1. Chapter 2 Semiconductor Materials and Diodes

2. Classification of Materials
Classification according to the way materials react to the current when a voltage is applied across them:
Insulators
Materials with very high resistance - current can’t flow
mica, rubber
Conductors
Materials with very low resistance – current can flow easily
copper, aluminum
Semiconductors
Neither good conductors nor insulators (silicon, germanium)
Can be controlled to either insulators by increasing their resistance or conductors by decreasing their resistance

3. Semiconductor Materials and Properties
An atom is composed of a nucleus, which contains positively charged protons and neutral neutrons, and negatively charged electrons that orbit the nucleus.
Electrons in the outermost shell are called valence electrons.

4. Elemental Semiconductors
A portion of the periodic table in which the more common semiconductors are found
Elemental Semiconductors
Silicon (Si) and germanium (Ge) are in group IV. Hence, they have 4 electrons in their outer shells
Do you still remember?
A stable atoms need ? electrons at its outermost shell 8

5. Si have 4 electrons in their outer shells
needs another 4 to become stable
So, when there are 4 other Si nearby = 4 electrons: Si
Sharing of electrons occurred; and this bond is known as the covalent bond Si Si Si Si
Atoms come into close proximity to each other and so the valence electrons interact to form a crystal.

6. BANDGAP ENERGY, Eg
Now, in order to break the covalent bond, a valence electron must gain enough energy to become free electrons.
The minimum energy required is known as the bandgap energy, Eg

7. ILLUSTRATION WHEN A VALENCE ELECTRON IS FREE
1. Becomes free electron
3. Electron moves to fill space
5. Electron moves to fill space
2. Becomes empty
4. Becomes empty

8. Intrinsic Semiconductor
A single-crystal semiconductor material with no other types of atoms within the crystal.
The densities of electrons and holes are equal.
The notation ni is used as intrinsic carrier concentration for the concentration of the free electrons as well as that of the hole:
B = a coefficient related to the specific semiconductor material
Eg = the bandgap energy (eV)
T = the temperature (Kelvin) remember that K = °C
k = Boltzmann’s constant (86 x 10-6 eV/K)

9. Intrinsic Semiconductor
The values of B and Eg for several semiconductor materials:
Example:
Calculate the intrinsic carrier
concentration in silicon
at T = 300 K.

10. Example 2
Find the intrinsic carrier concentration of Gallium Arsenide at temperature = 300K
k = Boltzmann’s constant (86 x 10-6 eV/K)
Answer: 1.8 x 106 cm-3

11. Example 3
Answer: 1.4 eV

12. Extrinsic Semiconductor
Since intrinsic concentration, ni is very small, so, very small current is possible
So, to increase the number of carriers, impurities are added to the Silicon/Germanium.
The impurities will be from Group V and Group III

13. Group V + Si = n-type semiconductor
Extra electron
Group V – 5 electrons in the outer shell; Example, Phosphorus, Arsenic
The 5th electron are loosely bound to the Phosphorus atom
Hence, even at room temperature, the electron has enough energy to break away and becomes free electron.
Atoms from Group V are known as donor impurity (because it donates electrons)
Group V + Si = n-type semiconductor

14. Group III + Si = p-type semiconductor
Extra hole
Group III – 3 electrons in the outer shell; Example, Boron
The valence electron from outer shells are attracted to fill the holes added by the insertion of Boron
Hence, we have movement of holes
Atoms from Group III are known as acceptor impurity (because it accept electrons)
Group III + Si = p-type semiconductor

15. Effects of doping process
The materials containing impurity atoms are called extrinsic semiconductors, or doped semiconductors.
Effects of doping process
controls the concentrations of free electrons and holes
determines the conductivity and currents in the materials.
The relation between the electron and hole concentrations in thermal equilibrium:
no = the thermal equilibrium concentration of free electrons
po = the thermal equilibrium concentration of holes
ni = the intrinsic carrier concentration

16. For N-type – electrons are the majority carriers
At room temperature (T = 300 K), each donor atom donates a free electron to the semiconductor.
If the donor concentration Nd is much larger than the intrinsic concentration, approximately:
Then, the hole concentration:

17. For P-type – holes are the majority carriers
Similarly, at room temperature, each acceptor atom accepts a valence electron, creating a hole.
If the acceptor concentration Na is much larger than the intrinsic concentration, approximately:
Then, the electron concentration:

18. Example 1
Calculate the thermal equilibrium electron and hole concentrations.
Consider silicon at T = 300 K doped with phosphorous at a concentration of Nd = 1016 cm-3 and ni = 1.5 x 1010 cm-3.

19. Example 2
Calculate the majority and minority carrier concentrations in silicon at T = 300K if
Na = 1017cm-3
Nd = 5 x 1015cm-3
Calculate ni
For part (a) – it is p-type
For part (b) – it is n-type
Answer: a) majority = 1017cm-3 minority 2.25x 103 cm-3
b) Majority 5 x 1015cm-3, minority 4.5 x 104 cm-3

20. Calculate the intrinsic carrier concentration of Silicon at T = 250K
k = Boltzmann’s constant (86 x 10-6 eV/K)
EXAMPLE 1
Calculate the intrinsic carrier concentration of Silicon at T = 250K
EXAMPLE 2
A silicon is doped with 5 x 1016 arsenic atoms
Is the material n-type or p-type?
Calculate the electrons and holes concentration of the doped silicon at T=300K
Answer: ni = 1.6 x 108 cm-3
Answer:
n-type
no = 5 x 1016 cm-3 and po = 4.5 x 103 cm-3