Basic Electronics

Lecture # 1
Introduction to Semiconductors
 Atomic Structure
• Atom is the smallest particle of an element
that retains the characteristics of that
element.
• Atom consist of Nucleus and Shells
• Nucleus contains two particles inside
– Proton (+ve charge)
– Neutron (Neutral – No charge)
• Shells contain
– Electrons (-ve Charge)
 Atomic Number
• The atomic number equals the number of Protons in the
nucleus, which is the same as the number of electrons in
an electrically balanced (Neutral) atoms.
 Electron Shells and Orbits
• Each Discrete (Orbit) from the nucleus corresponds to a certain
energy level. In an atom, the orbits are grouped into energy bands
known as Shells.
– Shells are, K, L, M, N, and so on.
– Number of Electrons in each Shell
• Ne = 2n2

• Valence Electrons
• The outer most shell of atom is known as
Valence shell and electrons in this shell
are called valence electrons.

• Ionization
• The process of losing/gaining a electron
is known as ionization.
 Core of an Atom
• All shells except the valence shell is
known as core.
 Conductors
• A conductor is a material that conducts electrical current.
• Best conductors contain one valence electron very
loosely bound to the atom. These loosely bound valence
electrons can easily break away from their atoms and
become free electrons. Therefore, a conductive material
has many free electrons that, when moving in the same
direction, make up current.
• Insulators
– An insulator is a material that does not conduct electrical current
under normal conditions.
• Valance electrons are tightly bound to the atoms; therefore, there
are very few free electrons in an insulator.
 Energy Bands
• Valence shell of an atom represents a band of
energy levels and the valence electrons are
confined to that band.
• Energy required by a electron to leave the
valence shell and become a free electron and
exist in what is known as the conduction band.
• The difference in energy between the valence
band and the conduction band is called an
energy gap.
Comparison of a Semiconductor Atom to a Conductor Atom
Silicon and Germanium
 Covalent Bonds
• Two atoms share the same number of
electrons.
 Silicon Crystal

An Intrinsic crystal is one that has no impurities.

 Conduction Electrons and holes
• An intrinsic (pure) silicon crystal at room temperature derives heat
(thermal) energy from the surrounding air, causing some valence
electrons to gain sufficient energy to jump the gap from the valence
band into the conduction band, becoming free electrons not bound to
any one atom but free to drift. Free electrons are also called
conduction electrons.

• When an electron jumps to the conduction band, a vacancy is left in

the valence band. This vacancy is called a hole.

• For every electron raised to the conduction band by external energy,

there is one hole left in the valence band, creating what is called an
electron-hole pair.

• Recombination occurs when a conduction-band electron loses

energy and falls back into a hole in the valence band.
Energy band diagram for an unexcited silicon atom. There are no electrons in the
conduction band.
Creation of an electron-hole pair in excited silicon atom. An electron in the
conduction band is a free electron.
Electron-hole pairs in a silicon crystal. Free electrons are being generated
continuously while some recombine with holes.
 Electron and Hole current
• When a voltage is applied across a piece of intrinsic
silicon, the thermally generated free electrons in the
conduction band, are easily attracted toward the positive
end. This movement of free electrons is one type of
current in a semi conductive material and is called
Electron Current.
• The holes created by the free electrons exist. Electrons
remaining in the valence band, a valence electron can
move into near by hole, thus laving another hole where it
came from. Effectively the hole has moved from one
place to another in the crystal structure, this is called
Hole Current.
 Doping
• The conductivity of silicon and germanium
can be drastically increased by the
controlled addition of impurities to the
intrinsic (pure) semi conductive material.
This process, called Doping.
• There are two categories of impurities
– N – Type
– P – Type
 N – Type Semiconductor
• To increase the number of conduction band electrons in intrinsic
silicon, pentavalent impurity atom are added.
1. Arsenic (As)
2. Phosphorus (P)
3. Bismith (Bi)
4. Antimony (Sb)

• Each pentavalent atom forms covalent

bond with four adjacent silicon atom.

• Leaving one extra electron.

• extra electron becomes conduction

electron.

• electrons are in majority in result of

pentavalent doping.

• Electrons are called Majority carriers

and Holes are called Minority carriers in
N-Type Material.
 P – Type Semiconductors
• To increase the number of Holes in intrinsic silicon, trivalent
impurity atom are added.
• Aluminum (Al)
• Boron (B)
• indium (In)
• gallium (Ga)

• Each trivalent atom forms covalent

bond with four adjacent silicon atom.

• Leaving one extra hole.

• Holes are in majority in result of

trivalent doping.

