Download BIPOLAR JUNCTION TRANSISTORS

 TRANSISTOR
TRANSISTOR
Background and Introduction
A semiconductor device that Amplifies, Oscillates, or Switches
the flow of current between two terminals
Invention of the Transistor
American physicists John Bardeen, Walter H. Brattain, and
William Shockley(later jointly awarded a Nobel Prize)
It was also independently developed nearly simultaneously
by Herbert Mataré and Heinrich Welker, German physicists
working
at Westinghouse Laboratory in Paris.
First Transistor
Model, 1947
Transistor Material
The transistor is an arrangement of semiconductor materials that
share common physical boundaries.
Materials most commonly used are
Silicon
gallium-arsenide
and germanium
Impurities have been introduced by a process called “doping”
TYPES OF TRANSISTOR
 BJT
 Bi Polar Junction transistor
 FET
 Field effect transistor
BIPOLAR JUNCTION TRANSISTORS
BJT
A BJT (Bipolar Junction Transistor) transistor has inside two
similar semi conductive materials, and between them there is a
third semi conductive material of different type
if the two similar materials are P and the middle one is N, then we
have a P-N-P or PNP transistor.
if the two materials are N and the middle one is P, then we have a
N-P-N material or NPN
Each transistor has 3 leads which we call base, collector and
emitter, and we use the symbols b, c and e respectively
The symbol of the transistor has an arrow on the emitter.
If the transistor is a PNP, then the arrow points to the base of the
transistor, otherwise it points to the output.
Types Of BJT
Two basic types of bipolar junction transistor construction,
1. PNP
2. NPN,
which basically describes the physical arrangement of the P-type
and N-type semiconductor materials
n-type semiconductors the impurities result in an excess of
electrons, or negative charges
p-type semiconductors the material lead to a deficiency of
electrons and therefore an excess of positive charge carriers or
“holes.”
current regulating devices that control the amount of current
flowing through them
NPN and PNP Transistor
BJT
Basic construction
The BJT is a three terminal device that produce two PN junctions
Emitter ( E )
Base ( B )
Collector ( C )
Principle of operation of the two transistor types PNP and
NPN
Biasing
Collector
polarity of the power
Base
supply for each type
Emitter
EMITTER
 It is highly doped
 To inject a large number of charge carriers
 Main function is to supply the majority carriers to the base.
 It is always forward biased with respect to base
BASE
 It is a middle section of a transistor
 It is lightly doped
 So that most of the charge carriers pass to the collector.
 It controls flow of charges
 It forms two PN junctions with Emitter and Collector
COLLECTOR
 IT IS situated opposite to the emitter
 It is always reversed biased
 So that it can collect the majority carriers
 Size of collector is larger than emitter
 Its doping level is in the middle of base and emitter
NPN TRANSISTOR
 It is constructed by two N type and One P type material.
 Emitter and collector are of N type material
 Base is of P type material
 It consist of two PN junctions
Sandwiching a P-type layer between two n-type layers.
OPERATION OF NPN TRANSISTOR
•The base-emitter diode is forward biased
•The base-collector diode is reverse biased
•VBE injects the electron to the Emitter.
•Emitter is highly doped
•Base is lightly doped
•Collector creates electrostatic field which
Attracts the electrons
•95 to 99% electrons diffuses in
collector region
C
backward
VCB
B
E
Forward
VBE
Transistor operation
With no power applied to the transistor areas
There are two depletion zones between the two P-N contacts.
Power source Connected b/w base and collector in reverse-bias
With the positive of the source connected to the collector and the negative to
the base.
The depletion zone of the P-N contact between the base and the collector will
be widened.
A slight current will flow
within this contact (due to impurities).
This current is the reverse contact
current symbol ICBO
Transistor operation
Voltage supply between the emitter and the base in forward bias
With the positive of the source connected to the base and the negative
connected to the emitter.
The depletion zone between the emitter and the base will be shortened
current (electrons) will flow when the voltage exceeds a specific level.
This level depends on the material that the transistor is made of.
Some of the electrons that go through the e-b depletion zone,
will re-connect with holes in the base.
This is the base current IB symbol for reference.
In real life, this current is at the scale
of micro-amperes (μA ):
Cont’d
Most of the electrons will flow through the base (due to spilling)
and will be directed to the collector.
When these electrons reach the depletion area between the base
and the collector, they will experience a force from the electric
field which exists in this zone,
The electrons will pass through the depletion zone.
The electrons will then re-connect with holes in the collector.
The re-connected holes will be replaced with holes coming from
the base-collector power supply (VCC).
The movement of these holes equals to a movement of electrons in
the opposite direction, from the collector to the supply.
In other words, the current that flows to the emitter will be
divided into the small base current and the larger collector
current:
IE = IB + IC
Cont’d
Generally, the number of electrons that arrive at the collector is the 99% of the
total electrons, and the rest 1% causes the base current.
At the collector, except the electrons that come from the emitter, there is also
the reverse current from the base-collector contact
Both currents flow at the same direction, so they are added
IC' = IC + ICBO
Transistor Parameters and Ratings
The ratio of the dc collector current (IC) to the dc base current (IB) is the dc beta (bDC).
bDC is called the gain of a transistor:
bDC = IC/IB
Typical values of bDC range from less than 20 to 200 or higher.
bDC is usually designated as an equivalent hybrid (h) parameter:
hFE = bDC
The ratio of the collector current (IC) to the dc emitter current (IE) is the dc alpha (aDC).
This is a less-used parameter than beta.
aDC = IC/IE
Typical values range from 0.95 to 0.99 or greater.
aDC is always less than 1.
This is because IC is always slightly less than IE by the amount of IB.
Cont’d
There are a number of standard parameters that are used to define the performance of a
transistor. Some of them are given below
Type number
Case
Material
Polarity
VCEO Collector emitter voltage with base open circuit
VCBO Collector base voltage with the emitter open circuit
VEBO Emitter base voltage with collector open circuit I
C
Collector current
ICM Peak collector current
IBM Peak base current
PTOT Total power dissipation
Tamb Ambient temperature T
Cont’d
Stg Storage temperature.
ICBO Collector base cut-off current
IEBO Emitter base cut-off current
hFE Forward current gain
VCEsat Collector emitter saturation voltage
VBEsat Base emitter saturation voltage
Cc Collector capacitance
Ce Emitter capacitance
Ft Frequency Transition
Introduction to Amplifiers
The BJT is an excellent amplifier when biased in the forward-active region.
The FET can be used as an amplifier if operated in the saturation region.
In these regions, the transistors can provide high voltage, current and power gains.
DC bias is provided to stabilize the operating point in the desired operation region.
The DC Q-point also determines
The small-signal parameters of the transistor
The voltage gain, input resistance, and output resistance
The maximum input and output signal amplitudes
The overall power consumption of the amplifier