Transcript Chapter1 introduction Semiconductor(a)

DMT 121
ELECTRONIC 1
PN. SITI NOR DIANA BT ISMAIL
Rumah PLV, KKF Taman Kuala Perlis
Emel : [email protected]
H/P : 012-3272514
Schedules

Lectures
- Monday(8.00 – 9.00 am), BKS 1
- Wednesday(2.00 – 3.00 pm), BKS 1

Laboratories
- Thursday (8.00 am – 9.00 am), MMES
- Friday (8.00 am – 9.00 am), MMES
Topics
1. Introduction to Semiconductor
2. Diode Applications
3. Bipolar Junction Transistors (BJTs)
4. DC BJT Biasing
5. Field – Effects Transistors (FETs)
Reference book :
Electronic Devices 8th Edition
By Thomas L. Floyd
Laboratory sessions
Lab 1 : Introduction to Basic Laboratory
Equipment
Lab 2 : Introduction to Diode
Lab 3 : Diode as Rectifiers
Lab 4 : Limiter and Clamper Circuit
Lab 5 : Current & Voltage Characteristics
of BJT
Lab 6 :Voltage Divider Biasing
Lab 7 : JFET Characteristics
Evaluation Contribution
Final Examination : 50 %
Course Works : 50 %
Details of course work contribution
Lab works
: 25 %
Test
: 10 %
Lab Test
: 10 %
Assignment
: 5%
Semiconductor Materials

Definition: Semiconductors are a special
class of elements having a conductivity
between that of a good conductor and
that of an insulator
Semiconductor Materials
Single crystal – Germanium (Ge) and
Silicon (Si)
 Compound Semiconductor – Gallium
Arsenide (GaAs), Cadmium Sulfide (CdS),
Gallium Nitride (GaN) and Gallium
Arsenide phosphide (GaAsP).
 Mostly used : Ge, Si and GaAs

Semiconductor Materials



Ge – First discovered. Used as Diode in
1939, transistor in 1947. Sensitive to changes
in temperature – suffer reliability problem.
Si – Introduced in 1954 (as transistor), less
sensitive to temperature. Abundant materials
on earth. Over the time – its sensitive to
issue of speed.
GaAs – in 1970 (transistor), 5x speed faster
than Si. Problem – difficult to manufacture,
expensive, had little design support at the
early stage.
F
Ne
S
Cl
Ar
Sc
Se
Br
Kr
Y
Te
I
Xe
Po
At
Rn
Be
Na
Mg
K
Ca
Rb
Sr
Cs
Ba
Fr
Ra
H
inert gases
accept 1e
O
Li
give up 3e
accept 2e
• Columns: Similar Valence Structure
give up 2e
give up 1e
Periodic Table
Electropositive elements:
Readily give up electrons
to become + ions.
Metal
Nonmetal
Intermediate
Electronegative elements:
Readily acquire electrons
to become - ions.
He
Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.
Semiconductors, Conductors &
Insulators
Conductors
 Material that easily conducts electrical current.
 The best conductors are single-element material (e.g
copper,silver,gold,aluminum,ect.)
 One valence electron very loosely bound to the atom- free electron
Insulators
 Material that does not conduct electric current under normal conditions.
 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
Covalent Bonding &
Intrinsic Materials
Atom = electron + proton + neutron
 Nucleus = neutrons + protons

Protons
(positive charge)
Neutrons
(uncharged)
Electrons
(negative charge)
Nucleus
(core of atom)
ATOM
Atomic Structure
No. of electron in each shell
Ne = 2(n)2
n = no of shell.
Covalent Bonding
Covalent bonding of the Silicon atom
Covalent bonding of the GaAs crystal
Intrinsic Carrier
Table 1.1
Intrinsic Carriers



Semiconductor
Intrinsic Carriers
(per cubic centimeter)
GaAs
1.7 x 106
Si
1.5 x 1010
Ge
2.5 x 1013
Intrinsic (pure) carriers – The free electrons in a material due to only
external causes
Ge has the highest number of carriers and GaAs has the lowest
intrinsic carriers.
The term intrinsic (pure) is applied to any semiconductor material
that has carefully refined to reduce the number of impurities to a
very low level – essentially as pure as can be made available through
modern technology
Relative Mobility Factor µn
Table 1.2
Relative Mobility Factor
Semiconduct
or



µn (cm2/V-s)
Si
1500
Ge
3900
GaAs
8500
Relative mobility – the ability of the free carriers to move
throughout the material.
GaAs has 5X the mobility of free carriers compared to Si, a factor
that results in response times using GaAs electronic devices is 5X
those of the same device made from Si.
Ge has more than twice the mobility of electrons in Si, a factor that
results in the continued of Ge in high-speed radio frequency
applications.
Difference between Conductors &
Semiconductors

Conductors – Resistance increases with the increase in
heat, because their vibration pattern about relatively fixed
location makes it increasingly difficult for a sustained flow of
carriers through the material – positive temperature
coefficient.

