Transcript Bipolar Junction Transistors

Bipolar Junction Transistors
(BJT)
EBB424E
Dr. Sabar D. Hutagalung
School of Materials & Mineral Resources Engineering,
Universiti Sains Malaysia
Transistors
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Two main categories of transistors:
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bipolar junction transistors (BJTs) and
field effect transistors (FETs).
Transistors have 3 terminals where the application of
current (BJT) or voltage (FET) to the input terminal
increases the amount of charge in the active region.
The physics of "transistor action" is quite different for
the BJT and FET.
In analog circuits, transistors are used in amplifiers
and linear regulated power supplies.
In digital circuits they function as electrical switches,
including logic gates, random access memory (RAM),
and microprocessors.
The First Transistor: Point-contact transistor
A point-contact transistor
was the first type of solid
state electronic transistor
ever constructed.
It was made by researchers
John Bardeen & Walter
Houser Brattain at Bell
Laboratories in December
1947.
The point-contact transistor was
commercialized and sold by Western
Electric and others but was rather
quickly superseded by the junction
transistor.
The Junction Transistor
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First BJT was invented early in 1948, only
weeks after the point contact transistor.
Initially known simply as the junction
transistor.
It did not become practical until the early 1950s.
The term “bipolar” was tagged onto the name to
distinguish the fact that both carrier types play
important roles in the operation.
Field Effect Transistors (FETs) are “unipolar”
transistors since their operation depends
primarily on a single carrier type.
Bipolar Junction Transistors (BJT)
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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.
The First BJT
Transistor Size (3/8”L X 5/32”W X 7/32”H)
No Date Codes. No Packaging.
Modern Transistors
BJT Fabrication
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BJT can be made either as discrete devices
or in planar integrated form.
In discrete, the substrate can be used for one
connection, typically the collector.
In integrated version, all 3 contacts appear
on the top surface.
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The E-B diode is closer to the surface than the
B-C junction because it is easier make the
havier doping at the top.
BJT Structure - Discrete
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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.
BJT Structure - Planar
The “Planar Structure” developed by
Fairchild in the late 50s shaped the basic
structure of the BJT, even up to the present
day.
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In the planar process, all steps are performed
from the surface of the wafer
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BJTs are usually constructed vertically
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Controlling depth of the emitter’s n doping sets the
base width
E
p
n
n
B
C
Advanced BJT Structures
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The original BJT structure survived, practically
unchanged, since the mid 60’s.
As the advances in MOS development appears,
some of the fabrication technology are also applied
to the BJT.
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Low defect epitaxy
Ion implant
Plasma etching (dry etch)
LOCOS (local oxidation of Si)
Polysilicon layers
Improved lithography
Isolation Methods
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The most significant advances in reducing overall device
size and packing density have come from improved
isolation methods.
The traditional junction isolation technique requires the
p+ deep diffusion to be aligned to the n+ buried layer
that is covered by a thick epitaxial layer.
The area (and hence junction capacitance) is determined
by alignment tolerance, area for side diffusion, and
allowance for the spread of the depletion region.
Modern isolation techniques: oxide isolation, and trench
isolation.
Oxide & Trench Isolation
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Oxide isolation processes were intorduced in the late 70’s. They utilize wet
anisotropic etch (KOH) of the <100> Si wafer with Si3N4 as mask.
The KOH etch will erode the <111> plane. Oxide is either deposited or
grown to fill the V-grooves.
The base and emitter are formed on the large mesa and the collector on the
small mesa.
To further reduce the area between adjacent mesa, trench isolation can be
used, making use of trench etching.
The trench is typically 2µm wide and 5µm deep. The trench walls are
oxidized and the remaining volume is filled with polysilicon.
Double Poly Transistors
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A further extension of the self-aligned BJT structure is to use
double polysilicon (n+ for emitter, p+ for base) to reduce the
area required for contacts.
Example of BJT Specification Sheet
How the BJT works
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NPN Bipolar Transistor
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 PNJunctions, so no charge
wants to move from one layer
to another.
How the BJT works
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Apply a Collector-Base voltage
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 Ptype holes apart.
This widens the depletion
zone between the collector
and base and so no current
will flow.
In effect we have reversebiassed the Base-Collector
diode junction.
Charge Flow
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Apply an Emitter-Base voltage
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What happens when we apply a
relatively small Emitter-Base voltage
whose polarity is designed to forwardbias the Emitter-Base junction.
