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Title: Bipolar Junction Transistors (BJT)


1
Bipolar Junction Transistors (BJT)
  • EBB424E
  • Dr. Sabar D. Hutagalung
  • School of Materials Mineral Resources
    Engineering, Universiti Sains Malaysia

2
Transistors
  • Two main categories of transistors
  • 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.

3
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rather quickly superseded by the junction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.
4
The Junction Transistor
  • 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.

5
Bipolar Junction Transistors (BJT)
  • 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.

6
The First BJT
Transistor Size (3/8L X 5/32W X 7/32H) No Date
Codes. No Packaging.
7
Modern Transistors
8
BJT Fabrication
  • 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.
  • 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.

9
BJT Structure - Discrete
  • 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.

10
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.
  • In the planar process, all steps are performed
    from the surface of the wafer

11
  • BJTs are usually constructed vertically
  • Controlling depth of the emitters n doping sets
    the base width

12
Advanced BJT Structures
  • The original BJT structure survived, practically
    unchanged, since the mid 60s.
  • As the advances in MOS development appears, some
    of the fabrication technology are also applied to
    the BJT.
  • Low defect epitaxy
  • Ion implant
  • Plasma etching (dry etch)
  • LOCOS (local oxidation of Si)
  • Polysilicon layers
  • Improved lithography

13
Isolation Methods
  • 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.

14
Oxide Trench Isolation
  • Oxide isolation processes were intorduced in the
    late 70s. They utilize wet anisotropic etch
    (KOH) of the lt100gt Si wafer with Si3N4 as mask.
  • The KOH etch will erode the lt111gt 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.

15
Double Poly Transistors
  • 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.

16
Example of BJT Specification Sheet
17
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
18
How the BJT works
  • 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
19
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.
  • Hence a Emitter-Collector current magnitude is
    set by the chosen Emitter-Base voltage applied.
  • Hence an external current flowing in the circuit.

Apply an Emitter-Base voltage
20
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
21
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
22
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.

23
Terminals Operations
24
Operation Mode
25
Operation Mode
  • Active
  • Most importance mode, e.g. for amplifier
    operation.
  • The region where current curves are practically
    flat.
  • Saturation
  • Barrier potential of the junctions cancel each
    other out causing a virtual short.
  • Ideal transistor behaves like a closed switch.
  • Cutoff
  • Current reduced to zero
  • Ideal transistor behaves like an open switch.

26
Operation Mode
27
BJT in Active Mode
  • Operation
  • 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

28
Circuit Symbols
29
Circuit Configuration
30
Band Diagrams (In equilibrium)
  • No current flow
  • Back-to-back PN diodes

31
Band Diagrams (Active Mode)
  • EBJ forward biased
  • Barrier reduced and so electrons diffuse into the
    base
  • Electrons get swept across the base into the
    collector
  • CBJ reverse biased
  • Electrons roll down the hill (high E-field)

32
Minority Carrier Concentration Profiles
  • 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
  • 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

33
Diffusion Current Through the Base
  • Diffusion of electrons through the base is set by
    concentration profile at the EBJ
  • Diffusion current of electrons through the base
    is (assuming an ideal straight line case)
  • 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

34
Collector Current
  • 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.
  • Note that iC is independent of vCB (potential
    bias across CBJ) ideally
  • Saturation current is
  • inversely proportional to W and directly
    proportional to AE
  • Want short base and large emitter area for high
    currents
  • dependent on temperature due to ni2 term

35
Collector Current
  • 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.
  • Note that iC is independent of vCB (potential
    bias across CBJ) ideally
  • Saturation current is
  • inversely proportional to W and directly
    proportional to AE
  • Want short base and large emitter area for high
    currents
  • dependent on temperature due to ni2 term

36
Collector Current
  • 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.
  • Note that iC is independent of vCB (potential
    bias across CBJ) ideally
  • Saturation current is
  • inversely proportional to W and directly
    proportional to AE
  • Want short base and large emitter area for high
    currents
  • dependent on temperature due to ni2 term