• Holes are called Majority carriers

and Electrons are called Minority
carriers in P-Type Material.
 PN – Junction
• If a piece of intrinsic silicon is doped so that half
is N-type and the other half is P-type, a PN –
junction form b/w two regions.
• The P – region has many holes ( majority
carriers) and only a few thermally generated free
electrons ( minority carriers).
• The N – region has many electrons ( majority
carriers) and only a few thermally generated
holes (minority carriers).
 Formation of Depletion Region
• When PN – Junction is formed, the P region loses free
electrons as they diffuses across the junction.
• This create a layer of +ve charges near the junction.
• As the electron move across the junction, the P region
loses holes as the electrons and holes combine.
• This create a layer of –ve charges near the junction.
• These two layers of +ve and –ve charges form the
DEPLETION REGION.
The potential difference of the electric field across the depletion region is the
amount of energy required to move electrons through the electric field. This
potential difference is called the Barrier Potential.
• Barrier Potential for Silicon at 25oC is 0.7V
• Barrier Potential for Germanium 25oC is 0.3V
 Energy Diagram of the PN junction and
Depletion Region

• The valence band and conduction band in an n-type material are at

slightly lower energy levels than the valence and conduction band in
p-type material.

• This is due to the differences in the atomic characteristics of the

pentavalent and trivalent impurity atom.

• The electrons in the n-region that occupy the upper part of the
conduction band in terms of their energy can easily diffuse across
the junction and temporarily become free electrons in the lower part
of the p-region conduction band.

• After crossing the junction, the electrons quickly lose energy and fall
into the holes in the p-region valence band.
 Biasing the PN-Junction
• Applying DC voltage across PN-Junction
is called Biasing.
– Forward Bias
• This is the condition that allows current through
PN-Junction.
– Reverse Bias
• This is the condition that prevents current through
the PN-Junction.
 Forward Bias
• Connecting a DC voltage source with PN-Junction in
such a way that –ve terminal connects with N-region and
+ve terminal connects with P-region.
• The Applied voltages are called Bias Voltage (VBIAS).
 Effect of Forward Bias
• Negative side of the bias-voltage source “pushes” the
free electrons, this flow of free electrons is called
Electron Current.
• The bias voltage source imparts sufficient energy to the
free electrons for them to overcome the barrier potential
of the depletion region and move on through into p-
region
• Once in the P-region, electrons have lost energy to
combine with holes.
• The +ve side of the bias voltage source attracts the
electrons.
• Electrons moves one hole to the next, effective flow of
holes is called Hole Current.
As more electrons flow into the depletion region, the number of positive ions is
reduced, as more holes effectively flow into the depletion region on the other side
of the PN-Junction, the number of –ve ions is reduced. This causes the depletion
region to narrow.
 The effect of Barrier Potential During Forward
Bias

• In forward bias the sufficient amount of energy required

to electrons to break the potential barrier and cross the
depletion layer. This energy is always greater than the
barrier potential (0.7V for Silicon and 0.3V for
germanium).
• Internal resistance of the material is called Dynamic
Resistance.
 Reverse Bias
• Connecting a DC voltage source with PN-Junction in
such a way that +ve terminal connects with N-region and
-ve terminal connects with P-region.
• The Applied voltages are called Bias Voltage (VBIAS).
 Effect of Reverse Bias
• Negative side of the bias-voltage source “pulls” the free
electrons.

• As the electrons flow toward the positive side of the

voltage source, additional positive ions are created. This
result in a widening the depletion region.

• In the p-region, electrons from the negative side of the

voltage source enter as valence electrons and move
from hole to hole toward the depletion region where they
created additional negative ions.
The extremely small reverse current in a reverse-biased pn-junction is due to the
minority carriers from thermally generated electron-hole pairs.
 Reverse Breakdown
• The external reverse-bias voltage is increased to
a value called the breakdown voltage, the
reverse current will drastically increase.
• Free minority electrons gain enough energy to
move from valence band to conduction band
with enormously high speed and These
electrons collide with conduction band electrons
and repeat the process.
• The multiplication of conduction electrons is
known as Avalanche and result in a very high
reverse current that can damage the pn
structure because of excessive heat dissipation.
Energy Diagram for Forward and Reverse Bias
I-V Characteristics of Forward Bias
I-V Characteristics of Reverse Bias
Temperature Effects on the I-V
characteristics
Diode
Diode in Forward/Reverse Bias
Ideal Diode Model

Forward Voltage VF 0
VBIAS
Forward Current IF
RLIMIT
Reverse Current IR 0A
Reverse Voltage VR VBIAS
The Practical Diode Model

Silicon VF 0.7V
Germanium VF 0.3V

VBIAS VF
Forward Current IF
RLIMIT
Reverse Current IR 0A
Reverse Voltage
VR VBIAS
The Complex Diode Model

VF 0.7V I F rd
VBIAS 0.7V
IF
RLIMIT rd