Transcript V - wayansupardi

1) Introduction
2) Transistor Theory
3) Naming the
Transistor Terminal
4) Transistor Action
5) Transistor Symbol
What is a transistor?
 A transistor is a 3 terminal electronic device made of semiconductor
material.
 Transistors have many uses, including amplification, switching, voltage
regulation, and the modulation of signals
HISTORY
Before transistors were invented, circuits used vacuum tubes:
Fragile, large in size, heavy, generate large quantities of heat, require a large
amount of power
 The first transistors were created at Bell Telephone Laboratories in 1947
William Shockley, John Bardeen, and Walter Brattain created the transistors in
and effort to develop a technology that would overcome the problems of tubes

The first patents for the principle of a field effect transistor were registered in
1928 by
Julius Lillenfield.

Shockley, Bardeen, and Brattain had referenced this material in their work
The word “transistor” is a combination of the terms “transconductance” and
“variable resistor”

Today an advanced microprossesor can have as many as 1.7 billion transistors.

What Are Diodes Made Out Of?
• Silicon (Si) and Germanium (Ge) are the two most common
single elements that are used to make Diodes. A
compound that is commonly used is Gallium Arsenide
(GaAs), especially in the case of LEDs because of it’s large
bandgap.
• Silicon and Germanium are both group 4 elements,
meaning they have 4 valence electrons. Their structure
allows them to grow in a shape called the diamond lattice.
• Gallium is a group 3 element while Arsenide is a group 5
element. When put together as a compound, GaAs creates
a zincblend lattice structure.
• In both the diamond lattice and zincblend lattice, each atom
shares its valence electrons with its four closest neighbors.
This sharing of electrons is what ultimately allows diodes to
be build. When dopants from groups 3 or 5 (in most cases)
are added to Si, Ge or GaAs it changes the properties of
the material so we are able to make the P- and N-type
materials that become the diode.
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
The diagram above shows
the 2D structure of the Si
crystal. The light green lines
represent the electronic
bonds made when the
valence electrons are shared.
Each Si atom shares one
electron with each of its four
closest neighbors so that its
valence band will have a full
8 electrons.
N-Type Material:-
+4
+4
+4
+4
+5
+4
+4
+4
+4
When extra valence electrons are
introduced into a material such as silicon
an n-type material is produced. The extra
valence electrons are introduced by putting
impurities or dopants into the silicon. The
dopants used to create an n-type material
are Group V elements. The most commonly
used dopants from Group V are arsenic,
antimony and phosphorus.
The 2D diagram to the left shows the extra
electron that will be present when a Group
V dopant is introduced to a material such
as silicon. This extra electron is very
mobile.
P-Type Material:-
+4
+4
+4
+4
+3
+4
+4
+4
+4
P-type material is produced when the
dopant that is introduced is from Group III.
Group III elements have only 3 valence
electrons and therefore there is an electron
missing. This creates a hole (h+), or a
positive charge that can move around in the
material. Commonly used Group III
dopants are aluminum, boron, and gallium.
The 2D diagram to the left shows the hole
that will be present when a Group III dopant
is introduced to a material such as silicon.
This hole is quite mobile in the same way
the extra electron is mobile in a n-type
material.
P-N Junction Diodes
Forward Bias:
Current flows from P to N
Reverse Bias:
No Current flows
Excessive heat can cause dopants in a
semiconductor device to migrate in
either direction over time, degrading
diode
Ex: Dead battery in car from rectifier
short
Ex: Recombination of holes and
electrons cause rectifier open circuit
and prevents car alternator form
charging battery
●
Practical Diode Circuit
• Diode charges
capacitor.
• The diode is assumed
ideal. It will only
conduct when vI is
more than vo
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Early
BJTs were fabricated
using alloying - an complicated
and unreliable process.
The structure contains two p-n
diodes, one between the base
and the emitter, and one
between the base and the
collector.
A bipolar transistor essentially
consists of a pair of PN Junction
diodes that are joined back-to-back.
There are therefore two kinds of
BJT, the NPN and PNP varieties.
The three layers of the sandwich
are conventionally called the
Collector, Base, and Emitter.
BJTs
are usually constructed vertically
›Controlling depth of the emitter’s n doping sets the
base width
E
p
n
n
B
C
.The Bipolar Junction
Transistor
The transistor is a versatile device usually configured to perform as a switch or as an
amplifier. The bipolar junction transistor (BJT) is the most common type and has
three leads:
3
3
Collector
Base
2
Collector
Base
1
Emitter
PNP Transistor
2
1
Emitter
NPN Transistor
In a transistor, the flow of current from the collector to the emitter is controlled
by the amount of current flowing into the base of the transistor. If no current
flows into the base, no current will flow from the collector to the emitter (it acts
like an open switch). If current flows into the base, then a proportional amount
of current flows from the collector to the emitter (somewhat like a closed switch).
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Transistor Size (3/8”L X 5/32”W X 7/32”H)
No Date Codes. No Packaging.