This 'pushes' electrons from the
Emitter into the Base region and sets
up a current flow across the EmitterBase 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.
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
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Some electron fall into a hole
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.
Charge Flow
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Some electron fall into a hole
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.
Terminals & Operations
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Three terminals:
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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):
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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.
Terminals & Operations
Operation Mode
Operation Mode
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Active:
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Saturation:
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Most importance mode, e.g. for amplifier operation.
The region where current curves are practically flat.
Barrier potential of the junctions cancel each other out
causing a virtual short.
Ideal transistor behaves like a closed switch.
Cutoff:
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Current reduced to zero
Ideal transistor behaves like an open switch.
Operation Mode
BJT in Active Mode
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Operation
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Forward bias of EBJ injects electrons from emitter into base
(small number of holes injected from base into emitter)
Most electrons shoot through the base into the collector across
the reverse bias junction (think about band diagram)
Some electrons recombine with majority carrier in (P-type) base
region
Circuit Symbols
Circuit Configuration
Band Diagrams (In equilibrium)
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No current flow
Back-to-back PN diodes
Emitter
Base
Collector
Ec
Ef
Ev
N
P
N
Band Diagrams (Active Mode)
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EBJ forward biased
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Barrier reduced and so electrons diffuse into the base
Electrons get swept across the base into the collector
CBJ reverse biased
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Electrons roll down the hill (high E-field)
Emitter
Base
Collector
Ec
Ef
Ev
N
P
N
Minority Carrier Concentration Profiles
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Current dominated by electrons from emitter to base (by design) b/c of the
forward bias and minority carrier concentration gradient (diffusion) through
the base
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some recombination causes bowing of electron concentration (in the base)
base is designed to be fairly short (minimize recombination)
emitter is heavily (sometimes degenerately) doped and base is lightly doped
Drift currents are usually small and neglected
Diffusion Current Through the Base
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Diffusion of electrons through the base is set by concentration profile at the EBJ
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Diffusion current of electrons through the base is (assuming an ideal straight line
case):
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Due to recombination in the base, the current at the EBJ and current at the CBJ are
not equal and differ by a base current
Collector Current
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Electrons that diffuse across the base to the CBJ junction are swept across
the CBJ depletion region to the collector b/c of the higher potential applied
to the collector.
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Note that iC is independent of vCB (potential bias across CBJ) ideally
Saturation current is
 inversely proportional to W and directly proportional to AE
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Want short base and large emitter area for high currents
2
 dependent on temperature due to ni term
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Collector Current
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Electrons that diffuse across the base to the CBJ junction are swept across
the CBJ depletion region to the collector b/c of the higher potential applied
to the collector.
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Note that iC is independent of vCB (potential bias across CBJ) ideally
Saturation current is
 inversely proportional to W and directly proportional to AE
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Want short base and large emitter area for high currents
2
 dependent on temperature due to ni term
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Collector Current
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Electrons that diffuse across the base to the CBJ junction are swept across
the CBJ depletion region to the collector b/c of the higher potential applied
to the collector.
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Note that iC is independent of vCB (potential bias across CBJ) ideally
Saturation current is
 inversely proportional to W and directly proportional to AE
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Want short base and large emitter area for high currents
2
 dependent on temperature due to ni term
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Base Current
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Base current iB composed of two components:
 holes injected from the base region into the emitter region
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holes supplied due to recombination in the base with diffusing electrons
and depends on minority carrier lifetime tb in the base
And the Q in the base is
So, current is
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Total base current is
Beta
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Can relate iB and iC by the following equation
and b is
Beta is constant for a particular transistor
 On the order of 100-200 in modern devices (but can be higher)
 Called the common-emitter current gain
For high current gain, want small W, low NA, high ND
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Emitter Current
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Emitter current is the sum of iC and iB
a is called the common-base current gain
I-V Characteristics
IC
IC
VBE3
VCE
VBE2
VBE
VBE1
VBE3 > VBE2 > VBE1
VCE
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Collector current vs. vCB shows the BJT looks like a
current source (ideally)
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Plot only shows values where BCJ is reverse biased and so BJT
in active region
However, real BJTs have non-ideal effects
I-V Characteristics
Base-emitter junction looks
like a forward biased diode
Collector-emitter is a family of
curves which are a function of
base current.
