Semiconductors
ECE 2210
John Stahl
Western Michigan University
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
For Silicon, which is atomic number 14 there are
14-protons. This means Silicon has the
electrostatic draw to attract 14-electrons.
Silicon:
Protons (+): 14
Electrons (-): 14
↑↓
↑↓
1S
2S
Core Electrons
↑↓
↑↓
2P
↑↓
↑↓
3S
↑
↑
3P
Valance Electrons
While there is enough electrons to balance the protons there are still openings in 3P
orbital. Full P-Orbitals represent a stable configuration of electrons. Since Silicon does not
have the raw positive draw to fill the 3P orbital it must share electrons. Each level within
the shell usually holds 2 electrons.
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
A common partner for Silicon is Oxygen, which is seeking two electrons to fill its
outer 2P-Orbital. For the stable orbital and charge balance there are typically 8valance electron opening to fill with the adjoining elements providing their valance
electrons.
Oxygen: 6 Valance electrons
Silicon: 4 Valance electrons
O
Si
Si
O
O
Each Oxygen molecule shares two electrons with Silicon so each element is surrounded
by 8-electrons. The combination is Silicon Oxide or Quartz.
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
Silicon Crystal
Pure Silicon can bond with other Silicon atoms to form a stable arrangement. A
shared electron pair is called a Covalent Bond. Each Silicon atom brings four electrons
to bond with allowing a stable arrangement of 8 electrons around a Silicon atom. The
complete bonding makes pure crystal Silicon a poor conductor of charge.
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
Si
Si
Doping
Adding impurities to the Silicon Crystal can
greatly alter the electrical properties of the
material. We’ll start with Aluminum which is the
neighboring element of Silicon.
Aluminum: 3 Valance electrons
Silicon: 4 Valance electrons
Si
Si
Si
Si
Si
Si
Si
Al
Si
Si
Si
Si
Al
Al
Si
Si
Si
Al
Since Aluminum does not have enough
electrons to fill the Silicon crystal
matrix there are holes in some of the
Silicon shells. These opening appear to
be weakly positive to an electron or PType material.
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
Next we dope with Phosphorus which will
bring 5 electrons to bond with resulting in a
left over electron.
Aluminum: 5 Valance electrons
Silicon: 4 Valance electrons
Si
P
Si
Si
P
Si
Si
Si
Si
Si
Si
P
Si
Si
Si
Si
Si
P
The resulting material has an excess
amount of electrons and has an overall
negative charge or N-Type material.
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
Electrons from the N-type material move into the openings of the p-type material.
The openings or Holes move toward the n-type material. The center forms a resistive
area called a Depletion Region.
P-Type
N-Type
Si
Si
Si
Si
P
Si
Si
P
Si
Si
Al
Si
Si
Si
Si
Si
Si
Si
Si
Al
Si
Si
P
Si
Al
Si
Si
Si
Si
Si
Si
Si
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
-
-
-
-
-
-
-
+
+
-
+
+
+
- +
+
+
-
+
+
-
N-Type
+
+
+
P-Type
Depletion Region
Applying a positive voltage to the anode and a negative voltage to the cathode causes
negative charges separate and shrink the depletion region. Charge can then flow
through the material.
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
BASE
PNP Transistor
COLLECTOR
+
+
+
+
-
+
+ +
+
+ +
+
+ +
+
+
+
- P-Type
+
+
+
+
+
- +
-
+
+
+
-
-
+
+
+
+
- +
- +
-
N-Type
+
+
+
+
+
+
+
+
+
EMITTER
+
P-Type
By placing a Negative Voltage at the BASE the negative charge in the depletion region is
pushed away leaving a small positively charged channel connecting the EMITTER and
COLLECTOR.
E
B
pnp
Copyright: John Stahl.
C For educational use.
Do not distribute or reproduce without
permission
BASE
NPN Transistor
COLLECTOR
-
-
-
-
-
-
-
-
-
+
+
- +
+
+
+
+
N-Type
+ -
+
+
-
-
-
+
+
+
+
-
-
+ + + -
P-Type
-
-
-
-
EMITTER
-
-
N-Type
By placing a Negative Voltage at the BASE the negative charge in the depletion region is
pushed away leaving a small positively charged channel connecting the EMITTER and
COLLECTOR.
C
B
npn
Copyright: John Stahl.
E For educational use.
Do not distribute or reproduce without
permission
MOSFET Transistor
DRAIN
+
+
+
+ +
+
+
+
+
+
+
+
+
P-Type
+
-
+
-
+ -
+
-
+
+
-
-
- +
GATE
+
+
+
-
-
-
-
+
+
+
+
-
+
+
- +
- +
-
N-Type
+
+
+
+
+
+
+
+
+
SOURCE
+
P-Type
The P MOSFET works by separating the GATE terminal from the n-type material by a metal
oxide layer. Applying a negative voltage to the GATE causes free negative charges to move
away from the GATE and positive holes to accumulate. This leaves a positive channel for
charge to flow through.
S
G
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
PFET
D
GATE
DRAIN
-
-
-
-
-
-
-
+
-
-
N-Type
-
+
-
-+
+
+
- +
+ -
+
- +
+
+
-
+
+
+
+
P-Type
-
+ -
-
-
-
-
SOURCE
-
-
N-Type
The N MOSFET works by separating the GATE terminal from the n-type material by a metal
oxide layer. Applying a positive voltage to the GATE causes holes to move away from the
GATE and electrons to accumulate. This leaves a negative channel for charge to flow
through.
D
G
Copyright: John Stahl. For educational use.
Do not distribute or reproduce without
permission
NFET
S