Basics of semiconductor physics and chemistry

Question 1

What are valence electrons?

Answer 1

The electrons in the outermost shell

Question 2

How many electrons can a shell contain?

Answer 2

2*n^2 where n stands for the period

Question 3

What are the three types of bonding?

Answer 3

Atomic bond, Ionic bond, Metallic Bond

Question 4

How does the atomic/co-valent bond work?

Answer 4

Atoms share electrons in order to achieve the lowest energy state

Question 5

How does the ionic bond work?

Answer 5

One atom donates an electron whereas another accepts an electron. The atoms stick together due to the charge difference between the,

Question 6

What are cations and anions?

Answer 6

A charged atom
is known as an ion, while a positive ion is called cation and a negative ion anion,
respectively

Question 7

How does Metallic bonding work?

Answer 7

The outer energy levels of atoms overlap, and valence electrons are shared between atoms, they can move around

Question 8

What properties do greater attraction forces and number of free electrons have

Answer 8

greater attraction forces mean higher melting and boiling points, the
number of free electrons affects the conductivity.

Question 9

What energy state do atoms want to achieve

Answer 9

The “octet” - 8 valence electrons, apart from helium with 2. Nobles gasses exist in this state and are unreactive

Question 10

Why are insulators non-conductive

Answer 10

They have no free charge carriers

Question 11

Are atoms in an atomic bond conductors?

Answer 11

No, there are no free electrons due to sharing electrons so atoms in atomic bonds are insulators

Question 12

Are atoms in ionic bonds conductors?

Answer 12

In the solid state, ions are arranged in a grid network. By electrical forces, the particles are held together. There are no free charge carriers to enable a current flow. Thus
substances composed of ions can be both conductor and insulator.

Question 13

What are semiconductors?

Answer 13

Semiconductors are solids whose conductivity lies between the conductivity of conductors and insulators. Due to exchange of electrons - to achieve the noble gas configuration - semiconductors arrange as lattice structure. Unlike metals, the conductivity
increases with increasing temperature.

Question 14

How are holes formed?

Answer 14

An electron detaches from an atom, a hole is left. Another electron jumps to fill this hole, this happens in a chain. Holes and atoms move in opposite directions

Question 15

What is the band gap?

Answer 15

The valence electrons - which serve as
charge carriers - are located in the valence band, in the ground state the conduction
band is occupied with no electrons. Between the two energy bands there is the band
gap, its width affects the conductivity of materials.

Question 16

What is the valence band?

Answer 16

with a very
large number of atoms, a single electron can no longer be assigned to one single atom.
As a result, the energy bands of the individual atoms merge to a continuous band, the
valence band.

Question 17

What occurs in the band model of conductors?

Answer 17

In conductors, the valence band is either not fully occupied with electrons, or the filled
valence band overlaps with the empty conduction band. In general, both states occur
at the same time, the electrons can therefore move inside the partially filled valence
band or inside the two overlapping bands.

In conductors there is no band gap between
the valence band and conduction band.

Question 18

What occurs in the band model of insulators?

Answer 18

In insulators the valence band is fully occupied with electrons due to the covalent
bonds. The electrons can not move because they are “locked up” between the atoms.
To achieve a conductivity, electrons from the valence band have to move into the conduction band. This prevents the band gap, which lies in-between the valence band and
conduction band.
Only with considerable energy expenditure (if at all possible) the band gap can be
overcome; thus leading to a negligible conductivity.
(Very large band gap)

Question 19

Is the valence band higher than the conduction band?

Answer 19

No, the conduction band is above the valence band

Question 20

How large is the band gap in semiconductors?

Answer 20

In semiconductors, there is a band gap, but compared to insulators it is so small
that even at room temperature electrons from the valence band can be lifted into the
conduction band

Question 21

Why are there positive charge carriers in semiconductors?

Answer 21

The electrons can move freely and act as charge carriers. In addition,
each electron also leaves a hole in the valence band behind, which can be filled by
other electrons in the valence band. Thus one gets wandering holes in the valence
band, which can be viewed as positive charge carriers.

Question 22

Compare the amount of holes and electrons in a semiconductor

Answer 22

There are always pairs of electrons and holes, so that there are as many negative as positive charges, the semiconductor crystal as a whole is neutral.

Question 23

What is an intrinsic semiconductor?

Answer 23

A pure undoped semiconductor is known as intrinsic semiconductor.

Question 24

Why aren’t intrinsic semiconductors used for components?

Answer 24

Conductivity comparable to metals is only possible at very high temperatures

Question 25

How can we influence the number of free holes and electrons?

Answer 25

Doping

Question 26

How many electrons in the valence for P doping?

Answer 26

3 valence electrons eg. boron

Question 27

How many electrons in the valence for n doping?

Answer 27

5 valence electrons eg. phosphorus

Question 28

Do p dopants accept or donate electrons?

Answer 28

P dopants have 3 electrons so can catch an extra electron, leaving a hole in the valence band. Therefore they accept.

Question 29

Do n dopants accept or donate electrons?

Answer 29

N dopants have 5 electrons so can donate an electrons. Therefore the dopants become positively charged

Question 30

Electronic band structure in doped semiconductors

Answer 30

Through the introduction of a dopant with five outer electrons, in n-doped semiconductors there is an electron in the crystal which is not bound and therefore can be
moved with relatively little energy into the conduction band. Thus in n-doped semiconductors one finds a donator energy level near the conduction band edge, the band
gap to overcome is very small.
Analog, through introduction of a 3-valent dopant in a semiconductor, a hole is available, which may be already occupied at low-energy by an electron from the valence
band of the silicon. For p-doped semiconductors one finds an acceptor energy level
near the valence band.

Question 31

What is the fermi level?

Answer 31

The fermi level is the highest energy state an electron can exist in at absolute 0. It is also the energy required to add or remove an electron from a body

Question 32

In an N doped extrinsic semi-conductor, is the fermi level closer to the conduction or valence band?

Answer 32

The fermi level is closer to the conduction band

Question 33

In an N doped extrinsic semi-conductor, is the fermi level closer to the conduction or valence band?

Answer 33

The fermi level is closer to the valence band