Transcript lecture 1:introduction to semiconductor

Discuss basic structures of atoms
Discuss properties of insulators,
conductors, and semiconductors
Discuss covalent bonding
Describe the conductions in semiconductor
Discuss N-type and P-type semiconductor
Discuss the diode
Discuss the bias of a diode
1.1 Atomic structure
1.2 Semiconductor, conductors and
insulators
1.3 Covalent bonding
1.4 Conduction in semiconductors
1.5 N-type and P-type semiconductors
1.6 Diode
1.7 Biasing the diode
1.8 Voltage-current characteristic of a
diode
1.9 Diode models
1.10 Testing a diode
• Electronics
are easy to move/control
- Easy to move/control electrons than real
physical stuff
• Move information not things
- Phone, fax, internet
- Takes much less energy and money
Atomic
number
Basic
structure
Electron shells
ATOM
Valence electron
Free electron
Ionization

smallest particle of an element contain 3
basic particles:
ATOM
Protons
(positive charge)
Nucleus
(core of atom)
Neutrons
(uncharged)
Electrons
(negative charge)
This model was
proposed by
Niels Bohr in
1915.
-electrons circle
the nucleus that
consists of
protons and
neutrons.
Figure 1.1 Bohr model of an atom
 Atomic Number
- Element in periodic table are arranged according to atomic
number
- Atomic number = number of protons in nucleus
 Electron Shells and Orbits
- Electrons near the nucleus have less energy than those in more
distant orbits.
- Each distance (orbits) from the nucleus corresponding to a
certain energy level.
- In an atom, the orbits are group into energy bands – shells
- Diff. in energy level within a shell << diff. in energy between
shells.
 Valence Electrons
- Electrons with the highest energy levels exist in the outermost
shell and loosely bound to the atom. The outermost shell –
valence shell.
- Electron in the valence shell called valence electrons.
 Ionization
- When atoms absorb energy (e.g heat source) – losing valence
electrons called ionization.
Escape electron called free electron.
 The Number of Electrons in Each Shell
-
-
The maximum number of electrons (Ne) in each shell is
calculated using formula below:
n = number of shell
Example for 2nd shell
N e  2n
2
N  2n  2(2)  8
2
e
2
•Atom can be represented by the valence shell and a core
•A core consists of all the inner shell and the nucleus
Carbon atom:
-valence shell – 4 e
-inner shell – 2 e
Nucleus:
-6 protons
-6 neutrons
+6 for the nucleus
and -2 for the two
inner-shell electrons
(net charge +4)
Conductors
• material that easily conducts electrical current.
• The best conductors are single-element material (e.g copper, silver, gold,
aluminum)
• Only one valence electron very loosely bound to the atom- free electron
Insulators
• material does not conduct electrical current
• valence electron are tightly bound to the atom – very few free electron
Semiconductors
• material between conductors and insulators in its ability to conduct
electric current
• in its pure (intrinsic) state is neither a good conductor nor a good
insulator
• most common semiconductor- silicon(Si), germanium(Ge), and
carbon(C) which contains four valence electrons.
1.2 Semiconductors, Conductors, and Insulators (cont.)
Energy Bands
1-2 Semiconductors, Conductors, and Insulators (cont.)
Energy Bands
• Energy gap-the difference between the energy levels of any two orbital
shells
• Band-another name for an orbital shell (valence shell=valence band)
• Conduction band –the band outside the valence shell where it has free
electrons.
Comparison of a Semiconductor Atom & Conductor Atom
A Silicon atom:
• 4 valence electrons
• A semiconductor
• Electron conf.: 2:8:4
14 protons
14 nucleus
10 electrons
in inner shell
A Copper atom:
• Only 1 valence electron
• A good conductor
• Electron conf.:2:8:18:1
29 protons
29 nucleus
28 electrons in
inner shell
1-3 Covalent Bonding
Covalent bonding – holding atoms together by sharing
valence electrons
sharing of
valence electron
produce the
covalent bond
To form Si crystal
Result of the bonding:
1. The atom are held together forming a solid
substrate.
2. The atoms are all electrically stable, because
their valence shells are complete.
