Electronic Circuit and Devices

Lesson 1

Introduction to Semiconductor
Introduction to Semiconductor -
Chapter Outline :
1.1 Atomic Structures
1.2 Semiconductors, Conductors, and Insulators
1.3 Covalent Bonds
1.4 Conduction in Semiconductor
1.5 N-Type and P-Type Semiconductor
1.6 The Diode
1.7 Biasing the Diode
1.8 Voltage Current Characteristic of a Diode
1.9 Diode Models
1.10 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

➢ An atom is a smallest particle of an element.

➢ contain 3 basic particles:

Protons Neutrons
(positive charge) (uncharged)

Nucleus Electrons
(core of atom) (negative charge)

ATOM
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
 1.1 Atomic Structures
Atomic Number
➢ Element in periodic table are arranged according to atomic number
➢ Atomic number = number of protons in nucleus
➢ At balanced (neutral) atom, number of electron= number of protons

➢ Hydrogen = group 1
➢ Helium = group 2
 1.1 Atomic Structures

Electron Shells and Orbits

- The orbits are group into energy bands - shells
- Valence - the outermost shell , electrons in this shell – valence electrons
- Valence electrons contribute to chemical reactions and bonding within the structure of
material and determine its properties.
- 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
Ionization
- A process of an atom either losing or gaining an electron to become positive
ion or negative ions.
- For example,
i) Positive ion - in a neutral hydrogen atom, the valence electron
acquires a sufficient amount of energy to jump out from the outmost
shell, this will leave the atom with less number of electron and more
a proton. (H become H+)
ii) Negative ion – a free electron fall into the outer shell of a neutral
hydrogen atom. (H become H-)
 1.1 Atomic Structures

Number of electrons in each shell

Number of electrons (Ne) that can exist in each shell of an atom can be calculated by
the formula:

Ne=2n2

n is the number of shell

1.2 Semiconductors, Conductors and Insulators
 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
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

• 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 Net charge = +4
-inner shell – 2 e +6 for the nucleus
Nucleus: and -2 for the two
-6 protons inner-shell electrons
-6 neutrons
 1.2 Semiconductors, Conductors and Insulators
Energy Level

• Each discrete shell (orbit) corresponds to a certain energy level.

• Electrons are bounded to their respective shells because of the attraction force between
proton (+) and electron (-).
• The electrons in orbits further from nucleus are less tightly bound (loose) compare to the
atom closer to the nucleus due to the attractive force.
• The distance between each shell to the nucleus depends on the energy of the respective
electrons.
• Therefore, electrons in the highest energy level exist in the outermost shell of an atom.
• Energy level increases as distance from nucleus.
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: A Copper atom:

•4 valence electrons •only 1 valence electron
•a semiconductor •a good conductor
•Electron conf.: 2:8:4 •Electron conf.:2:8:18:1

14 protons 29 protons
14 nucleus 29 nucleus
10 electrons 28 electrons in
in inner shell inner shell
 1.3 Covalent Bonding
Covalent bonding –1-3 Covalent
holding atoms Bonding
together by sharing
valence electrons

sharing of valence
electron To form Si crystal
produce the
covalent bond
 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.
- Example Pentavalent: Antimony(Sb), Phosphorus(Ph), Arsenic(As), Bismuth(Bi)
- 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 (donate) 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
- Example Trivalent : Boron(B), Aluminium(Al), Gallium(Ga), Indium(In)
- The holes created by doping process
- The no. of holes can be controlled by the no. of trivalent impurity atoms
- The trivalent atom is take (accept) 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 a diode (pn junction) when joined together
-p region- majority carriers - holes
minority carriers - electron
-n region - majority carriers – electron
minority carriers - holes
- before the pn junction is formed -no net charge (neutral)
 1.6 The Diode (The Depletion Region)
- Depletion region – the area around a pn junction that is depleted of free carriers
due to diffusion across the junction
-Also known as depletion layer.

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)
 1.6 The Diode (The Depletion Region)

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 e- through 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 << p- type material (diff. 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).
 1.7 Biasing The Diode (Bias)
❖ At equilibrium state – no electron move through the pn-junction.
❖ 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 Junction Junction
Width Resistance Current
Min Min Max

Max Max Min

 1.7 Biasing The Diode ( Forward Bias)
Flow of majority carries and the voltage
Diode connection
across the depletion region

•The negative side of the bias voltage push

•Voltage source or bias connections the free electrons in the n-region -> pn
are + to the p material and – to the junction
n material
•Also provide a continuous flow of electron
•Bias must be greater than barrier through the external connection into n-
potential (0 .3 V for Germanium or region
0.7 V for Silicon diodes)
•Bias voltage imparts energy to the free e-
•The depletion region narrows. to move to p-region
•Electrons in p-region loss energy- positive
•R – limits the current to prevent side of bias voltage source attracts the e-
damage for diode left the p-region
•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 in depletion region prevent free e- from diffusing

at equilibrium state -> barrier potential
▪ When apply forward bias – free e- enough energy to 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 ( Reverse Bias)
Shot transition time immediately after
Diode connection
reverse bias voltage is applied

•+ side of bias pulls the free electrons in the n-

•Condition that prevents current through the region away from pn junction
diode • cause add. +ve ions are created , widening
the depletion region
•Voltage source or bias connections are – to
the p material and + to the n material •In the p-region, e- from – side of the voltage
source enter as valence electrons
•Current flow is negligible in most cases. •e- move from hole to hole toward the
•The depletion region widens 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

• 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 –combine with minority holes in n -
region – create small hole current
 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

dynamic resistance r’d decreases as you move up the curve
measurement in Figure 1-
26, you get the V-I
characteristic curve for a
forward bias diode
- VF Increase to the right zero VF  0.7V
bias
- I F increase upward

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 The Practical

Diode Model Diode Model

DIODE
MODEL

The Complete
Diode Model
 1.9 Diode Models
( The ideal Diode model)

Ideal model of diode-

simple switch:
•Closed (on) switch -> FB
•Open (off) switch -> RB
•Assume V = 0V
F IR = 0A
•Forward
current, by VR = VBIAS
Ohm’s law
VBIAS
IF = (1-2)
RLIMIT
 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

•Equivalent to close •Same as ideal diode
applying Kirchhoff’s voltage switch in series with a model
low to figure (a)] small equivalent voltage
IR = 0A
VBIAS − VF − VRLIMIT = 0 source equal to the barrier
potential 0.7V VR = VBIAS
VRLIMIT = I F RLIMIT •Represent by VF
produced across the pn
Ohm’s Law junction

VBIAS − VF
IF = (1-3)
RLIMIT
 1.9 Diode Models ( The Complete Diode model)

Complete model of diode

consists:
•Barrier potential
•Dynamic resistance, r ' d
•Internal reverse resistance, r ' R •acts as closed switch in •acts as open
series with barrier switch in parallel
•The forward voltage: potential and small r ' d with the large r ' R
VF = 0.7V + I F rd' (1-4)
•The forward current:
VBIAS − 0.7V
IF = (1-5)
RLIMIT + rd'
 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 Diode Models ( Example)

a) 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

(VBIAS − VF ) 10V − 0.7V
IF = = = 9.3mA
RLIMIT 1000
VRLIMIT = I F  RLIMIT = (9.3 10−3 A)(1103 ) = 9.3V
(c) Complete model:
VBIAS − 0.7V 10V − 0.7V
IF = = = 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
 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)

Defective 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 – due : next week class
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.

voltmeter

Figure 2.1