Transcript Bipolar Junction Transistors (BJT)

Bipolar Junction Transistors
(BJT)
BHAVIN V KAKANI
Electronics & Communication Engineering Department
IT-NU
First - BJTs
The transistor was
probably the most
important invention of
the 20th Century, and
the story behind the
invention is one of
clashing egos and top
secret research.
Reference:
Bell Labs Museum
B. G. Streetman & S. Banerjee ‘Solid State Electronic Devices’, Prentice Hall 1999.
Interesting story…
Picture shows the workbench of John
Bardeen (Stocker Professor at OU) and
Walter Brattain at Bell Laboratories. They
were supposed to be doing fundamental
research about crystal surfaces.
The experimental results hadn't been
very good, though, and there's a rumor that
their boss, William Shockley, came near to
canceling the project. But in 1947, working
alone, they switched to using tremendously
pure materials.
It dawned on them that they could
build the circuit in the picture. It was a
working amplifier! John and Walter submitted
a patent for the first working point contact
transistor.
Interesting story…
Shockley was furious and took their
work and invented the junction transistor
and submitted a patent for it 9 days later.
The three shared a Nobel Prize in 1955.
Bardeen and Brattain continued in
research (and Bardeen later won another
Nobel).
Shockley quit to start a semiconductor
company in Palo Alto. It folded, but its
staff went on to invent the integrated
circuit (the "chip") and to found Intel
Corporation.
By 1960, all important computers used
transistors for logic, and ferrite cores for
memory.
Point-Contact Transistor –
first transistor ever made
Qualitative basic operation of point-contact
transistor
Problems with first transistor…
First Bipolar Junction Transistors
W. Shockley invented the p-n junction transistor
The physically relevant region is moved to the bulk of the material
Bipolar Junction Transistors
Advantages Of Transistors Over Vacuum Tubes
• Much- Smaller And Lighter
• Consume Much less Power
• Do Not Get Hot
• More rugged – No Glass to Break
• No Warm Up Time Needed
Bipolar Junction Transistors
Disadvantages Of Transistors Over Vacuum Tubes
• Can Not Handle Same Amount of Power
• Sensitive To Temperature and Radiation
• Harder To Mass Produce
10
The Structure
The Bipolar Junction Transistor (BJT)
 Bipolar: both electrons and holes are involved in current flow.
 Junction: has two p-n junctions.
 Transistor: Transfer + Resistor.
 It can be either n-p-n type or p-n-p type.
 Has three regions with three terminals labeled as
i. Emitter (E)
ii. Base (B)
iii. Collector (C)
and
Transistor Types
• Because there are two junctions,
transistors are generally labeled with
the prefix “ 2N”:
• 2N3904
• 2N3906
• 2N2222
• 2N2907
Bipolar Junction Transistors
Schematic Symbols
NPN
PNP
Bipolar Junction Transistors
Terminals
Base
2N
Emitter
3904
Emitter
Collector
15
Base
Collector
Heat sink
Bipolar Junction Transistors
Terminals
17
force – voltage/current
water flow – current
- amplification
Understanding of BJT
The Structure: npn & pnp
 Base is made much narrow.
 Emitter is heavily doped (p+, n+).
 Base is lightly doped (p-, n-).
 Collector is lightly doped (p, n).
19
Bipolar Junction Transistors
Bias
• Base
• Used to control amount of collector current flow
• Changes the “resistance” of the transistor (E to C)
• Base-Emitter Junction
• Must be Forward Biased!
• Base-Collector Junction
• Must Be Reverse Biased!
Basic models of BJT