3. The complete valence shells cause the silicon to
act as an insulator-intrinsic (pure) silicon.
In other word, it is a very poor conductor.
• Covalent bonding in an intrinsic or pure silicon crystal.
An intrinsic crystal has no impurities.
Covalent bonds in a 3-D silicon crystal
Figure 1-10 Energy band diagram for a pure (intrinsic) silicon crystal with
unexcited (no external energy such as heat) atoms. There are no electrons
in the conduction band. This condition occurs only at a temperature of
absolute 0 Kelvin.
Absorbs enough energy
(thermal energy)
to jumps
a free electron and
its matching valence
band hole –
electron-hole pair
Recombination-when a conduction electron
loses energy and fall back into hole in
valence band
Figure 1-11 Creation of electron-hole pairs in a silicon crystal. Electrons in the
conduction band are free (also called conduction electrons).
Figure 1-12 Electron-hole pairs in a silicon crystal. Free electrons are being
generated continuously while some recombine with holes.
Electron current
free
electrons
Apply voltage
When a voltage is applied, free electrons are free to move randomly
and attracted toward +ve end. The movement of electrons is one type of current
in semiconductor and is called electron current.
Figure 1-13 Electron current in intrinsic silicon is produced by the movement of
thermally generated free electrons.
movement
of holes
Figure 1-14 Hole current in intrinsic silicon.
Doping
- The process of creating N and P type materials
- By adding impurity atoms to intrinsic Si or Ge to improve the
conductivity of the semiconductor
- Two types of doping – trivalent (3 valence e-) & pentavalent (5 valence e-)
p-type material – a semiconductor that has added trivalent impurities
n-type material – a semiconductor that has added pentavalent
impurities
Trivalent Impurities:
Pentavalent Impurites:
• Aluminum (Al)
• Phosphorus (P)
• Gallium (Ga)
• Arsenic (As)
• Boron (B)
• Antimony (Sb)
• Indium (In)
• Bismuth (Bi)
N-type semiconductor:
Pentavalent impurities are added to Si or Ge, the result is an increase of
free electrons
1 extra electrons becomes a conduction electrons because it is not
attached to any atom
No. of conduction electrons can be controlled by the no. of impurity atoms
Pentavalent atom gives up an electron -call a donor atom
Current carries in n-type are electrons – majority carriers
Holes – minority carriers (holes created in Si when generation of electronholes pair.
Sb
impurity
atom
Pentavalent impurity atom in a Si crystal
P-type semiconductor:
- Trivalent impurities are added to Si or Ge to increase number of holes.
- Boron, indium and gallium have 3 valence e- form covalent bond with 4
adjacent silicon atom. A hole created when each trivalent atom is added.
- The no. of holes can be controlled by the no. of trivalent impurity atoms
- The trivalent atom can take an electron- acceptor atom
- Current carries in p-type are holes – majority carries
- electrons – minority carries (created during electron-holes pairs
generation).
B
impurity
atom
Trivalent impurity atom in a Si crystal
- Diode is a device that conducts current only in one direction.
- n-type material & p-type material become extremely useful when
joined together to form a pn junction – then diode is created
- before the pn junction is formed -no net charge (neutral) since no
of proton and electron is equal in both n-type and p-type.
-p region: holes (majority carriers), e- (minority carriers)
-n region: e- (majority carriers), holes (minority carriers)
Summary:
When an n-type material is joined with a p-type material:
1. A small amount of diffusion occurs across the junction.
2. When e- diffuse into p-region, they give up their energy and fall
into the holes near the junction.
3. Since the n-region loses electrons, it creates a layer of +ve
charges (pentavalent ions).
4. p-region loses holes since holes combine with electron and will
creates layer of –ve charges (trivalent ion). These two layers
form depletion region.
5 Depletion region establish equilibrium (no further diffusion)
when total –ve charge in the region repels any further diffusion
of electrons into p-region.

