Lecture 5
Semiconductors
• Semiconductors are crystalline materials
• For electronics application: Germanium (Ge) and Silicon (Si)
Lecture 5
Semiconductors - Silicon
Silicon has 14 electrons, but only the outer most 4 are
available as "valence" electrons to help bond with other atoms
Silicon atom showing 4 electrons in outer valence shell
Lecture 5
Semiconductors - Silicon
Once the atoms have been
arranged like this, the outer
valence electrons are no longer
strongly bound to the host
atom. Therefore, in principle,
these outer electrons can easily
be "freed" from the lattice and
move through the material.
The movement of electrons
through material is a current.
Lecture 5
Semiconductors - Germanium
Germanium atoms showing 4 electrons in
outer valence shell
Lecture 5
Semiconductors – Germanium (Ge)
Lecture 5
Summary
Impurities
By adding certain impurities to pure (intrinsic) silicon,
more holes or more electrons can be produced within
the crystal.
To increase the number of conduction
band electrons, pentavalent impurities are
added, forming an n-type semiconductor.
These are elements to the right of Si on
the Periodic Table.
To increase the number of holes, trivalent
impurities are added, forming a p-type
semiconductor. These are elements to the
left of Si on the Periodic Table.
III IV V
B C N
Al Si P
Ga Ge As
In Sn Sb
Lecture 5
Chemistry Periodic Table of the Elements
Lecture 5
P-type Semiconductor – add Boron (B), Aluminum(Al),
Gallium (Ga) to Silicon (Si).
When a small amount of impurity atom is added to the
intrinsic semiconductor, it is called impurity.
P-type semiconductor has holes in majority and free electrons.
Lecture 5
N-type Semiconductor – add phosphorus (P), arsenic(As)
and antimony (Sb).
With donor impurities - donate one valence electron to the
semiconductor crystal
N-type semiconductor has creates many free electrons and
holes as minority
Lecture 5
P-N Junction and Diodes
Lecture 5
P-N Junction and Diodes
Lecture 5
P-N Junction and Diodes
Lecture 5
Summary
The pn junction diode
When a pn junction is formed, electrons in the n-material
diffuse across the junction and recombine with holes in
the p-material. This action continues until the voltage of
the barrier repels further diffusion. Further diffusion
across the barrier requires the application of a voltage.
The pn junction is basically a diode,
which is a device that allows current
in only one direction. A few typical
diodes are shown.
Lecture 5
Summary
Forward bias
When a pn junction is forward-biased, current is permitted.
The bias voltage pushes conduction-band electrons in the
n-region and holes in the p-region toward the junction
where they combine.
p-region n-region
The barrier potential in the depletion
region must be overcome in order
for the external source to cause
current. For a silicon diode, this is
about 0.7 V.
p
n
R
-
+
VBIAS
The forward-bias causes the depletion region to be narrow.
Lecture 5
Summary
Reverse bias
When a pn junction is reverse-biased, the bias voltage
moves conduction-band electrons and holes away from the
junction, so current is prevented.
p-region n-region
The diode effectively acts as an
insulator. A relatively few electrons
manage to diffuse across the
junction, creating only a tiny reverse
current.
p
n
R
-
+
VBIAS
The reverse-bias causes the depletion region to widen.
Lecture 5
Summary
Diode characteristics
The forward and reverse characteristics are shown on
a V-I characteristic curve.
In the forward bias region, current
increases dramatically after the
barrier potential (0.7 V for Si) is
reached. The voltage across the
diode remains approximately
equal to the barrier potential.
The reverse-biased diode
effectively acts as an insulator
until breakdown is reached.
IF
VBR (breakdown)
Forward
bias
VR
0.7 V
Reverse
bias
Barrier
potential
IR
VF
Lecture 5
Summary
Diode models
The characteristic curve for a diode can be approximated
by various models of diode behavior. The model you will
IF
use depends on your requirements.
The ideal model assumes the diode is
either an open or closed switch.
The practical model includes the VR
barrier voltage in the approximation.
Forward
bias
0.7 V
Reverse
bias
The complete model includes the
forward resistance of the diode.
IR
VF
Lecture 5
Summary
Special-purpose diodes
Special purpose diodes include
Zener diodes – used for establishing a reference voltage
Light-emitting diodes – used in displays
Photodiodes – used as light sensors
Lecture 5
Choosing The Resistor To Use With LEDs
Red LED: 2V 15mA
Green LED: 2.1V 20mA
Blue LED: 3.2V 25mA
While LED: 3.2V 25mA
Web site:
https://www.youtube.com/watch?v=k9jcHB9tWko
https://www.youtube.com/watch?v=ApQt1rz8urU
http://www.instructables.com/id/Choosing-The-Resistor-To-Use-With-LEDs/
https://en.wikipedia.org/wiki/LED_circuit
http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-seriesresistor
Lecture 5
Semicondu
Wavelen
ctor
gth
Material
Colour
VF @
20mA
GaAs
850940nm
Infra-Red 1.2v
GaAsP
630660nm
Red
1.8v
GaAsP
605620nm
Amber
2.0v
GaAsP:N
585595nm
Yellow
2.2v
AlGaP
550570nm
Green
3.5v
SiC
430505nm
Blue
3.6v
GaInN
450nm
White
4.0v
Lecture 5
Selected Key Terms
Majority carrier The most numerous charge carrier in a doped
semiconductor material (either free electrons
or holes.
Minority carrier The least numerous charge carrier in a doped
semiconductor material (either free electrons
or holes.
PN junction The boundary between n-type and p-type
semiconductive materials.
Diode An electronic device that permits current in
only one direction.
Lecture 5
Selected Key Terms
Barrier The inherent voltage across the depletion
potential region of a pn junction diode.
Forward bias The condition in which a diode conducts
current.
Reverse bias The condition in which a diode prevents
current.
Lecture 5
Selected Key Terms
Zener diode A type of diode that operates in reverse
breakdown (called zener breakdown) to
provide a voltage reference.
Photodiode
A diode whose reverse resistance changes
with incident light.
Lecture 5
Question
1. The breakdown voltage for a silicon diode is reached
when
a. the forward bias is 0.7 V.
b. the forward current is greater than 1 A.
c. the reverse bias is 0.7 V.
d. none of the above.
Lecture 5
Question
2. How much voltage must be applied to a silicon diode
before it starts to conduct?
a. the forward bias is 0.7 V.
b. the forward current is greater than 1 A.
c. the reverse bias is 0.7 V.
d. none of the above.
Lecture 5
Question
3. Why must an external resistor be connected to a diod
when connecting to a voltage source?