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EMT112
CHAPTER 1
Introduction to Semiconductor
By
En. Rosemizi B. Abd Rahim
Introduction to Semiconductor Chapter Outline :
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
Atomic Structures
Semiconductors, Conductors, and Insulators
Covalent Bonds
Conduction in Semiconductor
N-Type and P-Type Semiconductor
The Diode
Biasing the Diode
Voltage Current Characteristic of a Diode
Diode Models
Testing a Diode
Introduction to Semiconductor Chapter Objectives :
Discuss basic operation of a diode
Discuss the basic structure of atoms
Discuss properties of insulators, conductors and
semiconductors
Discuss covalent bonding
Describe the properties of both p and n type materials
Discuss both forward and reverse biasing of a p-n
junction
1.1
Atomic Structures
History of Semiconductor
1.1
Atomic Structures
Atomic
number
basic
structure
Electron shells
ATOM
Valence electron
Free electron
ionization
1.1
Atomic Structures
smallest particle of an element contain 3 basic particles:
Protons
(positive charge)
Neutrons
(uncharged)
Electrons
(negative charge)
Nucleus
(core of atom)
ATOM
1.1


Atomic Structures
Atomic Number
Element in periodic table are arranged according to atomic number
Atomic number = number of protons in nucleus
1.1
Atomic Structures
Electron Shells and Orbits
-
In an atom, the orbits are group into energy bands – shells
Diff. in energy level within a shell << diff. an energy between shells
Energy increases as the distance from the nucleus increases.
1.1
-
Atomic Structures
Valence Electrons
Electrons with the highest energy levels exist in the outermost shell.
Electron in the valence shell called valence electrons.
The term valence is used to indicate the potential required to removed any one
of these electrons.
1.1
Atomic Structures
Bohr model of an atom
This model was proposed by
Niels Bohr in 1915.
• electrons circle the nucleus.
• nucleus made of:
i) +protons
ii) Neutral:neutron
1.2
Semiconductors, Conductors and Insulators
• 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
1.2
Semiconductors, Conductors and Insulators
Conductors
material that easily conducts electrical current.
The best conductors are single-element material (copper, silver, gold, aluminum)
 One valence electron very loosely bound to the atom- free electron
Insulators
 material does not conduct electric current
 valence electron are tightly bound to the atom – less free electron
1.2
Semiconductors, Conductors and Insulators
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 commonly use semiconductor ; silicon(Si), germanium(Ge), and
carbon(C).
 contains four valence electrons
1.2
Semiconductors, Conductors and Insulators
1.2
Semiconductors, Conductors and Insulators
Energy Bands
1.2
Semiconductors, Conductors and Insulators
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
1.2
Semiconductors, Conductors and Insulators
Energy Bands
at room temperature 25°
eV (electron volt) – the energy absorbed by an electron when it is subjected
to a 1V difference of potential
1.2
Semiconductors, Conductors and Insulators
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
Covalent bonding –1-3
holding
atoms Bonding
together by sharing
valence electrons
sharing of valence
electron
produce the
covalent bond
To form Si crystal
1.3
Covalent Bonding
The 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 is a very poor conductor
1.3
Covalent Bonding
Certain atoms will combine in this way to form a crystal
structure. Silicon and Germanium atoms combine in this
way in their intrinsic or pure state.
Covalent bonds in a 3-D silicon crystal
1.4
Conduction in Semiconductor
(Conduction Electron and holes)
FIGURE 1-10 Energy band diagram for a pure (intrinsic) silicon crystal with unexcited
atoms. There are no electrons in the conduction band.
1.4
Conduction in Semiconductor
(Conduction Electron and holes)
Absorbs enough energy
(thermal energy)
to jumps
a free electron and
its matching valence
band hole
FIGURE 1-11 Creation of electron-hole pairs in a silicon crystal. Electrons in the
conduction band are free.
1.4
Conduction in Semiconductor
(Conduction Electron and holes)
FIGURE 1-12 Electron-hole pairs in a silicon crystal. Free electrons are being
generated continuously while some recombine with holes.
1.4
Conduction in Semiconductor
(Electron and holes currents)
Electron current
free
electrons
Apply voltage
FIGURE 1-13 Electron current in intrinsic silicon is produced by the movement of
thermally generated free electrons.