Barrier Potential:

In depletion region, many +ve and –ve charges on
opposite sides of pn junction.
The forces between the opposite charges form a
“field of forces "called an electric field.
This electric field is a barrier to the free electrons in
the n-region, need more energy to move an ethrough the electric field.
The potential difference of electric field across the
depletion region is the amount of voltage required
to move e- through the electric field. This potential
difference is called barrier potential. [ unit: V ]
Depends on: type of semicon. material, amount of
doping and temperature. (e.g : 0.7V for Si and 0.3 V
for Ge at 25°C).




Overlapping
Energy level for n-type (Valence and Cond. Band) <<
p- type material (difference in atomic characteristic :
pentavalent & trivalent) and significant amount of
overlapping.
 Free e- in upper part conduction band in n-region can
easily diffuse across junction and temporarily become
free e- in lower part conduction band in p-region. After
crossing the junction, the e- loose energy quickly & fall
into the holes in p-region valence band.

As the diffusion continues, the depletion region begins to
form and the energy level of n-region conduction band
decreases due to loss of higher-energy e- that diffused
across junction to p-region.
 Soon, no more electrons left in n-region conduction band
with enough energy to cross the junction to p-region
conduction band.

Figure (b), the junction is at equilibrium state, the
depletion region is complete diffusion has ceased (stop).
Create an energy gradient which act as energy ‘hill’
where electron at n-region must climb to get to the pregion.
 The energy gap between valence & cond. band – remains
the same

No electron move through the pn-junction at equilibrium
state.

 Bias is a potential applied (dc voltage) to a pn junction to
obtain a desired mode of operation – control the width of the
depletion layer.
 Two bias conditions : forward bias & reverse bias
The relationship between the width of depletion layer & the junction current
Depletion Layer
Width
Junction
Resistance
Junction Current
Min
Min
Max
Max
Max
Min
1. Voltage source or bias
connections are + to
the p region and – to the
n region.
2. Bias voltage must be
greater than barrier
potential (0 .3 V for
Germanium or 0.7 V for
Silicon).
› The depletion region
Diode connection
narrows.
› R – limits the current
which can prevent damage
to the diode
1.7 Biasing The Diode (cont.)
Forward bias

The negative side of the bias
voltage push the free electrons
in the n-region -> pn junction.
Flow of free electron is called
electron current.

Also provide a continuous flow
of electron through the external
connection into n-region.

Bias voltage imparts energy to
the free e- to move to p-region.

Electrons in p-region loss
energy-combine with holes in
valence band.
Flow of majority carries and
the voltage across the
depletion region

Since unlike charges attract,
positive side of bias voltage
source attracts the e- left
end of p-region.

Holes in p-region act as
medium or pathway for
these e- to move through
the p-region.

e- move from one hole to
the next toward the left.

The holes move to right
toward the junction. This
effective flow is called hole
current.
 As more electrons flow into the depletion region, the no. of +ve ion is
reduced.
 As more holes flow into the depletion region on the other side of pn
junction, the no. of –ve ions is reduced.
 Reduction in +ve & -ve ions – causes the depletion region to narrow.
Electric field between +ve & -ve ions in depletion region
creates “energy hill” that prevent free e- from diffusing at
equilibrium state -> barrier potential

 When apply forward bias – free e- provided enough energy
to climb the hill and cross the depletion region.
 Electron got the same energy = barrier potential to cross
the depletion region.
 An add. small voltage drop occurs across the p and n
regions due to internal resistance of material – called
dynamic resistance – very small and can be neglected
Diode connection
Reverse bias - Condition that prevents current through
the diode
 Voltage source or bias connections are – to the p material
and + to the n material
 Current flow is negligible in most cases.
 The depletion region widens than in forward bias.