Transistors can be
constructed as two
diodes that are
connected together.
npn transistor
Diode
Diode
pnp transistor
Diode
Diode
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Circuit Symbol
Layout and Circuit Symbol: n-p-n Transistor
 The arrow indicates the direction of current flow.
 The current flows from collector to emitter in an n-p-n
transistor.
 The arrow is drawn on the emitter.
 The arrow always points towards the n-type. So the emitter
is n-type and the transistor is n-p-n type.
Circuit Symbol
Layout and Circuit Symbol: p-n-p Transistor
 The arrow indicates the direction of current flow.
 The current flows from emitter to collector in an p-n-p
transistor.
 The arrow points towards the n-type.
 So the base is n-type and transistor is p-n-p type.
Terminal Currents
Reference Positive Current Directions
IC
Collector current
IB
Base current
IE
Emitter current
24
Modes of Operation
 Based on the bias voltages applied at the two p-n junctions,
transistors can operate in three modes:
1. Cut-off (both EB and CB junctions are reversed
biased)
2. Saturation (both EB and CB junctions are
forward biased)
3. Active mode (EBJ is forward biased and CBJ is
reversed biased)
 Cut-off and Saturation modes are used in switching operation.
 Active mode is used in amplification purposes.
Modes of Operation
Cut-off
 Both
the
junctions
are
reversed biased.
 No current can flow through
either of the junctions.
 So the circuit is open.
VBC
+
+
VBE
-
Ideal model of BJT
in cut-off.
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Modes of Operation
Saturation: Ideal Model
 Both
the
junctions
are
forward biased.
 So the equivalent circuit can
be represented by short-circuit
between the base, emitter and
collector.
VBC
+
+
VBE
Ideal model of BJT
in saturation.
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Modes of Operation
Saturation: Practical Model
VCE (sat)  VCB  VBE  VBC  VBE
VCE(sat) is in the range of 0.1 to 0.2 V, as VBC and VBE are both approximately
equal to the diode forward drop.
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Modes of Operation
Active Mode Operation
EBJ:
Forward Biased
CBJ:
Reverse Biased
◦ Forward bias of EBJ injects electrons from emitter into base (Emitter current).
◦ Most electrons shoot through the base into the collector
(Collector current).
◦ Some emitted electrons recombine with holes in p-type base (Base Current)
• Both biasing potentials have been applied to a pnp transistor and resulting
majority and minority carrier flows indicated.
• Majority carriers (+) will diffuse across the forward-biased p-n junction into
the n-type material.
• A very small number of carriers (+) will through n-type material to the
base terminal. Resulting IB is typically in order of microamperes.
• The large number of majority carriers will diffuse across the reverse-biased
junction into the p-type material connected to the collector terminal.
Hole
N
E
electron
P
N
+
-
-
+
+
-
-
+
+
-
-
+
+
-
-
+
+
-
-
+
B
C
Electron diffusion
Hole diffusion
N
E-Field
P
E
+ -
VBE
N
-
+
-
+
-
+
-
+
-
+
C
VCB
B
N
E-Field
P
E
+ -
N
-
+
-
+
-
+
-
+
-
+
C
Electron hole recombination
VBE
VCB
B
• Majority carriers can cross the reverse-biased junction
because the injected majority carriers will appear as
minority carriers in the n-type material.
• Applying KCL to the transistor :
IE = IC + IB
• The comprises of two components – the majority and
minority carriers
IC = ICmajority + ICOminority
• ICO – IC current with emitter terminal open and is called
leakage current.
Collector current
Electrons that diffuse across the base to the CBJ junction are swept
across the CBJ depletion to the collector because of the higher potential
applied to the collector
iC  I s e
vBE
VT
• The equation above shows that the BJT is indeed a voltagedependent current source; thus it can be used as an amplifier.
Common-Base Configuration
• Common-base terminology is derived from the fact that
the :
- base is common to both input and output of the
configuration.
- base is usually the terminal closest to or at
ground potential.
• All current directions will refer to conventional (hole) flow
and the arrows in all electronic symbols have a direction
defined by this convention.
• Note that the applied biasing (voltage sources) are such
as to establish current in the direction indicated for each
branch.
• To describe the behavior of common-base amplifiers
requires two set of characteristics:
- Input or driving point characteristics.
- Output or collector characteristics
• The output characteristics has 3 basic regions:
- Active region –defined by the biasing arrangements
- Cutoff region – region where the collector current is 0A
- Saturation region- region of the characteristics to the left of
VCB = 0V
• The curves (output characteristics) clearly indicate that
a first approximation to the relationship between IE
and IC in the active region is given by
IC ≈IE
• Once a transistor is in the ‘on’ state, the base-emitter
voltage will be assumed to be
VBE = 0.7V
• In the dc mode the level of IC and IE due to the
majority carriers are related by a quantity called alpha
= IC
IE
IC = IE + ICBO
• It can then be summarize to IC = IE (ignore ICBO due
to small value)
• For ac situations where the point of operation moves
on the characteristics curve, an ac alpha defined by
  IC
IE
• Alpha a common base current gain factor that shows
the efficiency by calculating the current percent from
current flow from emitter to collector.The value of  is
typical from 0.9 ~ 0.998.
Biasing
• Proper biasing CB configuration in active region by
approximation IC  IE (IB  0 uA)
Transistor as an amplifier
Simulation of transistor as an
amplifier
Common-Emitter Configuration
• It is called common-emitter configuration since :
- emitter is common or reference to both input and
output terminals.
- emitter is usually the terminal closest to or at
ground
potential.
• Almost amplifier design is using connection of CE due
to the high gain for current and voltage.
• Two set of characteristics are necessary to describe the
behavior for CE ;input (base terminal) and output
(collector terminal) parameters.
Proper Biasing common-emitter configuration in active region
• IB is microamperes compared
to miliamperes of IC.
• IB will flow when VBE > 0.7V
for silicon and 0.3V for
germanium
• Before this value IB is very
small and no IB.
• Base-emitter junction is
forward bias
• Increasing VCE will reduce IB
for different values.
Input characteristics for a
common-emitter NPN transistor
Output characteristics for a
common-emitter npn
transistor
• For small VCE (VCE < VCESAT, IC increase linearly with increasing
of VCE
•
•
•
VCE > VCESAT IC not totally depends on VCE  constant IC
IB(uA) is very small compare to IC (mA). Small increase in IB
cause big increase in IC
IB=0 A  ICEO occur.
• Noticing the value when IC=0A. There is still some value of
current flows.
Beta () or amplification factor
• The ratio of dc collector current (IC) to the dc base current
(IB) is dc beta (dc ) which is dc current gain where IC and
IB are determined at a particular operating point, Q-point
(quiescent point).
• It’s define by the following equation:
30 < dc < 300  2N3904
• On data sheet, dc=hFE with h is derived from ac hybrid
equivalent cct. FE are derived from forward-current
amplification and common-emitter configuration
respectively.
• For ac conditions an ac beta has been defined as the
changes of collector current (IC) compared to the
changes of base current (IB) where IC and IB are
determined at operating point.
• On data sheet, ac=hfe
• It can defined by the following equation:
Example
From output characteristics of common
emitter configuration, find ac and dc with an
Operating point at IB=25 A and VCE =7.5V.
Solution:
Relationship analysis between α and β
Active Mode: Terminal Currents
Current Relationships and Amplification
I C   .I E
I E  IC  I B
I B  I E  IC
IB 

