lec_2_p-n diode
DESCRIPTION
lecture 2 electronic devices and circuitsTRANSCRIPT
TODAY’S LECTURE
Semiconductors
p-n junction diode
Ideal diode behavior
Practical diode behavior
Changing future, one IC at a time!
SEMI CONDUCTORS
Smallest unit of an element
Three basic particles:
• Neutrons
• Protons
• Electrons
Protons and Neutrons make up the nucleus.
Electrons revolve around the nucleus
Atomic Theory
Path of electrons are called orbits or shells.
The shells are named K,L,…,Q.
Valence shell : outermost shell
Free electrons in the valence shell determine the conductivity of the atom.
Atomic Model
What’s he maximum no of
electrons in the valence shell?
What’s the relationship between
valence shell and conductivity?
ANS : Eight.
ANS: Conductivity decreases with an increase in the no. of electrons.
Atoms having four electrons in their valence shell
Neither good conductors, nor insulators
Most common semi- conductors:
• Carbon
• Silicon
• Germanium
Semi Conductors
With no external force: No. of electrons = No. of protons
Net charge on an atom = 0
If an atom gains one electron, it becomes negatively charged.
If an atom loses one electron, it becomes positively charged.
Charges
What do you call a charged atom?
ANS : Ion
Positive ion = Cation Negative ion = Anion
Each shell has an energy level associated with it.
When an electron receives energy equal to energy difference of two shells, it jumps up to the next shell.
When the electron releases the energy, it comes back to the lower level.
Energy Levels
Atomic Number : 14
Atomic Mass : 28
Group IV of the periodic table
Silicon
Atoms form covalent bonds to complete their valence shells by sharing electrons.
Covalent Bond
The completed valence shells make intrinsic (pure) silicon a bad conductor.
The process of adding impurity atoms to the intrinsic (pure) silicon or germanium to improve the conductivity of the semiconductor.
Doping
What type of elements are used for
doping?
ANS : Trivalent : 3 valence electrons
Pentavalent : 5 valence electrons
When pentavalent impurities are added to silicon or germanium, the material is known as n-type.
This results in free electrons that increase conductivity.
N-type Material
When trivalent impurities are added to silicon or germanium, the material is known as p-type.
This results in the formation of holes that increase conductivity.
P-type Material
The fusion begins!
P-N JUNCTION
Individually, p-type and n-type materials offer little use.
Their real use comes when we can control the width of the depletion region of a p-n junction.
p-n Junction
When the p-type material is at greater potential than the n-type material.
Causes a flow of current through the p-n junction, provided that applied potential difference is greater than the potential barrier of the p-n junction.
Forward Bias
The depletion region becomes very thin, or practically disappears.
When the p-type material is at lower or equal potential to the n-type material.
No current flow. The width of the depletion region effectively
increases.
Reverse Bias
TAKE A BREAK! You’re not in jail!
The one way switch
P-N DIODE
A semi-conductor device made by creating a p-n junction.
Can be used in two modes :
Forward Bias.
Reverse Bias.
Acts as an uncontrolled switch.
p-n Junction Diode
Switch
How many states does a switch have?
ANS : Two 1 : ON 2 : OFF
How does a switch behave in its states?
ANS: ON : short circuit OFF: open circuit
Closed state : Essentially acts as a short circuit. • No resistance to current
flow
• No voltage drop across it
Open state : Acts as an open circuit. • Maximum resistance to
current flow
• ALL voltage is dropped across it
Switch: Behavior
Forward Bias : Essentially acts as a short circuit.
• No resistance to current flow
• No voltage drop across it
Reverse Bias : Acts as an open circuit.
• Maximum resistance to current flow
• ALL voltage is dropped across it
Diode: Behavior
Same properties as an ideal switch:
Turns ON and OFF instantaneously.
Offers zero resistance in forward bias state.
Zero forward bias voltage drop.
Infinite resistance in reverse bias state.
Zero leakage current.
Ideal Diode: Properties
Find out which diode in forward biased and which is reverse biased.
Ideal Diode: Examples
Find out the voltage drop across the resistors.
Ideal Diode: Examples
Ans: 3 Volts Ans: 0 Volts
Ideal Diode: Use
Why do we need to represent diode ideally?
To understand and represent things better, we assume the components ideal. • Diode is supposed to be ideal in
early stages of trouble shooting, when we are only concerned with the direction of current flow.
The way things really work
PRACTICAL DIODE
To understand and represent things better, we assume the components ideal.
However, these ideal models do not represent what actually happens in life.
A real diode has a finite forward voltage drop :
Silicon : 0.7 Volts
Germanium : 0.3 Volts
Caused by the p-n junction potential barrier.
Practical Diode: Properties
For a diode to be able to conduct electricity, it must be supplied some initial voltage equal to the barrier potential.
This voltage is dropped across the diode.
Forward Voltage Drop
Diodes may vary in shapes and sizes depending upon its type and application
Real Diodes
Find out the voltage drop across the resistors.
Practical Diode: Example
Describes how current through diode varies with a change in applied voltage.
Voltage-Current Relation
Which variable should be on x-axis and why?
Ans : Voltage, because current is changing with
respect to voltage
No forward voltage drop
No resistance to current flow
Ideal Diode V-I Relation
Finite forward voltage drop
Finite resistance to current flow
0.7V for Si
0.3V for Ge
Practical Diode V-I Model
ANY QUESTIONS?
Anyone willing to present?
Time allowed : 5 mins
Topic : Any
Bonus Points : +3
• INTRODUCTORY ELECTRONIC DEVICES AND CIRCUITS – By Robert T. Paynter
P.S: Please note that I might not follow this book throughout the semester, but this book is a really good one with some nice examples that will help you understand things a little better. You may ask questions related to the book, or lectures during University hours. You know where to find me =]
Reference Book
Fun Facts Why do crackers have holes in them??