+ side of bias pulls the free electrons in the n-region away
from pn junction cause add. +ve ions are created, widening
the depletion region.
In the p-region, e- from – side of the voltage source enter as
valence electrons e- and move from hole to hole toward the
depletion region, then created add. –ve ions.
As the depletion region widens, the availability of majority
carriers decrease.
Extremely small current exist – after the transition current dies
out caused by the minority carries in n & p regions that are produced by
thermally generated electron hole pairs.
• Small number of free minority e- in p region are “pushed toward the pn
junction by the –ve bias voltage.
• e- reach wide depletion region, they “fall down the energy hill” combine
with minority holes in n -region as valence e- and flow towards the +ve bias
voltage – create small hole current.
• The cond. band in p region is at higher energy level compare to cond. band in
n-region e- easily pass through the depletion region because they require no
additional energy.
•
-When a forward bias
voltage is applied, there is
current called forward
current, IF .
-In this case with the
voltage applied is less than
the barrier potential so the
diode for all practical
purposes is still in a nonconducting state. Current
is very small.
-Increase forward bias
voltage – current also
increase.
FIGURE 1-26 Forward-bias measurements show
general changes in VF and IF as VBIAS is increased.
1.8 Voltage-Current Characteristic of a Diode (cont.)
V-I Characteristic for Forward Bias
- With the applied voltage
exceeding the barrier
potential (0.7V), forward
current begins increasing
rapidly.
- But the voltage across
the diode increase only
gradually above 0.7 V. this
is due to voltage drop
across internal dynamic
resistance of semicon
material.
FIGURE 1-26 Forward-bias measurements show
general changes in VF and IF as VBIAS is increased.
1.8 Voltage-Current Characteristic of a Diode (cont.)
V-I Characteristic for Forward Bias
dynamic resistance r’d
decreases as you move up
the curve
-Plot the result of
measurement in Figure 126, you get the V-I
characteristic curve for a
forward bias diode
- VF Increase to the right
- I F increase upward
-After 0.7V, voltage remains
at 0.7V but IF increase
rapidly.
-Normal operation for a
forward-biased diode is
above the knee of the
curve.
zero
bias
VF  0.7V
VF  0.7V
Below knee, resistance is
greatest since current increase
very little for given voltage, r ' d  VF / I F
Resistance become smallest above
knee where a large change in current
for given change in voltage.
1.8 Voltage-Current Characteristic of a Diode
(cont.)
V-I Characteristic for Reverse Bias
- VR increase to the left
along x-axis while IR
increase downward along yaxis.
- When VR reaches VBR , IR
begin to increase rapidly.
Breakdown voltage, VBR.
- not a normal operation of
pn junction devices.
- the value can be vary for
typical Si.
- Cause overheating and
possible damage to diode.
Reverse
Current
1.8 Voltage-Current Characteristic of a Diode
(cont.)
The Complete V-I Characteristic Curve
Combine-Forward bias & Reverse bias  CompleteV-I characteristic curve
1.8 Voltage-Current Characteristic of a Diode (cont.)
Temperature Effects on the Diode V-I Characteristic

Forward biased
diode : T , I F  for
a given value of VF

Barrier potential
decrease as T
increase.
For reverse-biased,
T increase, IR
increase.


Reverse current
breakdown – small
& can be neglected
anode
cathode
Direction of current
The Ideal
Diode Model
The Practical
Diode Model
DIODE
MODEL
The Complete
Diode Model
Ideal model of diodesimple switch:
•Closed (on) switch
-> FB
•Open (off) switch > RB
•Forward
current
determined
by Ohm’s
law
• Barrier potential,
dynamic resistance and
reverse current all
VBIAS
IF 
neglected.
• Assume to have zero
voltage across diode
when FB.
RLIMIT
VF  0V
IR  0A
VR  VBIAS
•Adds the barrier potential
to the ideal switch model
'
r
• ‘ d is neglected
•From figure (c):
VF  0.7V ( Si)
VF  0.3V (Ge)
The forward current [by
applying Kirchhoff’s voltage
law to figure (a)]
VBIAS  VF  VRLIMIT  0
VRLIMIT  I F RLIMIT
By Ohm’s Law:
•Equivalent to close
switch in series with a
small equivalent voltage
source equal to the
barrier potential 0.7V
•Represent by VF
produced across the pn
junction
VBIAS  VF I R  0 A
IF 
VR  VBIAS
RLIMIT
•Open circuit, same as
ideal diode model.
•Barrier potential
doesn’t affect RB
Complete model of diode
consists:
•Barrier potential
'
•Dynamic resistance, r d
•Internal reverse resistance, r ' R
•The forward voltage
consists of barrier potential
& voltage drop across r’d :
VF  0.7V  I F rd'
•The forward current:
VBIAS  0.7V
IF 
RLIMIT  rd'
•acts as closed switch
in series with barrier
potential and small r ' d
•acts as open
switch in
parallel with
the large r ' R
(1) Determine the forward voltage and forward current [forward
bias] for each of the diode model also find the voltage across
the limiting resistor in each cases. Assumed rd’ = 10 at the
determined value of forward current.
1.0kΩ
1.0kΩ
10V
5V
a)
Ideal Model:
VF  0
VBIAS
10V

