4.

0 ELECTRONIC CIRCUITS
4.1 Semiconductor Physics
 4.1 Semiconductor Physics
•The electric current flows through a material is known as
conductivity.
•Conductors (like copper) have very high conductivity.
•Insulators (like rubber) have very low conductivity.
•The conductivity of a semiconductor depends on its conditions.

For example, at low temperatures and low voltages a

semiconductor acts like an insulator. So it has negative
temperature coefficient of resistance.

When the temperature and/or the voltage is increased, the

conductivity increases and the material acts more like a
conductor.
• If there are many free electrons to carry current,
the semiconductor acts more like a conductor.
• If there are few electrons, the semiconductor acts
like an insulator.
• Silicon is the most commonly used semiconductor.
• Atoms of silicon have 14 electrons.
• Ten of the electrons are bound tightly inside the
atom.
• Four electrons are near the outside of the atom and
only loosely bound.
 Semiconductor
Electron
Energy

r3

r2 r1
r2 14P
r3
r1

Center of core Valence orbit

Energy levels in a single has four electrons

atom
Isolated silicon atom
Electrons in the same orbit has same energy
Covalent Bond
Electron

14P An electron
shared by two
neighboring atoms
14P 14P 14P to form a covalent
bond.
This way an atom
14P can have a stable
structure with
eight valence band
electrons.
Silicon crystal
Intrinsic and extrinsic semiconductor

Intrinsic = pure

Extrinsic = impure or doped

 Doping
Doping means mixing a pure semiconductor with
impurities to increase its electrical conductivity
Can be done in two ways:
Increasing the number of electrons by mixing
pentavalent elements such as phosphorous,
arsenic, antimony (means adding donor impurities)
• Increasing the number of holes by mixing
trivalent elements such as aluminum, boron,
gallium
(means adding acceptor impurities)
To make silicon N-type:
• Add valence 5 phosphorous (P) atoms to valence 4
silicon.
• N-type semiconductor is obtained by adding
pentavalent impurity to a pure Silicon semiconductor.
 N-type semiconductor
Has many free electrons in conduction band and few holes
In valence band

Free Electron
Phosphorous atom
14P

14P 15P 14P

14P
• A N-type semiconductor (N for Negative) is a
material obtained by carrying out a process of
doping, that is, by adding some amount of an
element with more electrons to a semiconductor
element with fewer electrons.

• The impurity is called a "donor material,"

because it gives away (donates) weakly-bound
outer electrons to the semiconductor atoms.
To make silicon P-type:
• Add valence 3 boron (B) to silicon.
• P-type semiconductor is obtained by adding
trivalent impurity to a pure Silicon semiconductor.
•
 P-type semiconductor
Has few free electrons in conduction band and many holes
In valence band
Hole
Aluminum atom
14P

14P 13P 14P

14P
•A P-type semiconductor (P for Positive) is
obtained by carrying out a process of doping, that
is adding a certain type of atoms to the
semiconductor in order to increase the number of
free charge carriers (in this case positive).

• When the doping material is added, it takes

away (accepts) weakly-bound outer electrons
from the semiconductor atoms. This type of
doping agent is also known as an acceptor
material and the vacancy left behind by the
electron is known as an hole.
Hole/electron flow through a semiconductor
+ -
+ Free Electron (in conduction band)
-
+ -
+ -
+ 14P 14P 14P -
+ A C D F -
+ -
B E
+
Hole 14P 14P 14P -
+ -
+ (in valence band) -

The electron moves F-E-D-C-B-A

The hole moves A-B-C-D-E-F (pseudo movement)
 Majority and minority carriers

Electrons are
• Majority carriers in N-type semiconductor
• Minority carriers in P-type semiconductor

Holes are
• Majority carriers in P-type semiconductor
• Minority carriers in N-type semiconductor
 PN junction
• A p–n junction is a junction formed by joining
P-type and N-type semiconductors together in very
close contact.
• The term junction refers to the region where the
two regions of the semiconductor meet. p-type and
n-type blocks as shown in the following diagram:
• A p-doped and n-doped semiconductor is relatively
conductive.
• But the junction between them is a nonconductor.
• This non conducting layer, called the depletion
zone, occurs because the electrical charge carriers
in doped n-type and p-type silicon (electrons and
holes, respectively) attract and eliminate each other
in a process called recombination.
• By manipulating this non-conductive layer, p–n
junctions are commonly used as diodes: circuit
elements that allow a flow of electricity in one
direction but not in the other (opposite) direction.