1.4
Conduction in Semiconductor
(Electron and holes currents)
movement
of holes
FIGURE 1-14 Hole current in intrinsic silicon.
1.5
N-types and P-types Semiconductors
(Doping)
Doping -the process of creating N and P type materials
-by adding impurity atoms to intrinsic Si or Ge to imporove 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)
1.5
N-types and P-types Semiconductors
N-type semiconductor:
- Pentavalent impurities are added to Si or Ge, the result is an
increase the free electrons
- 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 carries
- Holes – minority carries
Sb
impurity
atom
Pentavalent impurity atom in a Si crystal
1.5
N-types and P-types Semiconductors
P-type semiconductor:
- Trivalent impurities are added to Si or Ge to create a deficiency of
electrons or hole charges
- The holes created by doping process
- 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
B
impurity
atom
Trivalent impurity atom in a Si crystal
1.6
The Diode
-n-type material & p-type material become extremely useful when
joined together to form a pn junction – then diode is created
-p region- holes (majority carriers), e- (minority carriers)
-n region- e- (majority carriers), holes (minority carriers)
-before the pn junction is formed -no net charge (neutral)
1.6
The Diode (The Depletion Region)
1.6
The Diode (The Depletion Region)
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 in the
valance band covalent bonds.
3. Since the n-region have lost an electron, they have an overall +ve charge.
4. Since the p-region have gained an electron, they have an overall –ve charge.
5 The difference in charges on the two sides of the junction is called the barrier potential.
(typically in the mV range)
Barrier Potential:
•
The buildup of –ve charge on the p-region of the junction and of +ve charge on the
n-region of the junction-therefore difference of potential between the two sides of the
junction is exist.
•
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 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. [ unit: V ]
•
Depend on: type of semicon. material, amount of doping, temperature. (e.g : 0.7V for Si
and 0.3 V for Ge at 25°C)
1.6
The Diode (Energy Diagram of the PN Junction and
Depletion Region)






Energy level for n-type (Valence and Cond. Band) << p- type material
(difference in atomic characteristic : pentavalent & trivalent)
After cross the junction, the e- lose energy & 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 decrease.
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 –energy ‘hill’ – electron at n-region must climb
to get to the p-region.
The energy gap between valence & cond. band – remains the same
1.7
Biasing The Diode (Bias)
 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
1.7
Biasing The Diode ( Forward Bias)
Diode connection
•Voltage source or bias connections are + to
the p material and – to the n material
•Bias must be greater than barrier potential
(0 .3 V for Germanium or 0.7 V for Silicon
diodes)
•The depletion region narrows.
•R – limits the current to prevent damage
for diode
Flow of majority carries and the voltage
across the depletion region
•The negative side of the bias voltage push
the free electrons in the n-region -> pn
junction
•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- positive side
of bias voltage source attracts the e- left the pregion
•Holes in p-region act as medium or pathway
for these e- to move through the p-region
1.7
Biasing The Diode ( The Effect of Forward Bias on the
Depletion Region)
 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 – the no. of –ve
ions is reduced.
 Reduction in +ve & -ve ions – causes the depletion region to narrow
1.7
Biasing The Diode ( The Effect of the Barrier Potential
during Forward Bias)
 Electric field between +ve & -ve ions in depletion region creates “energy hill”
-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
1.7
Biasing The Diode ( ReverseBias)
Diode connection
•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
Shot transition time immediately after
reverse bias voltage is applied
•+ side of bias pulls the free electrons in the nregion 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- 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
1.7
Biasing The Diode ( Reverse Current)
• 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- (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
1.8
Voltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)