I-V Characteristics
Example:
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Calculate the
values of β
and α from the
transistor
shown in the
previous
graphs.
Early Effect
Saturation region
Active region
VBE3
VBE2
VBE1
-VA
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VCE
Early Effect
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Current in active region depends (slightly) on vCE
VA is a parameter for the BJT (50 to 100) and called the Early voltage
Due to a decrease in effective base width W as reverse bias increases
Account for Early effect with additional term in collector current equation
Nonzero slope means the output resistance is NOT infinite, but…
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IC is collector current at the boundary of active region
Early Effect
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What causes the Early Effect?
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Increasing VCB causes depletion region of CBJ to grow and
so the effective base width decreases (base-width
modulation)
Shorter effective base width  higher dn/dx
EBJ
CBJ
dn/dx
Wbase
VCB > VCB
Common-emitter
It is called the common-emitter configuration because (ignoring the
power supply battery) both the signal source and the load share the
emitter lead as a common connection point.
Common-collector
It is called the common-collector configuration because both the signal
source and the load share the collector lead as a common connection
point. Also called an emitter follower since its output is taken from the emitter
resistor, is useful as an impedance matching device since its input impedance is
much higher than its output impedance.
Common-base
This configuration is more complex than the other two, and is less
common due to its strange operating characteristics.
Used for high frequency applications because the base separates the
input and output, minimizing oscillations at high frequency. It has a high
voltage gain, relatively low input impedance and high output impedance
compared to the common collector.
Collector Resistance, rC
Emitter Resistance, rE
Base Resistance, rB
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Mainly effects small-signal and transient
responses.
Difficult to measure since it depends on bias
condition and is influenced by rE.
In the Ebers-Moll model (SPICE’s default
model for BJTs), rB is assumed to be
constant.
Breakdown Voltages
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The basic limitation of the max. voltage in a transistor is
the same as that in a pn junction diode.
However, the voltage breakdown depends not only on
the nature of the junction involved but also on the
external circuit arrangement.
In Common Base configuration, the maximum voltage
between the collector and base with the emitter open,
BVCBO is determined by the avalanche breakdown
voltage of the CBJ.
In Common Emitter configuration, the maximum voltage
between the collect and emitter with the base open,
BVCEO can be much smaller than BVCBO.
Breakdown Voltages
Breakdown Voltages
Breakdown Voltages
BJT Analysis
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Here is a
common
emitter BJT
amplifier:
What are the
steps?
Input & Output
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We would want to know the collector current (iC),
collector-emitter voltage (VCE), and the voltage across
RC.
To get this we need to fine the base current (iB) and the
base-emitter voltage (VBE).
Input Equation
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To start, let’s write Kirchoff’s voltage law (KVL)
around the base circuit.
Output Equation
Likewise, we can write KVL around the collector
circuit.
Use Superposition:
DC & AC sources
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Note that both equations are written so as to calculate the
transistor parameters (i.e., base current, base-emitter voltage,
collector current, and the collector-emitter voltage) for both the
DC signal and the AC signal sources.
Use superposition, calculate the parameters for each separately,
and add up the results:
 First, the DC analysis to calculate the DC Q-point
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Short Circuit any AC voltage sources
Open Circuit any AC current sources
Next, the AC analysis to calculate gains of the amplifier.
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Depends on how we perform AC analysis
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Graphical Method
Equivalent circuit method for small AC signals
BJT - DC Analysis
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Using KVL for the input and output circuits
and the transistor characteristics, the
following steps apply:
1. Draw the load lines on the transistor characteristics
2. For the input characteristics determine the Q point for
the input circuit from the intersection of the load line
and the characteristic curve (Note that some transistor
do not need an input characteristic curve.)
3. From the output characteristics, find the intersection of
the load line and characteristic curve determined from
the Q point found in step 2, determine the Q point for
the output circuit.
Base-Emitter Circuit Q point
The Load Line
intersects the
Base-emitter
characteristics
at VBEQ = 0.6 V
and IBQ = 20 µA
Collector-Emitter Circuit Q point
Now that we have
the Q-point for the
base circuit, let’s
proceed to the
collector circuit.