• This property is explained in terms of forward bias

and reverse bias, where the term bias refers to an
application of electric voltage to the p–n junction.
 Applying Voltage
There are 3 possible "biasing" conditions for the
standard Junction Diode and these are:
1. Zero Bias - No external voltage potential is
applied to the PN-junction.
2. Reverse Bias - The voltage potential is connected
negative, (-ve) to the P-type material and
positive, (+ve) to the N-type material across the
diode which has the effect of Increasing the
PN-junction width and does so increasingly with
increasing reverse-bias voltage
3. Forward Bias - The voltage potential is connected
positive, (+ve) to the P-type material and negative,
(-ve) to the N-type material across the diode which
has the effect of Decreasing the PN-junction width.
Characteristics of PN
• In N-type, Donor impurities (pentavalent) are
introduced into one-side and acceptor impurities into
the other side of a single crystal of an intrinsic
semiconductor to form a p-n diode with a junction
called depletion region. This region gives rise to a
potential barrier Vγ called Cut- inVoltage. This is the
voltage across the diode at which it starts conducting.
The P-N junction can conduct beyond this Potential.
• In P-type, Both the holes on p-side and electrons
on n-side tend to move away from the junction
thereby increasing the depleted region. However
the process cannot continue indefinitely, thus a
small current called reverse saturation current
continues to flow in the diode.

• This small current is due to thermally generated

carriers. Assuming current flowing through the
diode to be negligible, the diode can be
approximated as an open circuited switch.
Summary
1). Semiconductors contain two types of mobile
charge carriers, Holes and Electrons.

2). The holes are positively charged while the

electrons negatively charged.

3). A semiconductor may be doped with donor

impurities such as phosphorous (N-type doping), so
that it contains mobile charges which are primarily
electrons.
4). A semiconductor may be doped with acceptor
impurities such as Boron (P-type doping), so that it
contains mobile charges which are mainly holes.

5). When a diode is Zero Biased no external energy

source is applied and a natural Potential Barrier is
developed across a PN-junction which is about 0.7v
for Silicon diodes and about 0.3v for Germanium
diodes.
6). When a diode is Forward Biased the PN-
junction is "reduced" and current flows
through the diode.

7). When a diode is Reverse Biased the PN-

junction is "increased" and zero current flows.
 4.5 RECTIFIERS

A rectifier is an electrical device that converts alternating

current (AC) to direct current (DC), a process known as
rectification.

A device which performs the opposite function (converting

DC to AC) is known as an inverter
TYPES OF RECTIFIERS

Centre-tape
Bridge rectifier.
rectifier.
 4.5.2 Half wave rectifier

• In half wave rectification, either the positive or negative

half of the AC wave is passed, while the other half is
blocked.
4.5.3 Center tap rectifier
4.5.4 Bridge wave rectifier
 4.5.5 Regulated power supplies
• Power supplies made from these blocks are
described below :
• Transformer
• Transformer + Rectifier
• Transformer + Rectifier + Smoothing
• Transformer + Rectifier + Smoothing + Regulator
• Transformer - steps down high voltage AC mains to low
voltage AC.
• Rectifier - converts AC to DC, but the DC output is varying.
• Smoothing - smooths the DC from varying greatly to a small
ripple.
• Regulator - eliminates ripple by setting DC output to a fixed
voltage.
Regulated power supplies
 COMPARISON OF RECTIFIERS
Half-wave Centre-tap Bridge type

No. of diode 1 2 4

Transformer No Yes No
necessary
Maximum 40.6% 81.2% 81.2%
efficiency
• The primary application of rectifiers is to derive
DC power from an AC supply. Virtually all
electronic devices require DC, so rectifiers find
uses inside the power supplies.
• Rectifiers also find a use in detection of
amplitude modulated radio signals.
• Rectifiers are also used to supply polarised
voltage for welding.
• A rectifier used in high-voltage direct current
power transmission systems
 4.6 Transistors & Amplifier
4.6.1 Introduction
•A transistor is a semiconductor device commonly
used to amplify or switch electronic signals.

•Transistors are composed of three parts – a base, a

collector, and an emitter.
4.6.2 Junction transistor structure