bond in which
one or more pairs of
electrons are shared
by two atoms.
bond in which one or more electrons
from one atom are removed and
attached to another atom, resulting in
positive and negative ions which
attract each other.
-Scientists use the percent ionic character of a bond to find out the
difference between an ionic bond and polar covalent bond.
-Ionic character increases with the electronegativity difference.
-Between discrete pairs of atoms, there are no bonds that are
considered ionic solids.
-Compounds with more than 50% ionic character are usually
considered ionic solids.
-If a compound conducts an electric current after it’s been melted,
it’s considered ionic.
A plot of the potential energy versus
the separation distance.
r= distance between the charges
Constant ( )=9.0×109N·m2/C2
If the two charges have the same sign,
the PE is positive for all values of r
If the two charges are of opposite sign, the PE is
negative because the product is negative.
This is how much energy must be put into the
system to separate the two atoms to infinity,
where the PE=0.
It is also the energy equivalent of
the “missing mass” of a nucleus.
The additional energy that must be injected into the
system to get the atoms over the “hump” (or barrier)
in the potential energy diagram.
Potential Energy Diagram For The Formation
• Quantum mechanics has been a great tool for
understanding the structure of solids. This active
field of research today is called solid-state
physics, or condensed-matter physics so as to
include liquids as well.
Although some solid materials are amorphous in
structure (such as glass) in that the atoms and
molecules show no longer range order, we will
be interested here in the large class of
crystalline substances whose atoms, ions, or
molecules are generally accepted to form an
orderly array in a geometric arrangement
known as a lattice.
Figure 29-22 Arrangement of atoms in (a) a simple cubic crystal , (b) body-
centered cubic crystal, and (c) face-centered cubic crystal .
Each diagram shows the relationship of the bonds. Each of these “cells” is
repeated in three dimensions to the edges of the macroscopic crystal.
Bonds in solid
• Covalent Bonding (as between the carbon
atoms of the diamond crystal)
• A different type of bond occurs in metals. Metal
atoms have relatively loosely held outer
electrons. Present-day metallic bond theories
propose that in a metallic solid, these outer
electrons roam rather freely among all the metal
atoms which, without their outer electrons, act
like positive ions. The electrostatic attraction
between the metal ions and these negative
electrons “gas” is at least in part responsible for
holding the solid together. The binding energy of
metal bonds is typically 1-3eV somewhat
weaker than ionic or covalent bonds(5-10eV in
solids). The free electrons are responsible for
the high electrical and thermal conductivity of
Comparison Of Important Strong
• Ionic Bonding- an electron is stolen from one
atom by another.
• Covalent Bonding- electrons are shared by
atoms within a single molecule
• Metallic Bonding- electrons are shared by all
atoms in the metal.
• When two hydrogen atoms approach each
other, the wave functions overlap, and the
two 1s states (one for each atom) divide into
two states of different energy.
• If a large number of atom comes together to
form a solid, then each of the original atomic
levels becomes a band. the energy levels are
so close together in each band that they seem
• The crucial aspect of a good conductor is that the
highest energy band containing electrons is only
• The 3s band, is only half full.
• Only two electrons can be in the 3s state , one
with spin up and one with spin down. these two
states have slightly different energy.
• For a solid consisting of N atoms, the 3s band will
contain 2N possible energy states.
• When a potential difference is applied across the
metal, electrons can respond by accelerating and
increasing their energy.
• A current flows readily and sodium is a good
• A characteristics of a good conductors is that
the highest energy band is only partially filled.
• In a material that is a good insulator, the
highest band containing electrons, called the
conduction band, is separated from the
valence bond by a “ forbidden” energy gap
• The important class of materials known as
• The bands for a semi conductor, such as silicon
(Si ), or germanium(Ge).
• At higher temperatures more electrons have
enough energy to jump the gap, this effect can
often more than offset the effects of more
frequent collisions due to increased disorder
at higher temperature.
• Unfilled electron states are called holes.
P-n junction diode
Separately, the two semiconductors are
electrically neutral .
When joined, a few electron near the junction
diffuse from the negative type into the positive
The n-type is left with a positive charge, and
the p-type acquires a net negative charge.
