Introduction to semiconductors

basic of semiconductor and semiconductor device

1. INTRODUCTION TO
SEMICONDUCTORS
Presented By:
VasudevShrivastava
P.G.T.(Physics)
J.N.V. Chhatarpur(M.P.)

2. ATOMIC STRUCTURE
AND SEMICONDUCTORS
•The basic structure of semiconductors
–Silicon and Germanium Atoms

3. ATOMIC BONDING
•The atoms within the crystal structure are
held together by covalent bonds
•This sharing of valence electrons produces
the covalent bonds that hold the atoms
together

4. CONDUCTION IN
SEMICONDUCTORS
•An energy band
diagram for silicon
crystal occurs only at
a temperature of
absolute 0 K

5. Conduction Electrons and Holes
•An intrinsic (pure) silicon crystal at room
temperature has sufficient heat energy for
some valence electrons to jump the gap
from the valence band into the conduction
band, which become free electrons
•When an electron jumps to the conduction
band, a vacancy is left in the valence band
within the crystal, called a hole.

6. Creation of electron-hole

7. Electron-hole pairs
•Recombination occurs when a conduction-
band electron loses energy and fall back
into a hole in the valence band

8. Electron and Hole Current
•When a voltage is applied across a piece
of silicon, the movement of free electrons
is called electron current. The current
which flow opposite with electron current is
called hole current.

9. Hole current in intrinsic silicon

10. Comparison of Semiconductors to
Conductors and Insulators
•Pure semi conductive
materials are neither
insulators nor good
conductors because
current in a material
depends directly on
the number of free
electrons

11. N-TYPE AND P-TYPE
SEMICONDUCTORS
•The conductivities of silicon and
germanium can be increased and
controlled by the addition of impurities to
the intrinsic (pure) semi conductive
material called doping
•The two categories of impurities are n-
type and p-type

12. N-TYPE SEMICONDUCTOR
•To increase the
number of
conduction-band
electron in intrinsic
silicon, pentavalent
impurity atom with
five valence electrons
(such as arsenic (As),
phosphorus (P), and
antimony (Sb) are
added.
n-type

13. Majority and Minority Carriers of
N-type Semiconductor
•The electrons are called the majority
carriesin n-type material ( the n stand for
the negative charge on an electron)
•Holes which are not produced by the
addition of the penta valent impurity atoms
are called minority carries

14. P-TYPE SEMICONDUCTOR
•Trivalent impurity atom
(three valence
electrons, such as
aluminum (Al), Boron
(B), and gallium (Ga))
are added to increase
the number of holes in
intrinsic silicon
•Atoms with three
valence electrons are
known acceptor atoms
because they leave a
hole in the
semiconductor’s crystal
structure
p-type

15. Majority and Minority Carriers of
P-type Semiconductor
•The holes are the majority carries in p-type
material and Electron in p-type material
are the minority carries

16. THE PNJUNCTION
•The junction of silicon which it has doped
on one half with a trivalent impurity and
the other half with a pentavalent impurity is
called the pn junction
•The pn junction is the feature that allows
diodes , transistor, and other devices to
work

17. Formation of the Depletion Region
•The area on both sides of the junction is called
depletion region
•The existence of the positive and negative ions on
the opposite sides of the junction creates a barrier
potential (V
B) that is the amount of voltage required
to move electrons through the electric field

18. P-N Junction Diode
•A P-N Junction Diode is formed by doping one
side of a piece of silicon with a P-type do-pant
(Boron) and the other side with a N-type dopant
(phosphorus).Ge can be used instead of Silicon.
The P-N junction diode is a two-terminal device.
This is the basic construction of the P-N junction
diode. It is one of the simplest semiconductor
devices as it allows current to flow in only one
direction .The diode does not behave linearly
with respect to the applied voltage, and it has an
exponential V-I relationship.

19. What is P-N Junction diode?
•A P-N junction diode is a piece of silicon
that has two terminals. One of the terminals
is doped with P-type material and the other
with N-type material. The P-N junction is
the basic element for semiconductor
diodes. A Semiconductor diode facilitates
the flow of electrons completely in one
direction only –which is the main function
of semiconductor diode. It can also be used
as a Rectifier.

21. Energy Diagram of the PN Junction

22. PN Junction Diode Theory
There are two operating regions: P-type and N-type. And
based on the applied voltage, there are three possible
“biasing” conditions for the P-N Junction Diode, which are
as follows:
Zero Bias–No external voltage is applied to the PN junction
diode.
Forward Bias–The voltage potential is connected positively
to the P-type terminal and negatively to the N-type terminal of
the Diode.
Reverse Bias–The voltage potential is connected
negatively to the P-type terminal and positively to the N-type
terminal of the Diode.