37
Base Current
  • Base current iB composed of two components
  • holes injected from the base region into the
    emitter region
  • 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
  • Total base current is

38
Beta
  • 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

39
Emitter Current
  • Emitter current is the sum of iC and iB
  • a is called the common-base current gain

40
I-V Characteristics
  • Collector current vs. vCB shows the BJT looks
    like a current source (ideally)
  • Plot only shows values where BCJ is reverse
    biased and so BJT in active region
  • However, real BJTs have non-ideal effects

41
I-V Characteristics
Collector-emitter is a family of curves which are
a function of base current.
Base-emitter junction looks like a forward biased
diode
42
I-V Characteristics
43
Example
  • Calculate the values of ß and a from the
    transistor shown in the previous graphs.

44
Early Effect
  • Early Effect
  • 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
  • IC is collector current at the boundary of active
    region

45
Early Effect
  • What causes the Early Effect?
  • 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

46
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.
47
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.
48
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.
49
Collector Resistance, rC
50
Emitter Resistance, rE
51
Base Resistance, rB
  • 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 (SPICEs default model
    for BJTs), rB is assumed to be constant.

52
Breakdown Voltages
  • 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.

53
Breakdown Voltages
54
Breakdown Voltages
55
Breakdown Voltages
56
BJT Analysis
  • Here is a common emitter BJT amplifier
  • What are the steps?

57
Input Output
  • 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).

58
Input Equation
  • To start, lets write Kirchoffs voltage law
    (KVL) around the base circuit.

59
Output Equation
Likewise, we can write KVL around the collector
circuit.
60
Use Superposition DC AC sources
  • 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
  • Short Circuit any AC voltage sources
  • Open Circuit any AC current sources
  • Next, the AC analysis to calculate gains of the
    amplifier.
  • Depends on how we perform AC analysis
  • Graphical Method
  • Equivalent circuit method for small AC signals

61
BJT - DC Analysis
  • 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.

62
Base-Emitter Circuit Q point
The Load Line intersects the Base-emitter
characteristics at VBEQ 0.6 V and IBQ 20 µA
63
Collector-Emitter Circuit Q point
Now that we have the Q-point for the base
circuit, lets 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 ß 2.5m/20 µ 125
64
BJT DC Analysis - Summary
  • Calculating the Q-point for BJT is the first step
    in analyzing the circuit
  • To summarize
  • 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

65
BJT - AC Analysis
  • 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.

66
BJT - AC Analysis
  • Lets assume that Vin(t) 0.2 sin(?t).
  • 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
  • Maximum value 1.8 / 50k 36 µA
  • Minimum value 1.4 / 50k 28 µA

67
AC Analysis Base-Emitter Circuit
Note the asymmetry around the Q-point of the Max
and Min Values for the base current and voltage
which is due to the non-linearity of the
base-emitter characteristics
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
?i?max 24-20 4 µA ?iBmin 20-15 5 µA
68
AC Analysis Base-Emitter Circuit
69
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
70
AC Characteristics-Collector Circuit
71
BJT AC Analysis - Amplifier Gains
From the values calculated from the base and
collector circuits we can calculate the amplifier
gains
72
BJT AC Analysis - Summary
  • Once we complete DC analysis, we analyze the
    circuit from an AC point of view.
  • AC analysis can be performed via a graphical
    processes
  • 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

73
The pnp Transistor
  • Basically, the pnp transistor is similar to the
    npn except the parameters have the opposite sign.
  • 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.

74
The PNP Transistor
Current flow in a pnp transistor biased to
operate in the active mode.
75
The pnp Transistor
  • Two junctions
  • Collector-Base and Emitter-Base
  • Biasing
  • vBE Forward Biased
  • vCB Reverse Biased

76
(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.
77
The pnp Transistor
Current flow in an pnp transistor biased to
operate in the active mode.
78
The pnp Transistor
Two large-signal models for the pnp transistor
operating in the active mode.
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