The externally applied voltage opposes the
internal potential difference and the diode is
said to be forward biased.
The positive type holes in the p-type
semiconductor are repelled by the positive
terminal of the battery and the electrons in
the n-type are repelled by the negative
terminal of the battery.
The positive terminal of the battery is
continually pulling electrons off the p end,
forming new holes, and electrons are being
supplied by the negative terminal at the n
end. consequently, a large current flows
through the diode.
When the diode is reversed biased, the holes
in the p end are attracted to the battery’s
negative terminal and the electrons in the n
end are attracted to the positive terminal.
If a voltage across the diode connected in
reverse bias is increasely greatly, a point is
reached where breakdown occurs.
The voltage remains constant over a wide
range of currents, and a diode designed for
this purpose is called a zener diode .
Zener diode can be obtained corresponding to
voltages of a few volts to a hundreds of volts.
Rectifier circuits are important because most
line voltage is ac and most electronic devices
require a dc voltage for their operation.
Photodiodes and solar cells are p-n junctions
used in the reverse way.
Photons are absorbed , creating electron-hole
pairs if the photon energy is greater than the
band gap energy.
A diode is called a nonlinear device because
the current is not proportional to the voltage
A small electronic device containing a
semiconductor and having at least three
electrical contacts, used in a circuit as an
amplifier, detector, or switch.
a circuit of transistors, resistors, and
capacitors constructed on a single
semiconductor wafer or chip, in which the
components are interconnected to perform
a given function; microcircuit. Abbr.: IC
A simple Junction
• A bipolar transistor consists of a three-layer “sandwich” of
doped (extrinsic) semiconductor materials, either P-N-P in
Figure below(b) or N-P-N at (d). Each layer forming
the transistor has a specific name, and each layer is provided
with a wire contact for connection to a circuit. The schematic
symbols are shown in Figure below(a) and (d).
• BJT transistor: (a) PNP schematic symbol, (b) physical layout
(c) NPN symbol, (d) layout.
The functional difference between a PNP transistor and an NPN transistor is
the proper biasing (polarity) of the junctions when operating. For any given
state of operation, the current directions and voltage polarities for each kind of
transistor are exactly opposite each other.
is controlled goes from collector to emitter, or from emitter to
respectively). The small current that controls the main current
depending on the kind of transistor it is (PNP or NPN,
respectively). According to the standards of semiconductor
symbology, the arrow always points against the direction of
Small electron base current controls large
collector electron current flowing against
Bipolar transistors are called bipolar because the main flow of electrons
through them takes place in two types of semiconductor material: P and
N, as the main current goes from emitter to collector (or vice versa). In
other words, two types of charge carriers -- electrons and holes --
comprise this main current through the transistor.
• As you can see, the controlling current and
the controlled current always mesh together through the emitter
wire, and their electrons always flow against the direction of
the transistor's arrow. This is the first and foremost rule in the
use of transistors: all currents must be going in the proper
directions for the device to work as a current regulator. The
small, controlling current is usually referred to simply as the base
current because it is the only current that goes through the base
wire of the transistor. Conversely, the large, controlled current is
referred to as the collector current because it is the only current
that goes through the collector wire. The emitter current is the
sum of the base and collector currents, in compliance with
Kirchhoff's Current Law.
• No current through the base of the transistor,
shuts it off like an open switch and prevents
current through the collector. A base current,
turns the transistor on like a closed switch and
the collector. Collector current is primarily
limited by the base current, regardless of the
amount of voltage available to push it.
• Bipolar transistors are so named because the controlled current
must go through two types of semiconductor material: P and N. The
current consists of both electron and hole flow, in different parts of
• Bipolar transistors consist of either a P-N-P or an N-P-N
semiconductor “sandwich” structure.
• The three leads of a bipolar transistor are called the Emitter, Base,
• Transistors function as current regulators by allowing a small
current to control a larger current. The amount of current allowed
between collector and emitter is primarily determined by the
amount of current moving between base and emitter.
• In order for a transistor to properly function as a current regulator,
the controlling (base) current and the controlled (collector) currents
must be going in the proper directions: meshing additively at the
emitter and going against the emitter arrow symbol.