Semiconductors – Exhibit an increased level of
conductivity with the application of heat. As the temperature
rises, an increasing number of valence electron absorb
sufficient thermal energy to break the covalent bond and
contribute to the number of free carriers – negative
temperature effects
Energy Level
Figure: Energy levels: conduction and valence bands of an
insulator, a semiconductor, and a conductor.
Extrinsic Materials : n-Type and PType Materials
The characteristics of a semiconductor material
can be altered significantly by the addition of a
specific purity atoms to relatively pure
semiconductor materials – this process is
known as doping process
 A semiconductor that has been subjected to
the doping process is called an extrinsic
materials.
 Extrinsic Materials are n-type material [five
valence electrons (pentavalent)] and p-type
material [three valence electrons atom
(trivalent)]

N-Type Materials


n-Type material is created by introducing the impurity (bendasing)
elements that have five valence electrons (pentavalent).
There are antimony (Sb), Arsenic (As) and phosphorous (P).
Diffused impurities with five
valence electrons are called
donor atoms
Figure: Antimony impurity in n-type material
N-Type Materials


The effect of this doping cause the energy level (called the donor
level) appears in the forbidden band with Eg significantly less than
intrinsic material.
This cause less thermal energy to move free electron (due to added
impurity) into conduction band at room temperature.
Figure: Effect of donor impurities on the energy band structure
N-Type Material




Pentavalent atoms is an n-type semiconductor (n stands for the negative
charge on electrons).
The electrons are called the majority carrier in n-type materials.
In n-type material there are also a few holes that are created when
electrons-holes pairs are thermally generated
Holes in n-type materials are called minority carrier.
P-Type Material



Si or Ge doped with impurities atoms having three valence electrons.
Mostly used are boron (B), gallium (Ga) and indium (In).
The void (vacancy) is called ‘hole’ represented by small circle or a ‘+’
sign.
Diffused impurities with three
valence electrons are called
acceptor atoms
Figure: Boron impurity in p-type
material.
P-Type Material


In p-type materials the hole is the majority carrier and
the electron is the minority carrier.
Holes can be thought as +ve charges because the
absence of electron leaves a net +ve charge on the atom.
Electron vs Hole Flow

With sufficient kinetic energy to break its covalent bond,
the electron will fills the void created by a hole, then a
vacancy or hole, will be created in the covalent bond that
released the electron.
Assignment 1
1)
2)
3)
4)
What is an ATOM?
What is maximum no of electron that can
exist in the 3rd shell of an atom? Prove it.
________ has four electron valences.
Give 3 differences between n – types and
p – types
Submit tomorrow morning during lab
session(8-9,MMES)
Semiconductor Diode
Diode
 Simple construction of electronic device
 It is a joining between n-type and p-type
material (joining one with majority carrier
of electron to one with a majority carrier of
holes)
Diode @
No Bias (VD=0V)
Forward Bias (VD > 0 V)
Figure: Forward-biased p–n junction. (a) Internal distribution of
charge under forward-bias conditions; (b) forward-bias polarity
and direction of resulting current.
Reverse Bias (VD < 0 V)
Figure: Reverse-biased p–n junction. (a) Internal distribution of charge
under reverse-bias conditions; (b) reverse-bias polarity and direction of
reverse saturation current.
Diode Characteristics Curve
Figure: Silicon semiconductor diode characteristics.
Ge, Si and GaAs
Figure: Comparison of Ge,
Si, and GaAs diodes.
Temperature Effects
Figure: Variation in Si diode
characteristics with
temperature change.
Ideal Vs Practical
Semiconductor diode behaves in a
manner similar to mechanical switch that
can control the current flow between it’s
two terminal
 However, semiconductor diode different
from a mechanical switch in the sense
that it permit the current flow in one
direction

Ideal Vs Practical
Figure: Ideal semiconductor diode:
(a) forward-biased (b) reverse-biased.
RF 
VD
0V

 0
ID 5mA
RR 
VD 20V

 
ID 0mA
Figure: Ideal versus actual
semiconductor characteristics.
(Short circuit equivalent –fwd bias, actual case R ≠ 0)
(Open circuit equivalent – Reverse bias, actual case
saturation current Is ≠ 0)
Approximate Diode
Resistance Levels
DC or Static Response
 Application of dc voltage will result in an operating point
on the characteristic curve will not change with time.
RD
VD

ID
In general, the higher the current
through a diode, the lower is the
dc resistance level.
Figure: Determining the dc resistance
of a diode at a particular operating
point.
Resistance Levels
Average AC Response
Vd
rav 
Id
Figure: Determining the average ac resistance
between indicated limits.
Diode Equivalent Model
VF  0.7V  IFrd
VBIAS  IF RLIMIT  rd   0.7V
VBIAS  0.7V
IF 
RLIMIT  rd
VBIAS  IF[rR  RLIMIT ]
Example 1
Determine the forward voltage (VF) and forward current [IF]. Also
find the voltage across the limiting
resistor. Assumed rd’ = 10 at the determined value of forward.
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
Example 2
Determine the Reverse voltage (VR). Also
find the voltage across the limiting resistor. Assumed IR = 1 µA.
Answer:
VRLIMIT =1mV
VR=4.999V
Diode Testing

Analog MM (or Ohm meter testing)
Figure: Checking a diode with an
ohmmeter.
Diode Testing – Defective diode

Digital MM (Testing Defective Diode)
Diode failed open: get
open circuit reading
(2.6 V) or ‘OL’
Diode is shorted: get
0 V reading in both
forward and reverse
bias test.
Diode Notation
Zener Diode
Figure: Conduction direction: (a)
Zener diode (b) semiconductor diode
(c) resistive element.
Figure: Characteristics of Zener
region.
Zener Region
The Zener region is in
the diode’s reverse-bias
region.
 At some point the
reverse bias voltage is
so large the diode
breaks down and the
reverse current
increases dramatically.
 This maximum voltage
is called avalanche
(runtuhan) breakdown
voltage
 The current is called
avalanche current.