IC

 IC
1

IC

I B  I B
1
I


and C  
1
IB
IC 

As  is close to unity,  is very large, typically around 100.

 represents the current amplification factor from base to
collector.

The base current is amplified by a factor of  in the collector
circuit in the Active mode.

 is called the Forward Current Gain, often written as  F.
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Amplification Action
Voltage Amplification: Active Mode

As the base-emitter junction is
forward biased, the source at the
input between EBJ sees a low
resistance.

However, as the CBJ is reverse
biased, the output resistance is
very high, typically in the range of
hundreds of kΩ to MΩ.
Basic voltage amplification action of
the common-base configuration.

Therefore, it is unlikely that the value of collector current IC will be affected by
a load resistance usually in the range of a few kΩ.

As such, a large load resistance will result in a large output voltage.

Therefore, the transistor is capable of both voltage and current amplification.

Voltage amplification is achieved by transferring the current from low
resistance to high resistance circuit and, thereby, the name TRANSISTOR.
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Common – Collector Configuration
• Also called emitter-follower (EF).
• It is called common-emitter configuration since both the
signal source and the load share the collector terminal as
a common connection point.
• The output voltage is obtained at emitter terminal.
• The input characteristic of common-collector
configuration is similar with common-emitter.
configuration.
• Common-collector circuit configuration is provided with
the load resistor connected from emitter to ground.
• It is used primarily for impedance-matching purpose
since it has high input impedance and low output
impedance.
Notation and symbols used with the common-collector configuration:
(a) pnp transistor ; (b) npn transistor.
• For the common-collector configuration, the output
characteristics are a plot of IE vs VCE for a range of values of IB.
Limits of Operation
• Many BJT transistor used as an amplifier. Thus it is
important to notice the limits of operations.
• At least 3 maximum values is mentioned in data sheet.
• There are:
a) Maximum power dissipation at collector: PCmax
or PD
b) Maximum collector-emitter voltage: VCEmax
sometimes named as VBR(CEO) or VCEO.
c) Maximum collector current: ICmax
• There are few rules that need to be followed for BJT
transistor used as an amplifier. The rules are:
i) transistor need to be operate in active region!
ii) IC < ICmax
Note:
VCE is at maximum and IC is at minimum (ICmax=ICEO) in the
cutoff region. IC is at maximum and VCE is at minimum
(VCE max = VCEsat = VCEO) in the saturation region. The transistor
operates in the active region between saturation and cutoff.
Refer to the fig.
Step1:
The maximum collector
power dissipation,
PD=ICmax x VCEmax
(1)
= 18m x 20 = 360 mW
Step 2:
At any point on the
characteristics the product of
and must be equal to 360 mW.
Ex. 1. If choose ICmax= 5 mA,
subtitute into the (1), we get
VCEmaxICmax= 360 mW
VCEmax(5 m)=360/5=7.2 V
Ex.2. If choose VCEmax=18 V,
subtitute into (1), we get
VCEmaxICmax= 360 mW
(10) ICmax=360m/18=20 mA