-When a forward bias voltage is
applied – current called forward
current, I
F
-In this case with the voltage
applied is less than the barrier
potential so the diode for all
practical purposes is still in a
non-conducting 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
( 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 above
0.7 V.
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
( V-I Characteristic for forward bias)
-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
dynamic resistance r’d decreases as you move up the curve
zero
bias
VF  0.7V
VF  0.7V
r ' d  VF / I F
1.8
Voltage-Current Characteristic of a Diode
( V-I Characteristic for Reverse bias)
Breakdown
voltage
-not a normal
operation of pn
junction devices
- the value can be
vary for typical Si
Reverse
Current
1.8
Voltage-Current Characteristic of a Diode
( Complete V-I Characteristic curve)
Combine-Forward bias
& Reverse bias  Complete
V-I characteristic curve
1.8
Voltage-Current Characteristic of a Diode
( Temperature effect on the diode V-I Characteristic)
•
Forward biased
dioed : T , I F 
for a given value
of VF
•
For a given I F ,VF 
•
Barrier potential
decrease as T
increase
•
Reverse current
breakdown –
small & can be
neglected
1.9
Diode Models
( Diode structure and symbol)
anod
cathode
Directional of current
1.9
Diode Models
The Ideal
Diode Model
The Practical
Diode Model
DIODE
MODEL
The Complete
Diode Model
1.9
Diode Models
( The ideal Diode model)
Ideal model of diodesimple switch:
•Closed (on) switch -> FB
•Open (off) switch -> RB
•Assume V  0V
F
•Forward
current, by
Ohm’s law
IF 
VBIAS
(1-2)
RLIMIT
IR  0A
VR  VBIAS
1.9
Diode Models ( The Practical Diode model)
•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
low to figure (a)]
VBIAS  VF  VRLIMIT  0
VRLIMIT  I F RLIMIT
•Represent by VF
produced across the pn
junction
Ohm’s Law
VBIAS  VF
IF 
RLIMIT
•Equivalent to close
switch in series with a
small equivalent voltage
source equal to the barrier
potential 0.7V
(1-3)
•Same as ideal diode
model
IR  0A
VR  VBIAS
1.9
Diode Models ( The Complete Diode model)
Complete model of diode
consists:
•Barrier potential
•Dynamic resistance, r ' d
•Internal reverse resistance, r ' R
•The forward voltage:
VF  0.7V  I F rd'
(1-4)
•The forward current:
IF 
VBIAS  0.7V
RLIMIT  rd'
(1-5)
•acts as closed switch in
series with barrier
potential and small r ' d
•acts as open
switch in parallel
with the large r ' R
1.9
Diode Models ( Example)
(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
1.9
a)
Diode Models ( Example)
Ideal Model: VF  0
V
10V
I F  BIAS 
 10mA
R
1000
VRLIMIT  I F  RLIMIT  (10 10 3 A)(1103 )  10V
b) Practical Model: VF  0.7V
IF 
(c) Complete model:
(VBIAS  VF ) 10V  0.7V

 9.3mA
RLIMIT
1000
VRLIMIT  I F  RLIMIT  (9.3 10 3 A)(1103 )  9.3V
IF 
VBIAS  0.7V 10V  0.7V

 9.21mA
'
RLIMIT  rd
1k  10V
VF  0.7V  I F rd'  0.7V  (9.21mA)(10)  792mV
VRLIMIT  I F RLIMIT  (9.21mA)(1k)  9.21V
1.9
Diode Models ( Typical Diodes)
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.
1.10
Testing A Diodes ( By Digital multimeter)
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 .7 V or
.3 V when forward biased.
When checking in reverse
bias the full applied testing
voltage will be seen on the
display.
K A
A K
1.10
Testing A Diodes ( By Digital multimeter)
NG DIODE
1.10
Testing A Diodes ( By Analog multimeter – ohm
function )
Select OHMs range
Good diode:
Forward-bias:
get low resistance reading (10 to 100
ohm)
Reverse-bias:
get high reading (0 or infinity)
Summary
 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 .3 V for a
Germanium and .7 V for Silicon for conduction to take
place.
Summary
 A diode conducts when forward biased and does not
conduct when reverse biased
 When reversed biased a diode can only withstand
so much applied voltage. The voltage at which
avalanche current occurs is called reverse breakdown
voltage.
 There are three ways of analyzing a diode. These
are ideal, practical, and complex. Typically we use a
practical diode model.
Assignment
1. Describe the difference between:
a) n-type and p-type semiconductor materials
b) donor and acceptor impurities
c) majority and minority carries
2. Predict the voltmeter reading in Figure 2.1. (assumed voltage across the diode is 0.7V,
R1= 10kohm, V1 = 5V). Then, calculate current, I.
XMM1
voltmeter
R1
V1
10kohm
5V
I
Figure 2.1
D1
1N4148