6. 6
◼ Atom is stable → Filled or Empty valence shell
1- Gain (or) lose electrons → Ionic compounds
Semiconductor Materials
7. 7
◼ Atom is stable → Filled or Empty valence shell
1-share electrons-→ covalent bonds
Semiconductor Materials
8. Semiconductor Materials
◼ Intrinsic (pure) semiconductor
Pure silicon is the most commonly used
Pure carbon very expensive
Pure Germanium has poor temperature
stability.
◼ Extrinsic (impure) semiconductor
8
11. Intrinsic Semiconductor
11
◼ No Impurities.
◼ Complete bonds at 0 K.
◼ No charge carriers.
Is it conductor ?!
◼ T > 0 → free electrons
12. What happen if T>0 k
◼ Si has four valence electrons. Therefore, it can
form covalent bonds with four of its nearest
neighbors.
◼ When temperature goes up, electrons can
become free to move about the Si lattice.
12
14. There are 2 types of mobile charge carriers in Si:
Conduction electrons are negatively charged;
Holes are positively charged.
15. Charge Carriers
◼ Excited electron = Free electron = Conduction
electron:
Electron from valence band to conduction
band.
◼ Negative electrons (n)…. ” Conduction band: Ec”
◼ Positive holes (p)------” Valence band: Ev ”
◼ Bandgap energy: ----”No carriers”
Eg = (Ec – Ev)
15
16. Charge Carriers
Carrier generation process
◼ Carrier generation describes
processes by which electrons gain
energy and move from the valence
band to the conduction band,
producing two mobile carriers;
16
17. ◼ Recombination describes
processes by which a conduction
band electron loses energy and
re-occupies the energy state of an
electron hole in the valence band.
◼ The electron–hole pair is the
fundamental unit of generation
and recombination
17
Charge Carriers
Carrier recombination process
18. Charge Carriers
◼ For the intrinsic
material, since
electrons and
holes are always
created in pairs, n
= p = ni where ni is
the symbol for
”intrinsic carrier
concentration.”
◼ To quantify electron
concentration (n) we count
number of electrons in the
conduction band per unit
volume /cm3
◼ To quantify hole
concentration (p) we count
number of holes in the
valance band per unit
volume /cm3
18
19. 19
◼ Doping process
Adding impurities to intrinsic semiconductor
To increase number of electrons (negative doping)
To increase number of holes (positive doping)
◼ n-type semiconductor
◼ p-type semiconductor
Extrinsic (Impure) Semiconductors
23. n-type semiconductors
What happen when adding pentavalent
atoms to intrinsic semiconductor?
▪ As atom has five valance
electron.
▪ Four electron form a covalent
bound with four silicon
▪ Fifth electron has no chance to
form such a bound (Free electron)
▪ Each As atom donate one
electron to conduction band of Si
atom leaving a positive charged
atom behind (ionization)
23
24. All donor atoms included in the crystal will give an electron to conduction band
𝒏𝟎 ≅ 𝑵𝑫
Example : 1 mg of As atom will have 8x1014 atom
1 e- 8x1014 free electron
24
n-type semiconductors
What happen when adding pentavalent
atoms to intrinsic semiconductor?
25. +5
-
-
+4
+4 +4
- -
-
-
+4
- -
+4
- -
+4
-
-
+4
-
-
+4
-
-
- -
- -
-
-
- -
-
-
-
-
-
-
-
-
-
-
-
-
CB
VB
x
EG
E
EC
EV
ED
not in valence band !
ED donor energy level
As a result of pentavalent impurity there will always be more free electrons
in the conduction band than holes in the valence band
Majority carriers : electrons
Minority carriers : holes
n-type semiconductors
Energy Band Diagram
25
26. ◼ ni: intrinsic carrier
concentration
◼ ND: Donor concentration
◼ n: electron concentration
◼ p: hole concentration
◼ n-type
26
◼ Intrinsic semi-conductor
n = p = ni
n-type semiconductors
Carrier concentration (Low of mass action)
28. p-type semiconductors
What happen when adding triavalent atoms
to intrinsic semiconductor?
▪ As atom has three valance
electron.
▪ three electron form a covalent
bound with three silicon
▪ It need one more electron to be
stable
▪ Each atom accept one electron
from Si atom creating a negative
charged atom behind (ionization)
28
29. All acceptor atoms included in the crystal will accept an electron (adding hole in
valance band) 𝒑𝟎 ≅ 𝑵𝑨
29
p-type semiconductors
What happen when adding trivalent atoms to
intrinsic semiconductor?
30. p-type semiconductors
+3
-
+4
+4 +4
- -
-
-
+4
- -
+4
- -
+4
-
-
+4
-
-
+4
-
-
- -
- -
-
-
- -
-
-
-
-
-
-
-
-
-
-
-
-
CB
VB
E
x
EG
EC
EV
EA
EA acceptor energy level
31. ◼ ni: intrinsic carrier
concentration
◼ NA: Accaptor
concentration
◼ n: electron concentration
◼ p: hole concentration
◼ p-type
31
◼ Intrinsic semi-conductor
n = p = ni
p-type semiconductors
Carrier concentration (Low of mass action)
32. Carriers transfer
32
There are two distinctly different mechanisms
for the movement of charge carriers and hence
for current flow in semiconductors: drift and
diffusion.
Currents
Drift
Diffusion
33. Carriers transfer
Drift current
❑Due to electric field (E) applied.
33
Holes are accelerated in the direction of E,
and free electrons are accelerated in the
direction opposite to that of E.
34. Carriers transfer
Drift current
◼ The flow of charge carriers,
which is due to the applied
voltage or electric field is called
drift current.
◼ In a semiconductor, there are
two types of charge carriers,
(electrons and holes)
◼ When the voltage is applied to a
semiconductor, the free
electrons move towards the
positive terminal of a battery and
holes move towards the
negative terminal of a battery. 34
35. Carriers transfer
◼ Drift current
Due to electric field (E) applied.
Drift velocity
µp hole mobility & µn electron mobility
◼ Vn is the drift velocity representing the carrier concentration of electrons.
◼ µn is the mobility of the particular electrons and
◼ E represents the electric field applied to the n-type of the semiconductor
Mobility measures how easily electronsholes moves in response to the
applied electrical field 35
E
v p
p
= E
v n
n
−
=
36. Drift current density
n
n v
n
q
J −
= p
p v
p
q
J =
E
n
n μ
v −
= E
p
p μ
v =
E
n
μ
n
q
= E
p
μ
p
q
=
( )E
p
n
p
n μ
p
nμ
q
J
J
J +
=
+
=
36
q = 1.6X10-19 coulomb
µn is the mobility of electrons its units are cm2/ Vs
µp is the mobility of holes and its units are cm2/ Vs
E is the electric field applied on it and it is measured in terms of V/ cm
37. Conductivity of Semiconductor
( )
p
n μ
p
nμ
q
σ +
=
σ
ρ
1
=
E
=
J
( )E
p
n μ
p
nμ
q
J +
=
Ohm’s law:
( ) ( )
p
A
n
D
p
n μ
N
μ
N
q
μ
p
nμ
q +
+
=
1
1
resistivity of silicon
37
38. ◼ Carrier diffusion occurs when the density of charge
carriers in a piece of semiconductor is not uniform.
◼ Carrier will diffuse from the region of high
concentration to the region of low concentration.
◼ The diffusion of charge carriers gives rise to a net
flow of charge, or diffusion current.
◼ If uniform concentrations of (n & P) → no current.
38
Carriers transfer
Diffusion current
