Semiconductors

Introduction
Semiconductor is a solid substance that has conductivity between that of an insulator and that of most metals, either due to the addition of an impurity or because of temperature effects. Devices made of semiconductors, notably silicon, are essential components of most electronic circuits.
Examples: Silicon, Germanium, Carbon
Intrinsic & Extrinsic Semiconductor
Semiconductors are mainly classified into two categories: Intrinsic and Extrinsic. An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes). Where as an extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added.
The Doping of Semiconductors
The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors.
Pentavalent impurities
Impurity atoms with 5 valence electrons produce n-type semiconductors by contributing extra electrons.
Trivalent impurities
Impurity atoms with 3 valence electrons produce p-type semiconductors by producing a "hole" or electron deficiency.
N-Type Semiconductor
The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion of phosphine gas (PH3).
P-Type Semiconductor
The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons,called "holes". It is typical to use B2H6 diborane gas to diffuse boron into the silicon material.
Diodes
A device that blocks current in one direction while letting current flow in another direction is called a diode. Diodes can be used in a number of ways. For example, a device that uses batteries often contains a diode that protects the device if you insert the batteries backward. The diode simply blocks any current from leaving the battery if it is reversed -- this protects the sensitive electronics in the device.

1. SEMICONDUCTORS
Muhammad Usman
BSSE
Comsats Institute of Information Technology, Islamabad

2. Overview
Semiconductors
What are P-type and N-type semiconductors??
What are Diodes?
Forward Bias & Reverse Bias
Characteristics Of Ideal Diode
I – V Characteristics of Diodes
Rectification

3. Introduction
Semiconductors are materials
whose electrical properties lie
between Conductors and
Insulators.
Ex : Silicon and Germanium

4. Intrinsic & Extrinsic Semiconductor
Semiconductors that are free of doping impurities
are called intrinsic semiconductors.
Semiconductors formed after adding impurities are
called extrinsic semiconductors.

5. Doping is a process of adding a
certain amount of specific impurities
called dopants to a pure
semiconductor to increase its
electricity conductivity.
DOPING PROCESS

6. What are P-type and N-type ?
Semiconductors are classified in to P-type and
N-type semiconductor
 P-type: A P-type material is one in which
holes are majority carriers i.e. they are
positively charged materials (++++)
N-type: A N-type material is one in which
electrons are majority charge carriers i.e. they
are negatively charged materials (-----)

7. Diodes
Electronic devices created by bringing
together a p-type and n-type region within the
same semiconductor lattice. Used for
rectifiers, LED etc

8. Diodes
It is represented by the following symbol,
where the arrow indicates the direction of
positive current flow.

9. What is the p-n junction?
• A p-n junction is formed when a n-type and p-type
semiconductors are joined together.
• The boundary between the p-type and n-type
regions is called the junction.
• At the p-n junction, electrons from the n-side move
to the p-side and recombine with the holes.
• Holes from the p-side similarly move into the n-
side, where they recombine with electrons.
• As a result of this flow, the n-side has a net positive
charge, and the p-side has a net negative charge.

10. • The region around the junction is left with neither
holes nor free electrons.
• This neutral region which has no charge carriers is
called the depletion layer.
• This layer which has no charge carrier is a poor
conductor of electricity.
What is Depletion region?

11. Forward Bias and Reverse Bias
Forward Bias : Connect positive of the Diode
to positive of supply…negative of Diode to
negative of supply
Reverse Bias: Connect positive of the Diode
to negative of supply…negative of diode to
positive of supply.

12. Characteristics of Diode
Diode always conducts in one direction.
Diodes always conduct current when
“Forward Biased” ( Zero resistance)
Diodes do not conduct when Reverse Biased
(Infinite resistance)

13. I-V characteristics of Ideal diode

14. I-V Characteristics of Practical Diode

15. Rectification
Converting ac to dc is accomplished by the
process of rectification.
Two processes are used:
Half-wave rectification;
Full-wave rectification.

16. Half-wave Rectification
Simplest process used
to convert ac to dc.
A diode is used to clip
the input signal
excursions of one
polarity to zero.

17. Full-wave rectification by using four diodes
Input
ac current
A circuit that provides an entirely positive or
negative output voltage when an AC input
voltage is applied. A full wave rectifier
provides an output for both the positive and
negative alternations of the input voltage.