 10mA
R
1000
VRLIMIT  I F  RLIMIT  (10 10 3 A)(1103 )  10V
IF 
b) Practical Model: V  0.7V
F
IF 
(c) Complete model:
IF 
(VBIAS  VF ) 10V  0.7V

 9.3mA
RLIMIT
1000
VRLIMIT  I F  RLIMIT  (9.3 10 3 A)(1103 )  9.3V
VBIAS  0.7V 10V  0.7V

 9.21mA
'
RLIMIT  rd
1k  10
VF  0.7V  I F rd'  0.7V  (9.21mA )(10)  792mV
VRLIMIT  I F RLIMIT  (9.21mA )(1k)  9.21V
Diodes come in a variety of sizes and shapes. The design and structure is
determined by what type of circuit they will be used in.
- Testing a diode is quite simple, particularly if the multimeter
used has a diode check function. With the diode check function
a specific known voltage is applied from the meter across the
diode.
- With the diode check
function a good diode will
show approximately 0.7 V or
0.3 V when forward biased.
- When checking in reverse
bias, reading based on
meter’s internal voltage
source. 2.6V is typical value
that indicate diode has
extremely high reverse
resistance.
K A
A K
-When diode is failed open, open
reading voltage is 2.6V or “OL”
indication for forward and reverse
bias.
-If diode is shorted, meter reads 0V
in both tests. If the diode exhibit a
small resistance, the meter reading
is less than 2.6V.
Select OHMs range
Good diode:
Forward-bias:
get low resistance reading (10 to 100
ohm)
Reverse-bias:
get high reading (0 or infinity)
 Diodes, transistors, and integrated circuits are
all made of semiconductor material.
 P-materials are doped with trivalent impurities
 N-materials are doped with pentavalent impurities
 P and N type materials are joined together to form a
PN junction.
 A diode is nothing more than a PN junction.
 At the junction a depletion region is formed. This
creates barrier which requires approximately 0.3 V for
a Germanium and 0.7 V for Silicon for conduction to
take place.
 A diode conducts when forward biased and does not
conduct when reverse biased
 The voltage at which avalanche current occurs is
called reverse breakdown voltage. Reverse breakdown
voltage for diode is typically greater than 50V.
 There are three ways of analyzing a diode. These
are ideal, practical, and complete. Typically we use a
practical diode model.
There once was a wise man that was known
throughout the land for his wisdom. One day a
young boy wanted to test him to prove that the
wise man a fake.
 He thought to himself, “I will bring one live bird to
test the old man. I will ask him whether the bird in
my hand is dead or alive. If he says that it is alive, I
will squeeze hard to kill the bird to prove that he is
wrong.
 On the other hand if he says that it is dead, I will let
the bird fly off, proving that he is wrong. Either way
the wise man will be wrong.”




With that idea in mind, he approached the wise
man and asked, “Oh wise man, I have a bird in my
hand. Can you tell me if the bird is dead or alive?”.
The wise man paused for a moment and replied,
“Young man, you indeed have a lot t learn. That
which you hold in your hand, it is what you make of
it. The life of the bird is in your hand.
If you wish it to be dead, then it will die. On the
other hand if you desire it to live, it will surely live”.
The young boy finally realized that the answer
given was indeed that of a man of wisdom.

Our dreams are very fragile, just like the little
bird. It is our own decision, if we decide to
kill it, or allow others to steal it away from us.
However, it is also our own choice to nurture
it and let it grow to fruition. Success comes
to those who allow their dreams to fly high,
just like the little bird, which will soar into the
sky if the young boy released it from his
grasp.