidly with increasing temperature.

Question 6

Describe how the resistivity of a doped (extrinsic) semiconductor changes with temperature

Answer 6

The resistivity is approximately temperature independent.

Question 7

What is a bandgap?

Answer 7

A range of energy for which there are no allowed electron states. This allows well defined energy levels to be formed.

Question 8

Are the valence and conduction bands filled for semiconductors at T = 0?

Answer 8

Valence band: filled (the highest energy band that is filled)
Conduction band: unfilled (the lowest energy band that isn’t filled)

Question 9

Are the valence and conduction bands filled for semiconductors at T > 0?

Answer 9

Valence band: filled
Conduction band: partially filled (due to the thermal excitation of electrons across a relatively small bandgap)

Question 10

What are doped semiconductors?

Answer 10

Semiconductors with added impurities. These impurities add electrons to the conduction band or remove electrons from the valence band.

Question 11

Give the equation for the energy of a photon

Answer 11

E = energy
f = frequency
c = speed of light
λ = wavelength

Question 12

Where are elemental semiconductors found in the periodic table?

Answer 12

In group IV

Question 13

Give 2 examples of elemental semiconductors

Answer 13

• Silicon (Si)
• Germanium (Ge)

Question 14

How many valence electrons per atom do elemental semiconductors have?

Answer 14

4

Question 15

What crystal structure do elemental semiconductors have?

Answer 15

Diamond structure

Question 16

Where are compound semiconductors found in the periodic table?

Answer 16

They are either a combinations of groups III and V or a combination of groups II and VI

Question 17

Give 2 examples of compound semiconductors for each bonding combination

Answer 17

III-V: GaAs, InP, GaN
II-VI: ZnO, HgTe

Question 18

How many valence electrons per atom do compound semiconductors have on average?

Answer 18

4

Question 19

What crystal structure do compound semiconductors have?

Answer 19

Zincblende structure

Question 20

Electrons in a nearly filled band can behave as if they have ________ charge.

Answer 20

Positive

Question 21

What is a hole?

Answer 21

An empty state (vacancy) with positive energy, charge, and effective mass due to the absence of an electron. It is at the top of the valence band so when an electron is removed the total energy of the band increases positively (by +ε as an electron with -ε energy has been removed).

Question 22

When are the dynamical properties of electron states equivalent to those of hole states?

Answer 22

In the case of a nearly filled band with n empty electron states, this is equivalent to n hole states with a positive charge and a positive effective mass.

Question 23

The properties of a hole equal the properties of a _____ with one missing ________.

Answer 23

Band
Electron

Question 24

What is the energy of a hole?

Answer 24

Equal and opposite energy to that of an electron.

+ε

Question 25

What is the wavevector of a full energy band?

Answer 25

k = wavevector

Question 26

What is the wave vector of a hole?

Answer 26

Equal and opposite to the wave vector of an electron.

Question 27

What is the charge of a hole?

Answer 27

It behaves as a positively charged particle in response to an electric field. The charge is of the same magnitude as that of an electron.

+e

Question 28

Give the equation for the effective mass of an electron state

Answer 28

m_e* = effective mass
ε_e = electron energy state
k_e = electron wavevector

Question 29

What is the effective mass of an electron?

Answer 29

negative

Question 30

What is the effective mass of a hole?

Answer 30

positive

Question 31

The effective mass of a hole is _____ and ________ to that of an electron.

Answer 31

Equal
Opposite