BJT (Bipolar Junction Transistor)
› npn
 Base is energized to allow current flow
› pnp
 Base is connected to a lower potential to allow current
flow

3 parameters of interest
› Current gain (β)
› Voltage drop from base to emitter when VBE=VFB
› Minimum voltage drop across the collector and
emitter when transistor is saturated
High potential at
collector
 Low potential at
emitter
 Allows current flow
when the base is
given a high potential

High potential at
emitter
 Low potential at
collector
 Allows current flow
when base is
connected to a low
potential

Emitter is heavily doped compared to collector. So, emitter
and collector are not interchangeable.
The base width is small compared to the minority carrier
diffusion length. If the base is much larger, then this will
behave like back-to-back diodes.
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Field-effect transistors (FETs) are
probably the simplest form of transistor
widely used in both analogue and digital
applications
they are characterised by a very high input
resistance and small physical size, and they
can be used to form circuits with a low power
consumption
they are widely used in very large-scale
integration
two basic forms:
insulated gate FETs
junction gate FETs
An Overview of Field-Effect
Transistors
Many forms, but basic operation is the same
a voltage on a control input produces an electric field
that affects the current between two other terminals
when considering
amplifiers we looked
at a circuit using a
‘control device’
a FET is a suitable
control device


Analogous to BJT
Transistors
FET Transistors switch
by voltage rather
than by current
D
G
S
The “Field Effect”
 The resulting field at the plate causes electrons to
gather
 As an electron bridge forms current is allowed to flow

