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rather quickly superseded by the junction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.

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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.

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

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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

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• 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

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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.

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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.

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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.

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Example of BJT Specification Sheet
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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
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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
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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
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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
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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.

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Terminals Operations
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Operation Mode
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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.

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Operation Mode
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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

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Circuit Symbols
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Circuit Configuration
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Band Diagrams (In equilibrium)

• No current flow
• Back-to-back PN diodes

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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

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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

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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

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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

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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
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I-V Characteristics
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Example

• Calculate the values of ß and a from the
  transistor shown in the previous graphs.

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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

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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

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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.
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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.
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Collector Resistance, rC
50
Emitter Resistance, rE
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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.

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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.

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Breakdown Voltages
54
Breakdown Voltages
55
Breakdown Voltages
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BJT Analysis

• Here is a common emitter BJT amplifier
• What are the steps?

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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).

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Input Equation

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

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Output Equation
Likewise, we can write KVL around the collector
circuit.
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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

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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.

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Base-Emitter Circuit Q point
The Load Line intersects the Base-emitter
characteristics at VBEQ 0.6 V and IBQ 20 µA
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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
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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

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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.

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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

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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
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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
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AC Characteristics-Collector Circuit
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BJT AC Analysis - Amplifier Gains
From the values calculated from the base and
collector circuits we can calculate the amplifier
gains
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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

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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.

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The PNP Transistor
Current flow in a pnp transistor biased to
operate in the active mode.
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The pnp Transistor

• Two junctions
• Collector-Base and Emitter-Base
• Biasing
• vBE Forward Biased
• vCB Reverse Biased

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