The Load Line intersects the Collector-emitter characteristic, iB = 20 µA at
VCEQ = 5.9 V and ICQ = 2.5mA, then β = 2.5m/20 µ = 125
BJT DC Analysis - Summary
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Calculating the Q-point for BJT is the first step in
analyzing the circuit
To summarize:
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We ignored the AC (variable) source
 Short circuit the voltage sources
 Open Circuit the current sources
We applied KVL to the base-emitter circuit and using load line
analysis on the base-emitter characteristics, we obtained the
base current Q-point
We then applied KVL to the collector-emitter circuit and using
load line analysis on the collector-emitter characteristics, we
obtained the collector current and voltage Q-point
This process is also called DC Analysis
We now proceed to perform AC Analysis
BJT - AC Analysis
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How do we handle the variable source Vin(t) ?
When the variations of Vin(t) are large we will
use the base-emitter and collector-emitter
characteristics using a similar graphical
technique as we did for obtaining the Q-point.
When the variations of Vin(t) are small we will
shortly use a linear approach using the BJT
small signal equivalent circuit.
BJT - AC Analysis
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Let’s assume that Vin(t) = 0.2 sin(ωt).
Then the voltage sources at the base vary from a
maximum of 1.6 + 0.2 = 1.8 V to a minimum of 1.6 0.2 = 1.4 V
We can then draw two “load lines” corresponding
the maximum and minimum values of the input
sources
The current intercepts then become for the:
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Maximum value: 1.8 / 50k = 36 µA
Minimum value: 1.4 / 50k = 28 µA
AC Analysis Base-Emitter Circuit
From this graph, we find:
At Maximum Input Voltage:
VBE = 0.63 V, iB = 24 µA
At Minimum Input Voltage:
VBE = 0.59 V, iB = 15 µA
Recall: At Q-point:
VBE = 0.6 V, iB = 20 µA
Note the asymmetry around the Qpoint of the Max and Min Values for
the base current and voltage which
is due to the non-linearity of the
base-emitter characteristics
∆iΒmax = 24-20 = 4 µA;
∆iBmin = 20-15 = 5 µA
AC Analysis Base-Emitter Circuit
AC Characteristics-Collector Circuit
Using these max and min values for the base current on the collect
circuit load line, we find:
At Max Input Voltage: VCE = 5 V, iC = 2.7mA
At Min Input Voltage: VCE = 7 V, iC = 1.9mA
Recall: At Q-point: VCE = 5.9 V, iB = 2.5ma
AC Characteristics-Collector Circuit
BJT AC Analysis - Amplifier Gains
From the values calculated from the base and
collector circuits we can calculate the amplifier gains:
BJT AC Analysis - Summary
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Once we complete DC analysis, we analyze the
circuit from an AC point of view.
AC analysis can be performed via a graphical
processes
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Find the maximum and minimum values of the input
parameters (e.g., base current for a BJT)
Use the transistor characteristics to calculate the output
parameters (e.g., collector current for a BJT).
Calculate the gains for the amplifier
The pnp Transistor
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Basically, the pnp transistor is similar to the
npn except the parameters have the opposite
sign.
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The collector and base currents flows out of the
transistor; while the emitter current flows into the
transistor
The base-emitter and collector-emitter voltages
are negative
Otherwise the analysis is identical to the npn
transistor.
The PNP Transistor
Current flow in a pnp transistor biased to operate in the active
mode.
The pnp Transistor
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Two junctions
 Collector-Base and Emitter-Base
Biasing
 vBE Forward Biased
 vCB Reverse Biased
IE
p+ n
Input
circuit
p
IC
pnp
E
B I
B
(a)
Base
B
Emitter
E
x
np(0)
np(x)
E
VEB
(c)
V CB
C
B
E
pn(0)
pn(x)
IE
(b)
Collector
C
E
Output
circuit
C
IC
IE
Electron
Diffusion
Hole
IC
Hole
diffusion d ri f t
pno
npo
Re c obi nat ion
W EB
V EB
WB
IB
W BC
Ele c trons
Leakage
current
V CB
IB
(a) A schematic illustration of pnp BJT with 3 differently doped regions. (b)
The pnp bipolar operated under normal and active conditions. (c) The CB
configuration with input and output circuits identified. (d) The illustration of
various current component under normal and active conditions.
(d)
The pnp Transistor
Current flow in an pnp transistor biased to operate in the
active mode.
The pnp Transistor
Two large-signal models for the pnp transistor operating in
the active mode.