23. Zero Biased Condition (un biased condition)
In this case, no external voltage is applied to the P-N junction diode; and
therefore, the electrons diffuse to the P-side and simultaneously holes diffuse
towards the N-side through the junction, and then combine with each other. Due
to this an electric field is generated by these charge carriers. The electric field
opposes further diffusion of charged carriers so that there is no movement in
the middle region. This region is known as depletion width or space charge.

24. Forward Bias
In the forward bias condition, the negative terminal of the battery is connected to
the N-type material and the positive terminal of the battery is connected to the P-
Type material. This connection is also called as giving positive voltage. Electrons
from the N-region cross the junction and enters the P-region. Due to the attractive
force that is generated in the P-region the electrons are attracted and move
towards the positive terminal. Simultaneously the holes are attracted to the
negative terminal of the battery. By the movement of electrons and holes current
flows. In this condition, the width of the depletion region decreases due to the
reduction in the number of positive and negative ions.

25. V-I Characteristics of p-n unction diode in
forward bias
By supplying positive voltage, the electrons get enough energy to
overcome the potential barrier (depletion layer) and cross the junction
and the same thing happens with the holes as well. The amount of
energy required by the electrons and holes for crossing the junction is
equal to the barrier potential 0.3 V for Ge and 0.7 V for Si, 1.2V for
GaAs. This is also known as Voltage drop. The voltage drop across the
diode occurs due to internal resistance. This can be observed in the
below graph .

26. The Effect of the Barrier Potential
on Forward Bias

27. Reverse Bias
Inthereversebiascondition,thenegativeterminalofthebatteryisconnectedtotheP-type
materialandthepositiveterminalofthebatteryisconnectedtotheN-typematerial.Thisconnection
isalsoknownasgivingnegativevoltage.Hence,theelectricfieldduetoboththevoltageand
depletionlayerisinthesamedirection.Thicknessofdepilationlayerwillincreases.Thismakesthe
electricfieldstrongerthanbefore.Duetothisstrongelectricfield,electronsandholeswantmore
energytocrossthejunctionsotheycannotdiffusetotheoppositeregion.Hence,thereisno
currentflowduetothelackofmovementofelectronsandholes.
The electrons from the N-type semiconductor are attracted towards the positive terminal and the
holes from the P-type semiconductor are attracted to the negative terminal. This leads to the
reduction of the number of electrons in N-type and holes in P-type. In addition, positive ions are
created in the N-type region and negative ions are created in the P-type region. Therefore, the
depletion layer width is increased due to the increasing number of positive and negative ions

28. Energy Diagram for Reverse Bias
•When a p-n junction is reverse-biased, the n-region
conduction band remain at an energy level that
prevents the free electrons from crossing into the p-
region
•There are a few free minority electrons in the p-region
conduction band that flow down the ‘energy hill’ into
the n-region, and they combine with minority hole in
the valence band

29. BIASING THE PN JUNCTION
•Reverse Bias
–Reverse bias is the condition that prevents
current through the pnjunction
–Reverse currentis a very small current
produced by minority carries during reverse
bias

30. Circuit diagram for Reverse bias

31. V-I Characteristics in reverse bais
Due to thermal energy in crystal minority carriers are produced. Minority carriers mean a hole in N-
type material and electrons in P-type material. These minority carriers are the electrons and holes
pushed towards P-N junction by the negative terminal and positive terminal, respectively. Due to the
movement of minority carriers, a very little current flows, This current is called as reverse saturation
current. Saturation means, after reaching its maximum value, a steady state is reached wherein the
current value remains same with increasing voltage.
When the reverse voltage is increased beyond the limit, then the reverse current increases
drastically. This particular voltage that causes the drastic change in reverse current is called
reverse breakdown voltage. Diode breakdown occurs by two mechanisms: Avalanche breakdown
and Zener breakdown.

32. Reverse Breakdown
•If the external reverse-bias voltage is increased
to a large enough value, reverse breakdown
occurs
•When one minority conduction-band electron
goes toward the positive end of the pnjunction,
during its travel, it collides with an atom and
imparts enough to knock a valence electron into
the conduction band
•The rapid multiplication of conduction-band
electrons, known as an avalanche effect

33. V-I Characteristics of P-N
junction Diode

34. The graph will be changed for different
semiconductor materials used in the construction
of a P-N junction diode.

35. Applications of PN junction Diode
•P-N junction diode in the reverse-biased configuration is sensitive to light from a range
between 400nm to 1000nm, which includes VISIBLE light. Therefore, it can be used as a
photodiode.
•It can also be used as a solar cell.
•P-N junction forward bias condition is used in all LED lighting applications.
•The voltage across the P-N junction biased is used to create Temperature Sensors, and
Reference voltages.
•It is used in many circuits’ rectifiers, varactors for voltage-controlled oscillators.