Fundamentals

Question 1

What is a hole?

Answer 1

A ‘gap’ left behind when an electron escapes it’s bonds and jumps from the valence band to the conduction band.

Question 2

What are the gaps left behind by an escaping electron called?

Answer 2

A hole

Question 3

How does a hole move?

Answer 3

A hole is restricted to only move within the bonds

Question 4

What is the charge of a hole?

Answer 4

Positive

Question 5

What is the importance of holes, in regard to conductivity?

Answer 5

The importance of a hole is that it may serve as a carrier of electricity in the same manner as a free electron but in the opposite direction.

Question 6

What are the two energy domains in which current can flow?

Answer 6

The conduction band and the valence band

Question 7

What is the conduction band?

Answer 7

The range of energies that electrons must possess if they are to be free to conduct electricity (rather than held in a bond).

Question 8

What is the valence band?

Answer 8

The range of electron energies that electrons are said to possess when they take part in a hole conduction process.

Question 9

How does mobility vary between an electron and a hole?

Answer 9

The holes in the valance band are less mobile than electrons in the conduction band by a factor of up to 10.

Question 10

What is mobility?

Answer 10

The speed at which electrons or holes move under the influence of unit electric field.
(speed with which carriers move is proportional to the applied electric field)

Question 11

Define: hole-electron pair generation

Answer 11

This refers to the excitation of electrons out of crystal bonds leaving holes behind

Question 12

What is electron density?

Answer 12

The number of free electrons per unit volume in the semiconductor

Question 13

What is hole density?

Answer 13

The number of holes per unit volume in the semiconductor

Question 14

What is a band/energy gap?

Answer 14

The minimum energy needed by an electron to escape its bonds, and move from the valence band to the conduction band

Question 15

What is a valence shell of an atom?

Answer 15

The outermost shell. It represents a band of energy levels so is also known as the valence band

Question 16

What happens if we supply enough energy to an electron in the valence band?

Answer 16

The electron will escape from the valence band and go to the conduction band

Question 17

How does the band gap of a material relate to it’s conductivity?

Answer 17

If the band gap is very large, then electrons will require a lot more energy in order to jump from the valence band to the conduction band so the material will be a poor conductor.

If the band gap is smaller the electrons can jump to the conduction band more readily and therefore the material is a better conductor.

Question 18

What is recombination?

Answer 18

Recombination is the fall of electrons into the gaps (holes) in the chemical bonds in the crystal with a release of energy Eg(electron-volts) per electron.

Question 19

What does the intrinsic carrier density represent?

Answer 19

The number of holes or electrons in a pure semiconductor

Question 20

What is Boltzmann’s constant?

Answer 20

k = 1.38 x 10^(-23) JK^(-1)

Question 21

What is the intrinsic carrier density of silicon at 300K?

Answer 21

ni = 1.5 x 10^(16) m^(-3)

Question 22

What is the intrinsic carrier density of germanium at 300K?

Answer 22

ni = 2.5 x 10^(19) m^(-3)

Question 23

Describe qualitatively how the recombination rate is found.

Answer 23

The probability of an electron falling into a hole depends upon the density p0 of holes available. This probability must be multiplied by the density n0 of the electrons waiting to recombine.
This allows us to obtain the net average recombination rate.