Conductors, Insulators, and Semiconductors

 The ability of a material to conduct current is based

on its atomic structure.
Introduction to Semiconductors  The orbit paths of the electrons surrounding the
nucleus are called shells.
Devices  Each shell has a defined number of electrons it will
hold. This is a fact of nature and can be determined by
the formula, 2n2.
Lecture1
 The outer shell is called the valence shell.
Second Semester
 The less complete a shell is filled to capacity the
more conductive the material is.

Conductors, Insulators, and Semiconductors Covalent Bonding

The valence shell determines the ability of material to conduct Covalent bonding is a bonding of two or more atoms by the
current. interaction of their valence electrons.
A Copper atom has only 1 electron A Silicon atom has 4 electrons in its
in its valence ring. This makes it a valence ring. This makes it a
good conductor. It takes 2n2 semiconductor. It takes 2n2 electrons
electrons or in this case 32 electrons or in this case or 18 electrons to fill
to fill the valence shell. the valence shell.
 Covalent Bonding N-type and P-type Semiconductors
Certain atoms will combine in this way to form a crystal
structure. Silicon and Germanium atoms combine in this way in The process of creating N- and P-type materials
their intrinsic or pure state. is called doping.
Other atoms with 5 electrons such as Other atoms with 3 electrons such as Boron
Antimony are added to Silicon to are added to Silicon to create a deficiency
increase the free electrons. of electrons or hole charges.

N-type P-type

The Depletion Region

Forward and Reverse Bias

Forward Bias Reverse Bias

With the formation of the p This creates the depletion Voltage source or bias connections are + to Voltage source or bias connections are – to
region and has a barrier the p material and – to the n material. the p material and + to the n material.
and n materials combination of
electrons and holes at the potential. This potential Bias must be greater than .3 V for Bias must be less than the breakdown
junction takes place. cannot be measured with a Germanium or .7 V for Silicon diodes. voltage.
voltmeter but it will cause a The depletion region narrows. Current flow is negligible in most cases.
small voltage drop. The depletion region widens.
 Forward Bias Measurements Forward Bias Measurements With Applied
With Small Voltage Applied Voltage Greater Than the Barrier Voltage.

In this case with the With the applied voltage

voltage applied is less exceeding the barrier
than the barrier potential the now fully
potential so the diode forward-biased diode
for all practical conducts. Note that the
purposes is still in a only practical loss is the .7
non-conducting state. Volts dropped across the
Current is very small. diode.

Ideal Diode Characteristic Curve

Practical Diode Characteristic Curve

In most cases we consider

In this characteristic only the forward bias
curve we do not voltage drop of a diode.
consider the voltage Once this voltage is
drop or the resistive overcome the current
properties. Current increases proportionally
flow proportionally with voltage.This drop is
increases with voltage. particularly important to
consider in low voltage
applications.
 Troubleshooting Diodes
Complex Characteristic Curve of a Diode 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.

The voltage drop is not

the only loss of a
diode. In some cases With the diode check function
we must take into a good diode will show
account other factors approximately .7 V or .3 V
such as the resistive when forward biased.
effects as well as When checking in reverse bias
reverse breakdown. the full applied testing voltage
will be seen on the display.
Note some meters show an
infinite (blinking) display.

Troubleshooting Diodes
Troubleshooting Diodes
Open Diode
In the case of an open diode no current flows in either direction
An ohmmeter can be used to check the forward
which is indicated by the full checking voltage with the diode
and reverse resistance of a diode if the ohmmeter
check function or high resistance using an ohmmeter in both
has enough voltage to force the diode into
forward and reverse connections.
conduction. Of course, in forward- biased
connection, low resistance will be seen and in
reverse-biased connection high resistance will be Shorted Diode
seen. In the case of a shorted diode maximum current flows indicated by a
0 V with the diode check function or low resistance with an
ohmmeter in both forward and reverse connections.
 Diode Packages 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.
Diodes come in a variety of sizes and shapes. The
design and structure is determined by what type of  P and N type materials are joined together to form a PN
circuit they will be used in. junction.
 A diode is nothing more than a PN junction.
 At the junction a depletion region is formed. This creates
barrier that 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.