Plate
Semiconductor
Terminals & Operations
Three terminals:
Base (B): very thin and lightly doped central region (little
recombination).
Emitter (E) and collector (C) are two outer regions sandwiching B.
Normal operation (linear or active region):
B-E junction forward biased; B-C junction reverse biased.
The emitter emits (injects) majority charge into base region and
because the base very thin, most will ultimately reach the
collector.
The emitter is highly doped while the collector is lightly doped.
The collector is usually at higher voltage than the emitter.
WORKING OF TRANSISTOR
The emitter base junction of a transistor is forward
biased whereas collector base junction is reversed
biased.
In the absence of the emitter base junction no
current would flow in the collector circuit because
of the reversed biased and if it is present forward
biased on it possess the emitter current to flow.
Emitter current almost entirely flows in the collector
circuit,therefore the current in the collector circuit
depends upon the emitter current.
How the BJT works
Figure shows the energy levels in an
NPN transistor under no externally
applying voltages.
In each of the N-type layers
conduction can take place by the
free movement of electrons in the
conduction band.
In the P-type (filling) layer
conduction can take place by the
movement of the free holes in the
valence band.
However, in the absence of any
externally applied electric field, we
find that depletion zones form at
both PN-Junctions, so no charge
wants to move from one layer to
another.
NPN Bipolar Transistor
What happens when we apply a
moderate voltage between the
collector and base parts.
The polarity of the applied
voltage is chosen to increase the
force pulling the N-type electrons
and P-type holes apart.
This widens the depletion zone
between the collector and base
and so no current will flow.
In effect we have reverse-biassed
the Base-Collector diode
junction.
Apply a Collector-Base voltage
Charge Flow
What happens when we apply a relatively
small Emitter-Base voltage whose polarity is
designed to forward-bias the Emitter-Base
junction.
This 'pushes' electrons from the Emitter into
the Base region and sets up a current flow
across the Emitter-Base boundary.
Once the electrons have managed to get into
the Base region they can respond to the
attractive force from the positively-biassed
Collector region.
As a result the electrons which get into the
Base move swiftly towards the Collector and
cross into the Collector region.
Apply an Emitter-Base voltage
Hence a Emitter-Collector current magnitude
is set by the chosen Emitter-Base voltage
applied.
Hence an external current flowing in the
circuit.
Charge Flow
Some of free electrons crossing the
Base encounter a hole and 'drop into
it'.
As a result, the Base region loses one
of its positive charges (holes).
The Base potential would become
more negative (because of the removal
of the holes) until it was negative
enough to repel any more electrons
from crossing the Emitter-Base
junction.
The current flow would then stop.
Some electron fall into a hole
Charge Flow
To prevent this happening we use the
applied E-B voltage to remove the
captured electrons from the base and
maintain the number of holes.
The effect, some of the electrons
which enter the transistor via the
Emitter emerging again from the
Base rather than the Collector.
For most practical BJT only about 1%
of the free electrons which try to
cross Base region get caught in this
way.
Hence a Base current, IB, which is
typically around one hundred times
smaller than the Emitter current, IE.
Some electron fall into a hole
Charge Flow
To prevent this happening we use the
applied E-B voltage to remove the
captured electrons from the base and
maintain the number of holes.
The effect, some of the electrons
which enter the transistor via the
Emitter emerging again from the
Base rather than the Collector.
For most practical BJT only about 1%
of the free electrons which try to
cross Base region get caught in this
way.
Hence a Base current, IB, which is
typically around one hundred times
smaller than the Emitter current, IE.
Some electron fall into a hole
•Current flow in an npn transistor biased to operate in the active mode
•Forward bias of Emitter-Base Junction: current flows to emitter, electrons move
towards base, holes to emitter
•Reverse bias of Base-Collector Junction: IC independent of VCB
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•Current in PNP mainly due to holes injected from emitter to base
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Combine the two diodes!
VF
VR
h
p
p
n
e
I forward
I reverse
No transistor
action
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E
h
e
Hole concentration is zero here,
reverse biased
I E
IE
( 1   )I E
R
VF
VR
The collector current IC is almost equal to IE, and collector current
is controlled by the E-B junction bias. The loss, i.e.  < 1
corresponds to the recombination of holes in base.
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BJT Modes
 Cut-off Region: VBE < VFB, iB=0
Transistor acts like an off switch
 Active Linear Region: VBE=VFB, iB≠0, iC=βiB
Transistor acts like a current amplifier
 Saturation Region: VBE=VFB, iB>iC,max/ β
In this mode the transistor acts like an on switch
Power across BJT
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●
BJT Characteristics
iC-vCB characteristics for an npn transistor in the active mode.
•Collector is constant current source only controlled by emitter current iC
iC  I E
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Transistor as an amplifier. (b) The circuit of (a) with the signal source vbe
eliminated for dc (bias) analysis.
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IE = IB + IC
and
VEB + VBC + VCE = 0
VCE =  VEC
As shown, the currents are positive quantities when the
transistor is operated in forward active mode.
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Emitter is heavily doped compared to collector. So, emitter
and collector are not interchangeable.
The base width is small compared to the minority carrier
diffusion length. If the base is much larger, then this will
behave like back-to-back diodes.
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